Understanding Magnetic Flux Leakage Testing Reading 1

Understanding 

Magnetic Flux Leakage 

Reading 1 

My ASNT Level III Pre-Exam Study Note 

30th August 2015 

Charlie Chong/ Fion Zhang

Permafrost Zone Pipeline MFLT 


Offshore Pipeline MFLT 


Cross Country Pipeline MFLT 










Tank Bottom MFLT 






Wire Rope MFLT 


Drilling String MFLT 


Reading I 

Content 

• Reading One: E1571 (Revisiting) 

• Reading Two: Magnetic Flux and SLOFEC Inspection of Thick Walled 

Components (Revisited) 

• Reading Three: 

• Reading Four: 


Principle of MFL Testing 

MFL testing is a magnetic based NDT method. The method is used to detect 

corrosion and cracks in ferromagnetic materials, such as pipelines, storage 

tanks, ropes and cables [10,27–29]. The basic principle of MFL testing is that 

the flux lines pass through the steel wires when a magnetic field is applied to 

the cable. At areas where corrosion or missing metal exists, the magneticfield 

leaks from the wires. 

In an MFL tool, magnetic sensors are placed between the poles of the 

magnet to detect the leakage field. The signal of the leakage field is analyzed 

to identify the damaged areas and estimate the amount of metal loss. Thus, 

the transducer includes magnetizers and magnetic sensors. 

The magnetic-field can be produced by a permanent magnet yoke, or a 

solenoid with the direct current. The magnetic-field density needs to meet 

near saturation under the sensor. Figure 3 shows the principle of MFL testing. 

The permanent magnet yoke is used to produce the magnetic-field, and the 

coil is used to induce (detect?) the leakage field. 


Figure 3. Principle of MFL testing. (a) Undamaged cable; (b) Cable with 

metal loss. 



E1571 

Standard Practice for Electromagnetic 

Examination of Ferromagnetic Steel Wire 

Rope 


1. Scope 

1.1 This practice covers the application and standardization of instruments 

that use the electromagnetic, the magnetic flux, and the magnetic flux 

leakage examination method to detect flaws and changes in metallic crossectional 

areas in ferromagnetic wire rope products. 

1.1.1 This practice includes rope diameters up to 2.5 in. (63.5 mm). Larger 

diameters may be included, subject to agreement by the users of this practice. 

1.2 This standard does not purport to address all of the safety concerns, if 

any, associated with its use. It is the responsibility of the user of this standard 

to establish appropriate safety and health practices and determine the 

applicability of regulatory limitations prior to use. 


2. Referenced Documents 

2.1 ASTM Standards: 

E 543 Practice for Agencies Performing Nondestructive Testing 

E 1316 Terminology for Nondestructive Examinations2 


3. Terminology 

3.1 Definitions—See Terminology E 1316 for general terminology applicable 

to this practice. 

3.2 Definitions of Terms Specific to This Standard: 

3.2.1 dual- unction instrument—a wire rope NDT instrument designed to 

detect and display changes of metallic cross-sectional area on one channel 

and local flaws on another channel of a dual-channel strip chart recorder or 

another appropriate device. 

3.2.2 local flaw (LF)—a discontinuity in a rope, such as a broken or damaged 

wire, a corrosion pit on a wire, a groove worn into a wire, or any other 

physical condition that degrades the integrity of the rope in a localized 

manner. 

3.2.3 loss of metallic cross-sectional area (LMA)—a relative measure of the 

amount of material (mass) missing from a location along the wire rope and is 

measured by comparing a point with a reference point on the rope that 

represents maximum metallic cross-sectional area, as measured with an 

instrument. 


3.2.4 single-function instrument—a wire rope NDT instrument designed to 

detect and display either changes in metallic cross-sectional area or local 

flaws, but not both, on a strip chart recorder or another appropriate device. 

Keywords: 

changes in metallic cross-sectional area 

local flaws 


4. Summary of Practice 

4.1 The principle of operation of a wire rope nondestructive examination 

instrument is as follows: 

4.1.1 AC Electromagnetic Instrument—An electromagnetic wire rope 

examination instrument works on the transformer principle with primary and 

secondary coils wound around the rope (Fig. 1). The rope acts as the 

transformer core. The primary (exciter) coil is energized with a low frequency 

alternating current (ac), typically in the 10 to 30 Hz range. The secondary 

(search) coil measures the magnetic characteristics of the rope. Any 

significant change in the magnetic characteristics in the core (wire rope) will 

be reflected as voltage changes (amplitude and phase) in the secondary coil. 

Electromagnetic instruments operate at relatively low magnetic field strengths; 

therefore, it is necessary to completely demagnetize the rope before the start 

of an examination. This type of instrument is designed to detect changes in 

metallic crosssectional area. 


Keywords: 

• AC- Alternating Current System 

• Electromagnetic instruments operate at relatively low magnetic field 

strengths; 

• it is necessary to completely demagnetize the rope before the start of an 

examination. 

• This type of instrument is designed to detect changes in metallic 

crosssectional area. 


Alternating Field MFL method 

The Alternating Field MFL probe rotates at high speed around the 

longitudinally moved test material and scans its surface helically. The rotating 

probe scans „punctiform“ only a small area of the material surface at any 

moment, i.e. when testing, it focuses on a very small part of the overall 

surface. Thus, even an extremely small material flaw represents a major 

disturbance with respect to this relatively small material surface area. One 

other advantage of the rotating probe method: Long drawn-out material flaws 

are indicated over their full length. 

Charlie Chong/ Fion Zhang 

MAGNETIC FLUX LEAKAGE TESTING WITH CIRCOFLUX®







FIG. 1 Schematic Representation of an Electromagnetic Instrument Sensor- 

Head 


4.1.2 Direct Current and Permanent Magnet (Magnetic Flux) Instruments- 

Direct current (dc) and permanent magnet instruments (Figs. 2 and 3) supply 

a constant flux that magnetizes a length of rope as it passes through the 

sensor head (magnetizing circuit). The total axial magnetic flux in the rope 

can be measured either by Hall effect sensors, an encircling (sense) coil, or 

by any other appropriate device that can measure absolute magnetic fields or 

variations in a steady magnetic field. The signal from the sensors is 

electronically processed, and the output voltage is proportional to the volume 

of steel or the change in metallic cross-sectional area, within the region of 

influence of the magnetizing circuit. This type of instrument measures 

changes in metallic cross-sectional area. 


FIG. 2 Schematic Representation of a Permanent Magnet Equipped Sensor- 

Head Using a Sense Coil to Measure the Loss of Metallic Cross- ectional 

Area 


FIG. 2 Schematic Representation of a Permanent Magnet Equipped Sensor- 

Head Using a Sense Coil to Measure the Loss of Metallic Cross- ectional 

Area 

Sensor Head 


8.1.3 The sensor head, containing the energizing 

and detecting units, and other components, should 

be designed to accommodate different rope 

diameters. The rope should be approximately 

centered in the sensor head.

FIG. 3 Schematic Representation of a Permanent Magnet Equipped Sensorhead 

Using Hall Devices to Measure the Loss of Metallic Cross-Sectional 

Area 

Sensor Head 

Hall Devices 

Hall Devices 


4.1.3 Magnetic Flux Leakage Instrument- A direct current (DC) or permanent 

magnet instrument (Fig. 4) is used to supply a constant flux that magnetizes a 

length of rope as it passes through the sensor head (magnetizing circuit). The 

magnetic flux leakage created by a discontinuity in the rope, such as a broken 

wire, can be detected with a differential sensor, such as a Hall effect sensor, 

sensor coils, or by any appropriate device. The signal from the sensor is 

electronically processed and recorded. This type of instrument measures LFs. 

While the information is not quantitative as to the exact nature and magnitude 

of the causal flaws, valuable conclusions can be drawn as to the presence of 

broken wires, internal corrosion, and fretting of wires in the rope.” 


4.2 The examination is conducted using one or more techniques discussed in 

4.1. Loss of metallic cross-sectional area can be determined by using an 

instrument operating according to the principle discussed in 4.1.1 and 4.1.2. 

Broken wires and internal (or external) corrosion can be detected by using a 

magnetic flux leakage instrument as described in 4.1.3. The examination 

procedure must conform to Section 9. One instrument may incorporate both 

magnetic flux and magnetic flux leakage principles. 


5. Significance and Use 

5.1 This practice outlines a procedure to standardize an instrument and to 

use the instrument to examine ferromagnetic wire rope products in which the 

electromagnetic, magnetic flux, magnetic flux leakage, or any combination of 

these methods is used. If properly applied, the electromagnetic and the 

magnetic flux methods are capable of detecting the presence, location, and 

magnitude of metal loss from wear and corrosion, and the magnetic flux 

leakage method is capable of detecting the presence and location of flaws 

such as broken wires and corrosion pits. 

5.2 The instrument’s response to the rope’s fabrication, installation, and inservice-induced 

flaws can be significantly different from the instrument’s 

response to artificial flaws such as wire gaps or added wires. For this reason, 

it is preferable to detect and mark (using set-up standards that represent) real 

in-service-induced flaws whose characteristics will adversely affect the 

serviceability of the wire rope. 


6. Basis of Application 

6.1 The following items require agreement by the users of this practice and 

should be included in the rope examination contract: 

6.1.1 Acceptance criteria. 

6.1.2 Determination of LMA, or the display of LFs, or both. 

6.1.3 Extent of rope examination (that is, full length that may require several 

setups or partial length with one setup). 

6.1.4 Standardization method to be used: wire rope reference standard, rod 

reference standards, or a combination thereof. 

6.1.5 Maximum time interval between equipment standardizations. 


6.2 Wire Rope Reference Standard (Fig. 5): 

6.2.1 Type, dimension, location, and number of artificial anomalies to be 

placed on a wire rope reference standard. 

6.2.2 Methods of verifying dimensions of artificial anomalies placed on a wire 

rope reference standard and allowable tolerances. 

6.2.3 Diameter and construction of wire rope(s) used for a wire rope reference 

standard. 

6.3 Rod Reference Standards (Fig. 6): 

6.3.1 Rod reference standard use, whether in the laboratory or in the field, or 

both. 

6.3.2 Quantity, lengths, and diameters of rod reference standards. 


FIG. 5 Example of a Wire Rope Reference Standard 


FIG. 6 Example of a Rod Reference Standard 


7. Limitations 

7.1 General Limitations: 

7.1.1 This practice is limited to the examination of ferromagnetic steel ropes. 

7.1.2 It is difficult, if not impossible, to detect flaws at or near rope 

terminations and ferromagnetic steel connections. 

7.1.3 Deterioration of a purely metallurgical nature (brittleness, fatigue, etc.) 

may not be easily distinguishable. 

7.1.4 A given size sensor head accommodates a limited range of rope 

diameters, the combination (between rope outside diameter and sensor head 

inside diameter) of which provides an acceptable minimum air gap to assure 

a reliable examination. 

air gap 


7.2 Limitations Inherent in the Use of Electromagnetic and Magnetic Flux 

Methods (LMA) : 

7.2.1 Instruments designed to measure changes in metallic cross- sectional 

area are capable of showing changes relative to that point on the rope where 

the instrument was standardized. 

7.2.2 The sensitivity of these methods may decrease with the depth of the 

flaw from the surface of the rope and with decreasing gaps between the ends 

of the broken wires. 

Factor affecting measured LMA 


7.3 Limitations Inherent in the Use of the Magnetic Flux Leakage Method: 

7.3.1 It may be impossible to discern relatively smalldiameter broken wires, 

broken wires with small gaps, or individual broken wires within closely-spaced 

multiple breaks. It may be impossible to discern broken wires from wires with 

corrosion pits. 

7.3.2 Because deterioration of a purely metallurgical nature may not be easily 

distinguishable, more frequent examinations may be necessary after broken 

wires are detected to determine when the rope should be retired, based on 

percent rate of increase of broken wires. 

Keywords: 

■ Electromagnetic Method (AC-LMA) (electromagnet) 

■ Magnetic Flux Method (DC-LMA) (electromagnet or permanent magnet) 

■ Magnetic Flux Leakage Method 

(DC-LF) (electromagnet or permanent magnet) 


8. Apparatus 

8.1 The equipment used shall be specifically designed to examine 

ferromagnetic wire rope products. 

8.1.1 The energizing unit within the sensor head shall consist of (1) 

permanent or (2) electromagnets, or (2a) AC or (2b) DC solenoid coils 

configured to allow application to the rope at the location of service. 

8.1.2 The energizing unit, excluding the ac solenoid coil, shall be capable of 

magnetically saturating (except for electromagnetic AC method?) the range 

(size and construction) of ropes for which it was designed. 

8.1.3 The sensor head, containing the energizing and detecting units, and 

other components, should be designed to accommodate different rope 

diameters. The rope should be approximately centered in the sensor head. 


8.1.4 The instrument should have connectors, or other means, for transmitting 

output signals to strip chart recorders, data recorders, or a multifunction 

computer interface. The instrument may also contain meters, bar indicators, 

or other display devices, necessary for instrument setup, standardization, and 

examination. 

8.1.5 The instrument should have an (1) examination distance and (2) rope 

speed output indicating the current examination distance traveled and rope 

speed or, whenever applicable, have a proportional drive chart control that 

synchronizes the chart speed with the rope speed. 

8.2 Auxiliary Equipment The examination results shall be recorded on a 

permanent basis by either 

8.2.1 a strip chart recorder 

8.2.2 and/or by an other type of data recorder 

8.2.3 and/or by a multifunctional computer interface. 


9. Examination Procedure 

9.1 The electronic system shall have a pre-examination standardization 

procedure. 

9.2 The wire rope shall be examined for LFs or LMA, or both, as specified in 

the agreement by the users of this practice. The users may select the 

instrument that best suits the intended purpose of the examination. The 

examination should be conducted as follows: 

9.2.1 The rope must be demagnetized before examination (ALL- AC 

electromagnetic, DC/PM Magnetic flux and DC/PM Magnetic Flux Leakage methods) by an 

electromagnetic instrument. If a magnetic flux or a magnetic flux leakage 

instrument is used, it may be necessary to repeat the examination to 

homogenize the magnetization of the rope. 

9.2.2 The sensor head must be approximately centered around the wire rope. 

9.2.3 The instrument must be adjusted in accordance with a procedure. The 

sensitivity setting should be verified prior to starting the examination by 

inserting a ferromagnetic steel rod or wire of known cross-sectional area. This 

standardization signal should be permanently recorded for future reference. 

DC/PM = DC electromagnet of Permanent Magnet 


9.2.4 The wire rope must be examined by moving the head, or the rope, at a 

relatively uniform speed. Relevant signal(s) must be recorded on suitable 

media, such as on a strip chart recorder, on a tape recorder, or on computer 

file(s), for the purpose of both present and future replay/analysis. 


9.2.5 The following information shall be recorded as examination data for 

analysis: 

9.2.5.1 Date of examination, 

9.2.5.2 Examination number, 

9.2.5.3 Customer identification, 

9.2.5.4 Rope identification (use, location, reel and rope number, etc.), 

9.2.5.5 Rope diameter and construction, 

9.2.5.6 Instrument serial number, 

9.2.5.7 Instrument standardization settings, 

9.2.5.8 Strip chart recorder settings, 

9.2.5.9 Strip chart speed, 

9.2.5.10 Location of sensor head with respect to a welldefined reference point 

along the rope, both at the beginning of the examination and when 

commencing a second set-up run, 

9.2.5.11 Direction of rope or sensor head travel, 

9.2.5.12 Total length of rope examined, and 

9.2.5.13 examination speed. 


9.2.6 To assure repeatability of the examination results, two or more 

operational passes are required. 

9.2.7 When more than one setup is required to examine the full working 

length of the rope, the sensor head should be positioned to maintain the 

same magnetic polarity (?) with respect to the rope for all setups. For strip 

chart alignment purposes, a temporary marker should be placed on the rope 

at a point common to the two adjacent runs. (A ferromagnetic marker shows 

an indication on a recording device.) The same instrument detection signals 

should be achieved for the same standard when future examinations are 

conducted on the same rope. 


9.2.8 When determining percent LMA, it must be understood that 

comparisons are made with respect to a reference point on the rope 

representing maximum metallic cross sectional area. The reference point may 

have deteriorated such that it does not represent the original (new) rope. The 

reference point must be inspected visually to evaluate its condition. 

When determining percent LMA, it must be understood that comparisons are 

made with respect to a reference point on the rope that represents the rope’s 

maximum metallic crosssectional area. The reference point’s condition may 

have deteriorated during the rope’s operational use such that it no longer 

represents the original (new) rope values. The reference point must be 

examined visually, and possibly by other means, to evaluate its current 

condition. 


9.2.9 If the NDT indicates existence of significant rope deterioration at any 

rope location, an additional NDT of this location(s) should be conducted to 

check for indication repeatability. Rope locations at which the NDT indicates 

significant deterioration must be examined visually in addition to the NDT. 

9.3 Local flaw baseline data for LF and LMA/LF instruments may be 

established during the initial examination of a (new) rope. Whenever 

applicable, gain settings for future examination of the same rope should be 

adjusted to produce the same amplitude for a known flaw, such as a rod or 

wire attached to the rope. 


10. Reference Standard 

10.1 General: 

10.1.1 The instrument should be standardized with respect to the acceptance 

criteria established by the users of this practice. 

10.1.2 Standardization should be done the first time the instrument is used, 

during periodic checks, or in the event of a suspected malfunction. 

10.1.3 The instrument should be standardized using one or more of the 

following: 

■ wire rope reference standard with artificial flaws (see Fig. 5), or 

■ rod reference standards (see Fig. 6). 

For clarification, the following sections – 

10.2 and 10.3 – are useful for laboratory purposes to more fully understand 

instrument limitations. 


10.2 Wire Rope Reference Standard: 

10.2.1 The wire ropes selected for reference standards should be first 

examined to ascertain and account for the existence of interfering, preexisting 

flaws (if they exist) prior to the introduction of artificial flaws. The reference 

standard shall be that rope appropriate for the instrument and sensor head 

being used and for the wire rope to be examined unless rod reference 

standards are used. The reference standard shall be of sufficient length to 

permit the required spacing of artificial flaws and to provide sufficient space to 

avoid rope end effects. The selected configuration for the reference standard 

rope shall be as established by the users of this practice. 


10.2.2 Artificial flaws placed in the wire rope reference standard shall include 

gaps produced by removing, or by adding, lengths of outer wire. The gaps 

shall have typical lengths of 1/16 , 1/8 , 1/4 , 1/2 , 1, 2, 4, 8, 16, and 32 in. (1.6, 

3.2, 6.4, 12.7, 25.4, 50.8, 101.6, 203.2, 406.4, and 812.8 mm, respectively). 

The gaps shall typically be spaced 30 in. (762 mm) apart. There shall be a 

minimum of 48 in. (1219 mm) between gaps and the ends of the wire rope. 

Some of the gap lengths may not be required. All wire ends shall be square 

and perpendicular to the wire. 10.2.3 Stricter requirements than those stated 

above for local flaws and changes in metallic cross-sectional area may be 

established by the users if proven feasible for a given NDT instrument, 

subject to agreement by the users. 


10.3 Rod Reference Standard: 

10.3.1 Steel rods are assembled in a manner such that the total crossectional 

area will be equal to the cross-sectional area of the wire rope to be 

examined. The rod bundle is to be placed in the sensor head in a manner 

simulating the conditions that arise when a rope is placed along the axis of 

the examination head. Individual rods are to be removed to simulate loss of 

metallic area caused by wear, corrosion, or missing wires in a rope. This 

procedure gives highly accurate control of changes in instrument response 

and can be used to adjust and standardize the instrument. 

10.3.2 The rods for laboratory standardization procedures should be a 

minimum of 3 ft (Approx. 1 m) in length to minimize end-effects from the rod 

ends, or as recommended by the instrument manufacturer. 

10.3.3 Shorter rods or wires may be used for a preexamination check in the 

field. 


10.4 Adjustment and Standardization of Apparatus Sensitivity: 

10.4.1 The procedure for setting up and checking the sensitivity of the 

apparatus is as follows: 

10.4.1.1 The reference standard shall be fabricated as specified in the 

agreement by the users. 

10.4.1.2 The sensor head shall be adjusted for the size of material to be 

examined. 

10.4.1.3 The sensor head shall be installed around the reference standard. 

10.4.1.4 The reference standard shall be scanned, and, whenever applicable, 

gain and zero potentiometers, chart recording scale, or other apparatus 

controls shall be adjusted for required performance. 

10.4.1.5 If standardization is a static procedure, as with an electromagnetic 

instrument (see 4.1.1), the standard reference rope shall be passed through 

the detector assembly at field examination speed to demonstrate adequate 

dynamic performance of the examination instrument. The instrument settings 

that provide required standardization shall be recorded. 


11. Test Agency Qualification 

11.1 Nondestructive Testing Agency Qualification—Use of an NDT agency (in 

accordance with Practice E 543) to perform the examination may be agreed 

upon by the using parties. If a systematic assessment of the capability of the 

agency is specified, a documented procedure such as Practice E 543 shall be 

used as the basis for the assessment. 

12. Keywords 

12.1 electromagnetic examination; flux leakage; local flaws (LF); Magnetic 

flux; magnetic flux leakage; percent loss of metallic cross-sectional area 

(LMA); rod reference standards; sensor head; wire rope; wire rope reference 

standard 


End Of Reading 1 



Magnetic Flux and SLOFEC Inspection of 

Thick Walled Components 


http://www.ndt.net/article/wcndt00/papers/idn352/idn352.htm

Summary: 

Magnetic Flux Leakage (MFL) inspection of low-alloy carbon steel 

components is attractive while, contrary to ultrasonic inspection, no acoustic 

coupling is needed between the sensor system and the object. Furthermore 

MFL is a fast and reliable method to detect local corrosion. The well-known 

and widely used traditional MFL method however is, despite efforts to 

improve, limited to a thickness of up to no more than 15 mm. This paper 

describes an improved highly sensitive MFL method with an upper thickness 

limit of at least 30 mm. The extended thickness capability of the new MFL tool 

makes the method suitable for a much wider range of applications, not only 

for inspection of thick components but also for thinner walls covered with thick 

non-metallic protection layers such as glass fibre reinforced epoxy coatings 

on floors of (oil)storage tanks. Moreover this improved MFL method is able to 

differentiate surface from back wall defects, which is a unique and very useful 

feature. The new MFL method, known as "SLOFEC" in the meantime has 

successfully been applied in the field on a variety of components. Background 

and applications of this new intriguing MFL tool for the NDT industry are 

described in this paper. 


1. Introduction 

NDT is an essential activity to establish the integrity of (petro)chemical) plants 

as part of regular maintenance[1]. Because of stringent maintenance cost 

reduction programs, application of NDT is ever more rationalised. 

Conventional inspection programs are often not taken for granted any more 

when viewed from new and better understanding of safety and risk. So called 

Risk Based Inspection (RBI) philosophies gradually influence or dictate what 

is done by NDT, what method, qualitative or quantitative, to what extent but 

always at the lowest possible cost. As a consequence one can observe that 

some constructions are inspected over their full surface with NDT screening 

tools, [1], because all places are considered of equal risk, e.g. the floor of an 

(oil) storage tank. 


On other components, e.g. on a pressure vessel, NDT can be limited to 

certain critical areas. The increasing knowledge of risk, failure and fracture 

mechanics has influenced the need to improve or adapt the capabilities of 

some NDT methods. The MFL method to inspect steel components fits very 

well in a full surface coverage and low cost inspection approach. As such it 

has been the prevailing method to inspect long distance pipelines for decades. 

Over the past decade, despite its limited quantitative capabilities, MFL 

became the most common method to inspect tank floors [2][3]. Unfortunately 

this "traditional“ MFL method is limited to a thickness of 10 or at best 15 mm 

under favourable field conditions. The demand for MFL tools with a larger 

thickness range is known for decades, but considerable efforts to increase the 

range of traditional MFL tools so far were hardly successful. Only marginal 

improvements could be achieved at a high cost and weight penalty. 


2. Progress in NDT capabilities 

These days capabilities of common NDT methods are stretched to the limit, 

one does hardly observe "quantum leaps" in performance any more, most 

progress was achieved in the past. Of course the implementation of computer 

technology and signal analysis in NDT systems, all accomplished in the last 

decade have resulted in sometimes large technical steps forward. A good 

illustration of this impressive progress is Computer Tomography in 

combination with ultrasonic or radiographic inspection. Such large 

improvements are often at high cost and only suitable and affordable for 

laboratory type use. Moreover due to the complexity of such systems they are 

not suitable for industrial bulk work and reduce the use of these CT systems 

to niche applications. From this historical viewpoint it is remarkable that only a 

few years ago a "quantum leap" was achieved with the relatively simple MFL 

technique. 


All of a sudden the thickness range could be increased to at least 30 mm in 

combination with several other unique inspection features. Besides, the 

improved MFL technique is very suitable for prevailing field conditions. After a 

period of proof of principle and verification the new method is now gradually 

becoming known in industry. The now maturing improved MFL method, offers 

economically affordable NDT solutions until recently not available to industry, 

it fulfils a demand and fits very well in the current inspection approaches. 


3. Traditional MFL Inspection systems 

The need for a fast and simple NDT technique which does not require 

acoustic coupling liquid as required in traditional ultrasonic inspection , was a 

major incentive to develop tools based on the MFL principle. Moreover an 

MFL system is rather tolerant to surface condition, removal of loose and 

excessive debris prior to inspection is sufficient. Because of this and other 

merits MFL has become the premier method to inspect long distance 

pipelines from the inside. This is done on stream with so called "intelligent 

pigs". The full pipe surface is inspected to reveal local metal loss either inside 

or outside. Over a number of decades these tools have been optimised and 

reached capabilities near perfection [4]. In the eighties the method was 

selected to inspect floors of (oil)storage tanks. Such tools now are a 

commodity. Figure 1 shows a system in use to inspect a tank floor. In more 

recent years some derivative MFL tools to inspect pipes from the outside 

were introduced. 


Fig 1: MFL inspection of a tank floor 


Fig 2: Adjustable MFL pipe scanner 


Silverwing UK - Floormap VS2i - MFL tank inspection, Corrosion Mapping 

and Detection Floors Scanner 

■ https://www.youtube.com/watch?v=8JtVJJp3mc8 

■ https://www.youtube.com/watch?v=c22z9Mo0PVs 


For inspection of bare or painted pipe from the outside, instead of single 

diameter scanners sometimes adjustable yoke and scanner constructions are 

used to make them suitable for a range of diameters. With such adjustable 

scanners, of which one is shown in Figure 2, inspection cost per meter of pipe 

can be reduced, a factor of paramount importance in the maintenance 

inspection world. 

The MFL method is very suitable to detect local corrosion and is qualitative 

rather than quantitative. 

Gradual thinning can not be detected. 

Once one needs quantitative data complementary methods e.g. ultrasonic 

inspection has to be applied. Another considerable drawback of the MFL 

technique is that rather heavy and bulky scanners are needed, adapted to the 

geometry of the component. This limits the application to large constructions 

of uniform geometry such as storage tanks, pipe lines and long lengths of 

plant piping. Despite some of the described limitations MFL has obtained a 

good reputation in industry also due to its high reliability of defect detection. 


4. Principle of "traditional" MFL 

The MFL method can only be applied on low alloy carbon steels which have a 

high magnetic permeability. The well known principle is illustrated in Figure 3. 

Fig 3: Principle of MFL to detect metal loss 


MFL method 



http://idea-ndt.en.alibaba.com/product/578144516-212166760/Magnetic_flux_leakage_testing_instrument.html

A magnet within a yoke construction is used to establish a uniform magnetic 

flux in the material to be inspected. The magnetisation should be up to a high 

level close to magnetic saturation. Usually strong permanent magnets are 

used to generate the magnetic field, but sometimes electromagnets are used 

if sufficient power is available, even combinations of both to achieve 

superimposed magnetisation. In a defect free plate the magnetic flux is 

uniform. In contrast a metal loss type defect, such as local corrosion or 

erosion, not only distorts the uniformity of the flux but a small portion of the 

magnetic flux is forced to "leak" out of the plate. 

Sensors placed between the poles of the magnet or yoke construction can 

detect this small local "leakage". 

The amount of distortion and leakage is dependent on depth, orientation, type 

and position (topside/back wall) of the defect. Defects are often of erratic form. 

Various combinations of volume loss can result in the same flux leakage level 

although not having the same depth. 


This causes that the method is and remains rather qualitative and not 

quantitative, despite efforts to apply signal analysis and adaptive learning 

software programs to improve depth sizing. Very often this mainly qualitative 

character is acceptable for industry in return for its high speed , full surface 

coverage and in particular its high probability of defect detection. 

Most of the MFL inspection tools make use of "passive" Hall effect sensors to 

detect flux leakage as indication of metal loss. The systems using Hall 

elements we call "traditional" MFL tools. Due to physical limits of the size of 

magnets and total weight of the necessary scanner there is an optimum in 

performance of traditional MFL tools. As a consequence thickness range is 

limited to 10 or at best 15 mm under favourable circumstances. Sensitivity 

drops dramatically with increasing thickness. Thus the challenge to design a 

tool for a much greater wall thickness remained. 


5. Principle of "improved" MFL (SLOFEC) 

Trying to increase the range of MFL tools, another sensor type in combination 

with a few other essential equipment modifications ultimately solved the 

problem. Instead of the "passive" Hall sensors, as illustrated in figure 3, 

"active" eddy current sensors are used to detect flux leakage, even better, 

these sensors can detect changes in flux density inside the plate. The 

sensing is virtually "in the plate" and this explains its higher sensitivity for 

variations of the magnetic flux than a passive sensor "at the plate surface". 

The principle has been known and systems existed already for a considerable 

time [5]. It is applied for steel (boiler) tube inspection not exceeding say 5 mm 

wall thickness, thus not for extreme thicknesses up to 30 mm being the 

subject of this paper. Eddy currents in steel have a small penetration depth 

due to the high relative magnetic permeability, say 500 or more. This limits 

penetration of the eddy currents to the outer surface. 

δ= √ (2/ωσμ) = 1/ √(πfσμ) = (πfσμ) -½ 


This so called "skin effect" is strongly reduced by magnetic saturation of the 

wall, causing a low relative permeability, say close to 1. This allows the eddy 

currents to penetrate much deeper, up to the full wall thickness. 

Magnetic saturation not only creates a low permeability and uniform flux, it 

also suppresses the usual local permeability variations in the material. This 

eliminates an enormous source of noise, which can hardly be filtered out, and 

otherwise would prohibit proper functioning of flux sensing systems. 

Keywords: 

• Magnetic saturation 

• Magnetic permeability μ=1 

• Uniform flux 

• Deeper penetration 

• Skin effect 

• Local permeability variations 

• Hall sensor (passive!) 

• Eddy current sensor (active) 


Figure 4: shows the relative sensitivity curves for traditional and improved 

MFL. 


These curves are typical for inspection results achieved on plates. This 

extreme high sensitivity is achieved with the eddy current sensors in 

combination with special electronics and fast on-line signal processing. In 

eddy current testing, phase information is provided and from that it can be 

established whether the defect is at the top or back wall of the component. 

Using phase information the type of defect can automatically be sorted out 

including a reasonable level of defect severity. In addition, although there is 

still room for improvement, the new system provides some information on 

general wall thickness reduction. Despite all these merits it can not replace 

ultrasonic inspection in terms of absolute accuracy. Experiments in the 

laboratory and field trials proved that with the improved MFL system a 

thickness of up to 30 mm and probably more can be inspected with a much 

higher overall sensitivity than with traditional MFL, this applies certainly for 

thickness range beyond 5 mm. 


The technical thickness limit of this new system is determined by the 

combination of sufficient magnetic saturation ( bias field) of the full wall 

thickness of the component and a low enough eddy current frequency to 

penetrate the full wall without sacrificing on inspection speed. 

Most probably the weight of the magnetic yoke and scanner dictate the real 

physical upper limit. The limits have not fully been explored yet. The now 

existing system, suitable for approximately 30 mm wall thickness, seems to 

be an optimum. The "new" MFL technology is called "SLOFEC". The acronym 

SLOFEC stands for Saturation LOw Frequency Eddy Current. 

Keywords: 

■ phase information is provided and from that it can be established whether 

the defect is at the top or back wall of the component. 

■ low enough eddy current frequency to penetrate the full wall without 

sacrificing on inspection speed 


6. Development of tools - market demand 

At first SLOFEC systems were built for customer specific "one-off "solutions. 

In fact a specific inspection problem which could not economically be solved 

with regular NDT provided the challenge. The customer needed a system to 

inspect thick large diameter buried bullet tanks from the inside to detect 

corrosion under the external tar coating. SLOFEC offered an affordable 

solution to inspect the tanks which otherwise had to be lifted, cleaned and 

inspected ; a not attractive expensive procedure. Figure 5 shows the partly 

buried tanks with a diameter of 5 metres. Figure 6 shows the typical "oneoff“ 

scanner, with adjustable diameter, built for this job ……………. 

For more read: http://www.ndt.net/article/wcndt00/papers/idn352/idn352.htm 




Reading 3: 

A Comparison of the Magnetic Flux Leakage and 

Ultrasonic Methods in the detection and 

measurement of corrosion pitting in ferrous plate 

and pipe 



INTRODUCTION 

Magnetic Flux Leakage (MFL) and manual Ultrasonics (UT) have been used 

extensively for the detection and sizing of corrosion pits in ferrous plates and 

pipes. Users and providers of these inspection services may have different 

perceptions and expectations of the sensitivity and accuracy of the methods. 

This paper discusses the underlying principles of the methods and their effect 

on Probability of Detection (POD) and accuracy. 

CORROSION PITTING 

There are many types and mechanisms of corrosion but in this instance we 

deal exclusively with corrosion that is typical between the pad and the 

underside of tank bottoms or from water contamination inside the tank. The 

ultrasonic means of detecting erosion in pipework was so successful during 

the 1960's that it has given a false impression of the accuracy that will be 

obtained with pitting type corrosion. To help appreciate the difference we will 

illustrate erosion and some typical pit shapes. Figure 1 shows erosion 

whereas Figures 2 to 4 sketch corrosion shapes that have been given the 

terms "Lake Type", "Cone Type" and "Pipe Type". 



Figures 5 to 8 are photographs of erosion and typical corrosion of the lake 

and cone type. It is interesting to note the steps or 'terraces' formed as the 

corrosion progressed. 

Lake and pipe (cone?) types of corrosion are most commonly found in 

storage tank floors. 

They are usually the result of moisture ingress between the floor and the pad 

(underside) or water in the product (topside). 

Pipe type pitting is relatively uncommon (?)and is usually associated with 

water droplet erosion or Sulphur Reducing Bacteria (SRB). 



METHOD PRINCIPLES 

MFL: 

The principles of both the MFL method and the UT method have been 

described in detail elsewhere. For the purposes of this paper these are briefly 

summarised here. Figure 9 illustrates the basic principle of the MFL method. 

A magnet mounted on a carriage induces a strong magnetic field in the plate 

or pipe wall. In the presence of a corrosion pit, a magnetic flux leakage field 

forms outside the plate or pipe wall. An array of sensors is positioned 

between the magnet poles to detect this flux leakage. The sensors are usually 

(1) Hall Effect devices or (2) (Eddy current?) coils; there are advantages and 

limitations with either type of sensor. 


UT: 

Figure 10 illustrates a simple UT set-up using the pulse-echo principle and a 

twin crystal probe. In this configuration one crystal acts as transmitter and the 

other as the receiver. The transmitter is isolated from the receiving circuits so 

that the A-scan display is freed from the presence of a transmission signal. As 

a result the transmission pulse does not obscure the first back wall echo 

when testing relatively thin areas of plate or pipe. We shall see that simple 

digital thickness meters without an A-scan facility are not suitable for either 

detection or measurement of pitting. 


PROBABILITY OF DETECTION - MFL. 

The MFL method uses an array of sensors such that each sensing field 

overlaps with its neighbour. The probability of detection of any flux leakage 

signal depends on the amplitude of that leakage field in relation to any noise 

signals. In other words, the signal to noise ratio is the primary factor 

governing detection. Some of the parameters affecting the signal to noise 

ratio are related to the equipment design and performance, and some are 

related to the floor condition including the geometry of any pitting. 

Equipment parameters 

Magnet design 

Sensor type and layout 

Speed control 

Vibration damping 

Signal processing 

Detection notification 

Floor parameters 

Floor material 

Scanning surface condition 

Scanning surface coating 

Cleanliness 

Pit depth 

Pit volume 

Pit contour 


Equipment Parameters 

■ Magnet design 

The magnet must be strong enough to achieve a flux density in the material 

being tested that is close to saturation. The carriage design must be such that 

the magnet system can ride any undulations in the scanning surface without 

too much variation in the gap between the magnet poles and the test surface 

(lift off). Clearly, one advantage of using Electro-magnets is that the 

magnetising force can be adjusted to compensate for different material 

thicknesses and lift off changes. A practical advantage is also that the 

magnetic field can be switched off to aid removal of the scanning head from 

the test surface. The major disadvantages are size and weight. For this 

reason many scanners resort to permanent magnets using Neodymium – iron 

- boron in the magnet design. The result is a compact scanning head suitable 

for wall thicknesses up to 12.5 mm, or, at reduced sensitivity, up to 20 mm. 

Greater thicknesses could be achieved provided that a suitable and safe 

system to place and remove the carriage from the test surface is devised. 


■ Sensor type and layout 

Two types of sensor are in common use, 

- coils and 

- Hall effect devices. 

In either case the spacing between adjacent elements in the array must be 

small enough to ensure that there are no gaps in detection across the array. If 

sensors are arranged in differential pairs for noise cancelling purposes, the 

layout should take into account the fact that the leakage field may extend 3 

or 4 times the diameter of the pit across the array but only about the 

diameter of the pit in the scanning direction. (?) The voltage signal 

generated by a given leakage field in a coil sensor is a function of the rate of 

cutting lines of force. This will be a function of the number of turns in the coil 

and the forward speed of the scanner. Thus the coil type of sensor is speed 

sensitive and this should be taken into account in the equipment design. Coils 

are also more sensitive to lift off variation than some configurations of Hall 

effect devices. 


One distinct advantage of the coil sensor is that it appears to be less affected 

than Hall effect devices by the strong eddy current signal that is generated 

during the acceleration and deceleration phases of the scanner. 

Hall effect devices are in principle less sensitive to speed variation, however 

when filtration is used during signal processing to remove low and high 

frequency spurious signals, the resulting band pass window imposes some 

restriction on speed variation. When these devices are arranged to detect the 

Horizontal component of the leakage field, they are relatively insensitive to 

the eddy current signal mentioned above, but, like the coil, relatively sensitive 

to lift off variations. When arranged to detect the Vertical component, they are 

less sensitive to lift off variations but very sensitive to the eddy current signals. 

One advantage of this arrangement, however, is that a larger gap between 

the sensor housing and the test surface can be accommodated which 

reduces housing wear and allows the housing to clear some of the surface 

imperfections such as weld spatter. 

Keywords: 

acceleration and deceleration phases of the scanner 


■ Speed control 

Some degree of speed control is necessary with all types of sensor but there 

is less latitude when coils are used. 

■ Vibration damping 

One source of background noise and false indications is due to surface 

roughness of the scanning surface. This is very common in the case of 

storage tank floors and above ground pipelines that have not been coated. 

The resulting corrosion on those surfaces causes the scanning carriage to 

vibrate the magnet and sensor system. The resulting noise can be reduced in 

three ways: by fitting broader wheels, by incorporating shock absorbers and 

by signal processing since the vibration frequency is likely to be higher than 

that from pit signals. 

Keypoints: 

by signal processing since the vibration frequency is likely to be higher 

than that from pit signals. (unlike crack and weld defect!) 


■ Signal processing 

The signals from leakage fields are relatively small and need amplification. 

They also need to be discriminated from unwanted noise. Band pass filters 

are used to remove the low frequency (eddy current) (?) (lift off eddy current 

variation?) and high frequency (vibration) noise. Any residual noise can be 

countered by the use of thresholds set on the defect detection circuit or, in the 

case of dynamic detection notification displays, by the operator assessing the 

general noise level. 


■ Defect notification 

There are three ways in current use in which a defect may be drawn to the 

attention of the operator: - 

Autostop. The scanner automatically stops when a defect is encountered 

and a visual display indicates which sensors in the array have detected the pit. 

The scanner cannot be restarted until the operator has cancelled the 

indication. The operator marks the floor so that pit depth measurement can be 

performed. 

Dynamic display. The operator views a dynamic display indicating the 

current status of signals across the array. A signal above the general noise 

level indicates the presence of a pit. In these systems the operator may be 

assisted by an audible or visual alarm which triggers above a pre-set 

threshold. The operator marks the floor so that pit depth measurement can be 

performed. 

Computer data acquisition. Some systems use a computer to store data 

from the inspection for subsequent analysis and reporting. This may include 

software to allow mapping of the tank floor with colour coded indications of 

material loss. The operator can access the data at the end of each scan in 

order to mark the floor so that some cross checking of results can be 

performed. 


Floor Parameters 

■ Material 

Clearly a ferrous material is necessary for MFL, but the magnetic permeability 

of the ferrous material will affect the results. It follows that the calibration plate 

or pipe used to set up the equipment should be made of the same grade of 

steel as the material to be inspected. This is generally not a problem with 

storage tank floors since with very rare exceptions they are constructed using 

low carbon mild steels. Greater care is needed when selecting a calibration 

pipe to ensure that the correct grade of steel is selected. For a given 

magnetising field, material thickness will affect the degree of saturation 

achieved and this in turn will affect the flux leakage amplitude for a given pit 


■ Scanning surface condition 

The scanning surface should be clean and free from debris (particularly from 

corrosion products that may have fallen from the tank roof). Surface 

roughness may cause vibration noise requiring a relatively high threshold to 

be set (reduced pit sensitivity). In some cases laying a thin sheet (circa 1mm) 

of plastic over the scanning surface can alleviate this. Other anomalies such 

as weld spatter or weld repairs that have been ground flush will give large 

false indications. 

It must also be remembered that the MFL method does not discriminate 

between pitting on the scanning surface and that on the remote surface, 

however, for pits penetrating 50% or more through the material, the MFL 

method is more sensitive to remote surface pitting. (comment: some vendor 

provide eddy current probe with phase discrimination for depth analysis?) 


These curves are typical for inspection results achieved on plates. This 

extreme high sensitivity is achieved with the eddy current sensors in 

combination with special electronics and fast on-line signal processing. In 

eddy current testing, phase information is provided and from that it can be 

established whether the defect is at the top or back wall of the component. 

Using phase information the type of defect can automatically be sorted out 

including a reasonable level of defect severity. In addition, although there is 

still room for improvement, the new system provides some information on 

general wall thickness reduction. Despite all these merits it can not replace 

ultrasonic inspection in terms of absolute accuracy. Experiments in the 

laboratory and field trials proved that with the improved MFL system a 

thickness of up to 30 mm and probably more can be inspected with a much 

higher overall sensitivity than with traditional MFL, this applies certainly for 

thickness range beyond 5 mm. 


■ Scanning surface coating 

One major advantage of the MFL method is that it is able to function with 

relatively thick surface coating and maintain reasonable sensitivity. Fibreglass 

coatings up to 6mm thick on 6.32mm thick floors have been inspected and 

20% wall loss detected. 

■ Cleanliness 

MFL is less sensitive to floor surface condition that ultrasonics but heavily 

ribbed scale can cause false indications and corrosion products can build up 

on the magnet poles and then give false indications as they break away and 

pass under the sensor head. Generally removal of product and subsequent 

water jetting of the surface is sufficient. 

■ Pit depth 

Pit depth is one of the main factors affecting flux leakage amplitude at a 

particular distance above the test surface. Volume and contour also affect this 

amplitude and these are discussed below. However within prescribed 

limitations the amplitude of the flux leakage field can be used to assess the 

percentage wall loss and thus reduce the amount of cross checking needed. 


■ Pit Volume 

It has been claimed elsewhere that the volume of the pit is the most 

significant factor affecting signal amplitude and for this reason it is claimed 

that no quantitative information about the pit can be deduced from the MFL 

results. Since the claim mostly appears as a bald statement we decided to 

carry out a study of the effects of volume and depth using modelling 

techniques and some empirical trials on real corrosion. A series of models of 

pits of given depth and varying volumes were produced. The results for 

depths of 40%, 50% and 60% pits in 6.35mm plate are shown at Figure 11. 

These show that as the volume increases its affect on signal amplitude 

decreases. This suggests that for typical tank floor corrosion of the cone and 

lake type it should be possible to "band" corrosion severity with reasonable 

accuracy using MFL alone. Pipe - like pitting such as that encountered with 

Sulphur Reducing Bacteria attack, however, are likely to give inaccurate 

results because the volumes will correspond to the region where the curves in 

Figure 11 converge. 


60%, 

40%, 

50%, 


■ Pit contour 

Very often people producing test plates with machined pitting choose simple 

shapes such as flat-bottomed holes (borrowed from ultrasonics) or simple 

conical impressions using drill bits. It has been shown that the contour of the 

pit will affect the leakage field. Since corrosion pitting usually progresses in 

such a way as to produce "terracing" in its profile, we have used artificial pits 

for calibration purposes that mimic the terracing as shown in Figure 12. These 

have been used to calibrate the MFL system used in the empirical results 

shown below. 


■ Human factors 

As with other NDT methods, human factors must be considered in assessing 

probability of detection. Especially in the case of storage tanks, the 

environment is not friendly! The interior of the tank is dark, dirty and has the 

lingering smell of the product. It can at times be extremely hot (+50°C) or 

extremely cold (-20°C) depending on location and season. It is therefore 

essential that the demands made on the operator are as light as possible. 

However, the operator must also ensure that the equipment is maintained in 

the best possible condition and that the calibration routine is carried out with 

precision. 


POD Summary for MFL 

The probability of detection of pitting using the MFL is high within certain 

limits. With well-maintained equipment, trained and conscientious operators 

working on clean unpitted scanning surfaces on material thicknesses up to 

10mm thick losses of 20% (sometimes as low as 10%) can be reliably 

detected. On less clean surfaces and on thicknesses up to 13mm 40% losses 

can be detected. Within these limits MFL is able to scan at speeds around 

0.5m/sec with scan widths from 150mm to 450mm wide. The method is less 

influenced by surface condition than ultrasonics and for most MFL systems 

less operator dependant. 


PROBABILITY OF DETECTION - ULTRASONICS 

The probability of detection of corrosion pitting using the ultrasonic method is 

also dependent on many factors. Because the method is rather slower than 

MFL, it was common practice until recently to use spot checks on a grid 

pattern in the same way that was used for erosion detection on pipe bends. 

Clearly the probability of detecting isolated pitting using this technique is 

negligible. Area scanning is now preferred and can be applied manually using 

contact scanning or using automated scanning with water irrigated probes. 

The reflecting surface that is offered by typical corrosion pitting is often poor 

for ultrasonic purposes and the operator needs to be able to see the 

character of the signal to avoid errors. For this reason simple digital thickness 

meters are not suitable for corrosion detection. Equipment with an A-Scan 

presentation is preferred and this can be complimented by B-Scan and C- 

Scan facilities. As with the MFL, the factors affecting POD with Ultrasonics 

include those that relate to the equipment and technique and those that relate 

to the floor and any pitting that may be present. 


Equipment Parameters 

Flaw Detector 

Probe Type 

Couplant method and type 

Scanning technique 

Calibration 

Training and experience 


Floor Thickness 


Floor coating 

Pit characteristics 


Flaw detector 

As a minimum it should have an 

A-Scan display but the use of data 

storage techniques with facilities 

for producing both C-Scan and B- 

Scan images greatly enhances the 

probability of detection. In 

particular, these facilities 

demonstrate that continuous 

coupling has been achieved 

during the inspection. 


Probe Type 

In many cases the thickness of material being examined is less than 10mm 

and the scanning surfaces are not completely smooth. This means that the 

initial pulse of single crystal transducers will occupy a significant portion of the 

nominal thickness so these transducers are not suitable. Twin crystal (Dual) 

transducers overcome this problem but it must be remembered that the 

optimum distance at which the maximum amount of transmitted energy is 

able to be captured by the receiver is a function of the probe design. Figure 

13 illustrates this and shows clearly why reflectors below this distance will 

give reduced amplitude signals even when the reflecting surface in question 

is flat and parallel to the scanning surface. The operator should be aware of 

this possibility especially as corrosion pits are not ideal reflectors and should 

be prepared to vary the gain when backwall echoes are 'lost'. The rough 

surfaces encountered will rapidly wear Perspex shoes and change the beam 

angle so it is necessary to fit a wear ring to the probe. The crystal size should 

be between 10 and 15 mm diameter. 


Couplant method and type 

Two methods of coupling ultrasound to the material are in current use. For 

manual scanning the contact method is used, whilst for automated and semiautomated 

scanning, water irrigation is preferred. In either case it is essential 

that the couplant is able to 'wet' the surface. Suitable gels are available for 

manual scanning and for water irrigation it may be necessary to add a wetting 

agent (soap). 

Scanning technique 

It should be obvious that taking spot readings on a grid pattern is only suitable 

for detecting areas of general corrosion and is useless in detecting isolated 

pits. Therefore it is necessary to use an area scan technique with a suitable 

overlap to ensure coverage by the effective area of the probe. With manual 

scanning it is better to use a fairly rapid probe movement with suitable 

calibration than to use a slow painstaking approach to the detection phase. 

This is because the human eye naturally responds to a sudden change 

(movement) in signal pattern. Once the pit has been detected, a more careful 

investigation of pit depth can be carried out. 


Calibration 

For the detection phase of the inspection when using manual scanning, it is 

better to calibrate the flaw detector on the actual test material by selecting an 

area on the floor where the thickness is known to be at the nominal plate 

thickness. The timebase is then set to display 3 backwall echoes positioned 

at 3, 6 and 9. The gain should be set so that the third backwall echo is at 80% 

full screen height. With this arrangement, using the fast scanning movement 

described above, loss of couplant will show as a vertical drop in all three 

echoes. The presence of a pit will show as a progressive loss (3rd then 2nd 

and then 1st echo) coupled with a general movement of the signals towards 

zero. With practice the eye becomes well adapted to recognise these patterns. 

Training and experience 

The detection of corrosion pits is more difficult than simple thickness 

measurement or the detection of laminations or erosion. The slow scanning 

technique, with a timebase calibrated to display only one backwall echo, used 

by some operators is prone to miss pits that have poor reflectivity such as the 

conical types. Operators often say that they 'lost' the signal due to poor 

scanning surface when they have just encountered a pit. Specific training and 

experience is required for corrosion detection. 



Thickness 

Thinner wall thicknesses present the main difficulty when using the ultrasonic 

method. Below 6 mm the signal from a good reflector is reduced as described 

above and shown in Figure 13. The operator must be aware that more gain 

will be required. For thicker sections (above 12 mm) the ultrasonic method is 

far less restricted than MFL, however the POD limitations with respect to 

shape and reflectivity of pits still apply. 



The ultrasonic method is much more sensitive to the condition of the scanning 

surface than is the MFL method. This applies to both contact scanning and 

irrigated 'gap' scanning. Reflections in the couplant layer create 'noise' that 

obscures part of the timebase as shown in Figure 14. Since the velocity of 

sound in the couplant is about one quarter of the velocity in the material, top 

surface pits may give clear echoes that appear to show a reduced wall 

thickness. Figure 15 illustrates a lake type pit 1 mm deep. The echo from the 

bottom of the pit appears at a steel thickness of 4 mm. If unnoticed the 

operator may report a 6mm deep underfloor pit in a 10mm plate (60% loss). 

The same pit is likely to be misinterpreted with automated and semiautomated 

systems whether or not they use interface triggering and/or echoto-echo 

monitoring. 



Floor Coatings 

Painted and epoxy coated floors in which the coating is in good condition and 

has been applied from new present few problems to ultrasonic inspection and 

pit detection. The accuracy of measurement of remaining wall thickness is 

improved if the echo-to-echo method is used to eliminate paint thickness 

errors. Thicker, fibreglass coatings present more of a problem. Although in 

theory it may be possible to inspect through such a coating if the adhesion to 

the metal surface is good, it is seldom suitable for inspection. 


Pit characteristics 

The easiest pits to detect are the lake type because in the deepest region 

they are relatively parallel to the scanning surface and can be expected to 

give reasonable reflectivity. On the other hand the conical pits tend to reflect 

sound away from the receiver and the centre of the pit is often too small in 

area to give a strong signal (Figure 16). These are the pits that are most likely 

to be missed by the ultrasonic operator. Often one of the 'terrace' facets is the 

strongest reflector and the pit is detected but its depth is underestimated. 

Pipe like pits such as those typical of SRB attack present very small targets to 

the ultrasonic beam and may also be as difficult to detect. Where the 

reflectivity of the pit is favourable, the ultrasonic method is capable of 

detecting smaller changes of thickness than the MFL method but, since the 

corrosion allowance is often as much as 50%, this advantage is not always 

significant. 



POD Summary for Ultrasonics 

On good scanning surfaces the probability of detecting Lake Type pits is high. 

For poor scanning surfaces and for Cone Type pitting, the probability of 

detection is less satisfactory. To some extent the POD can be improved using 

the automated techniques with data storage and at least a C-scan 

presentation using colour coding to 'band' thickness. 


SOME PRACTICAL RESULTS 

Some sections of floor were cut from storage tank bottoms after MFL 

inspection. Sections were taken from areas where underfloor corrosion was 

reported and also from areas where there was said to be no corrosion that 

was deeper than 20%. Some of the sections had been inspected using the 

Silver Wing 'Floormap' system that produced a map of the floor with colour 

coded indications of corrosion, each colour representing a 'band' of 

percentage wall loss. The corroded sections were subjected to mechanical pit 

depth measurement and the results compared with the MFL report. The 

pitting included both lake and cone examples. The approximate locations of 

the pits were marked on the opposite side of the plates (scanning surface) 

and two teams of UT operators were asked to locate the pits and measure 

their depth. 




Figures 17 to 21 are photographs of some of the corrosion detected. Figures 

22 and 23 are graphs showing actual pit depth against reported depths for the 

two UT teams. Figure 24 show the same for the MFL results. It can be seen 

that on average the MFL system overestimates the depth of pitting by about 

10% whereas the ultrasonic method has underestimated by about 10%. 

However one UT team missed two of the pits even though the approximate 

location had been marked. 




CONCLUSIONS 

Both methods have limitations in the thickness range that can be reliably 

inspected and the smallest pit that can be detected. Within the limitations 

described for MFL, the probability of detection of isolated pitting is better than 

ultrasonics and the method is also quicker than ultrasonics so more economic. 

In terms of accuracy of depth measurement, both methods have the same 

percentage error though in opposite senses. Since there is a remote chance 

that the floor material may not be mild steel and thus may have a permeability 

that differs from the calibration plate, it is always necessary to carry out at 

least limited cross checking of MFL results with UT before relying on MFL 

depth assessment. 





The Truth About Magnetic Flux Leakage As Applied 

To Tank Floor Inspections 


The Truth About Magnetic Flux Leakage As Applied To Tank 

Floor Inspections 

Magnetic Flux Leakage Inspection techniques have been widely used in the 

Oil field Inspection Industry for over a quarter of a century for the examination 

of pipe, tubing and casing both new and used. It is only in the last fifteen 

years that this inspection technique has been applied to above ground 

storage tank floors in an attempt to provide a reliable indication of the overall 

floor condition within an economical time frame. In most cases these 

inspections are being carried out by Industrial Inspection NDT Companies 

who do not have the depth of experience in the technique that most of the Oil 

field Tubular Inspection Companies have. 

At the same time this relatively new application of Magnetic Flux Leakage 

brings with it some additional problems not evident in the inspection of 

tubulars where certain parameters can be quite closely controlled. Probably 

the greatest of these is that tank floors are never flat, whereas tubulars are 

generally always round. 


The ability to obtain any reasonably consistent quantitative information is 

seriously impacted by this general unevenness of most tank floors. The 

application of rigid accept/reject criteria based on signal amplitude thresholds 

has proved to be absolutely unreliable as regards truly quantitative 

information. A more realistic approach is required in the application of this 

inspection technique and in the design of the MFL inspection equipment to 

ensure that there are fewer incidences of significant defects being missed. 

The following information outlines some of the major considerations that need 

to be addressed in order to achieve reliable, fast and economical inspections 

of above ground storage tank floors. 


MAGNETIC FLUX LEAKAGE (MFL) 

In order to understand some of the problems associated with this particular 

application of Magnetic Flux Leakage (MFL), it is necessary to understand the 

basic principles of the technique. Most people are familiar with a magnet’s 

ability to “stick” to a carbon steel plate. This happens because the magnetic 

lines of force (flux) prefer to travel in the carbon steel plate rather than in the 

surrounding air. In fact, this flux is very reluctant to travel in air unless it is 

forced to do so by the lack of another suitable medium. For the purposes of 

this particular application, a magnetic bridge is used to introduce as near a 

saturation of flux as is possible in the inspection material between the poles 

of the bridge. Any significant reduction in the thickness of the plate will result 

in some of the magnetic flux being forced into the air around the area of 

reduction. Sensors which can detect these flux leakages are placed between 

the poles of the bridge. Figure 1 graphically illustrates this phenomenon. 


FIGURE “1” 


THE MFL INSPECTION ENVIRONMENT 

In order to optimize the effectiveness of the MFL inspection, it is necessary to 

consider the environment and address the physical restrictions imposed by 

the actual conditions found when examining the majority of tank floors. 

■ CLIMATIC CONDITIONS 

Invariably, the range of temperature and humidity conditions will vary 

enormously worldwide. The effect on both operator and equipment must be 

taken into consideration. Human beings do not function well in extremes of 

temperature. Use of the MFL equipment should not place too great a burden 

on them from either a physical or mental point of view. In other words, the 

simpler, more reliable and easy to use the MFL inspection equipment is made, 

the more reliable the inspection results. 


■ TANK FLOOR CLEANLINESS 

By their very nature, the majority of above ground storage tanks are dirty and 

sometimes dusty places to work. The conditions in this regard vary widely 

and are dependent upon how much effort the tank owner/operator is willing to 

expend in cleaning the floors in preparation for Magnetic Flux Leakage 

Scanning. As an absolute minimum, a good water blast is necessary and all 

loose debris and scale should be removed from the inspection surface. The 

surface does not necessarily have to be dry but puddles of standing water 

need to be removed. The cleaner the floor, the better the inspection. 


■ STORAGE TANK SURFACE CONDITION 

Significant top surface corrosion and/or buckling of the tank floor plates 

represents a serious limitation to both the achievable coverage in the areas 

concerned and also the achievable sensitivity. While it is understood that very 

little can be done to improve this situation prior to inspection, it must be 

considered in the design of the MFL inspection equipment and its effect on 

the sensitivity of the inspection appreciated by both the owner/operator of the 

tank as well as the person conducting the examination. Any physical 

disturbance of the MFL scanning system as it traverses the tank floor will 

result in the generation of noise. The rougher the surface, the greater the 

noise and, therefore, a reduction in achievable sensitivity. 


MFL EQUIPMENT DESIGN CONSIDERATIONS 

It is vital that Magnetic Flux Leakage NDT equipment used for storage tank 

floor inspection is designed to handle the environmental and practical field 

conditions that are consistently present. A piece of MFL equipment designed 

in a laboratory and tested in ideal conditions will invariably have significant 

short comings in real world applications. 

■ ELECTROMAGNETS/PERMANENT MAGNETS 

Powerful rare earth magnets are ideally suited for this application. They are 

more than capable of introducing the required flux levels into the material 

under test. Electromagnets by comparison are bulky and heavy. They do 

have an advantage in that the magnetic flux levels can be easily adjusted and 

“turned off” if necessary for cleaning purposes. Permanent magnet heights 

can be adjusted to alter flux levels but the bridge requires regular cleaning to 

remove ferritic debris. The buildup of debris can have a significant impact on 

system sensitivity. 


■ SENSOR TYPES 

MFL tools typically use one of two types of sensors: Coils and Hall Effect 

Sensors. They are both capable of detecting the magnetic flux leakage fields 

caused by corrosion on tank floors. There is a fundamental difference, 

however, in the way that they respond to leakage fields. 

COILS 

Coils are passive devices and follow Faraday’s Law in the presence of a 

magnetic field. As a coil is passed through a magnetic field, a voltage is 

generated in the coil and the level of this voltage is dependent on the number 

of turns in the coil and the rate of change of the flux leakage. From this, it is 

clear that speed will have some influence on the signals obtained from this 

type of sensor. 


HALL EFFECT SENSORS 

Hall Effect Sensors are solid state devices which form part of an electrical 

circuit and, when passed through a magnetic field, the value of the voltage in 

the circuit varies dependent on the absolute value of the flux density. It is 

necessary to carry out some cross referencing and canceling with this type of 

sensor in order to separate true signals from other causes of large variations 

in voltage levels generated by the MFL inspection process. 

There is disagreement within the industry as to which is the best type of 

sensor to use for this application. Hall Effect Sensors are undeniably more 

sensitive than coils. However, in this application, coils are more than 

adequately sensitive and are more stable and reliable. Hall Effect sensors 

prove to be too sensitive when surface conditions are less than perfect which 

results in an unreliable inspection and the generation of significant false calls. 


MFL TECHNIQUE APPLICATION CONSIDERATIONS 

COVERAGE LIMITATIONS 

It is virtually impossible to achieve 100% coverage using this technique due 

to the physical access limitations. The MFL inspection equipment should be 

designed so that it can scan as close as possible to the lap joint and shell. 

There are obviously compromises to be made as the wheel base of the 

scanner is an important consideration on tank floors that are not perfectly flat. 

Smaller scanning heads can be used in confined spaces to increase 

coverage. 


■ TOPSIDE/BOTTOM SIDE DIFFERENTIATION 

Magnetic Flux Leakage cannot differentiate between the response from 

topside and bottom side indications. Some attempt has been made to use the 

eddy current signals from topside defects for the purposes of differentiation 

based on frequency discrimination. This is unreliable on real tank floors due 

to the uneven nature and lack of cleanliness of the inspection surface. In most 

cases, visual techniques are perfectly adequate for this purpose. 

Contrary to what is expected, the Magnetic Flux Leakage response from a 

topside indication is significantly lower in amplitude than that from an 

equivalent bottom side indication. This means that, to some degree, the 

influence of the top side indications can be “tuned out” to allow a reliable 

assessment of the under floor condition. (?) 


■ QUANTITATIVE ASSESSMENT OF INDICATIONS 

Magnetic Flux Leakage is a qualitative, not quantitative inspection tool and is 

a reliable detector of corrosion on tank floors. Due to the environmental and 

physical restrictions encountered during real inspections, no reliable 

quantification of indications are possible. Amplitude alone is an unreliable 

indication of remaining wall thickness as it is more dependent on actual 

volume loss. Defects exhibiting various combinations of volume loss and 

through wall dimension can give the same amplitude signal. Couple to 

this the continually changing spatial relationship of magnets, sensor and 

inspection surface and it is absolutely clear that an accurate assessment of 

remaining wall thickness is virtually impossible. Truly quantitative results can 

only be obtained using a combination of Ultrasonic testing and Magnetic Flux 

Leakage. 


60%, 

40%, 

50%, 


■ THE SINGLE LEVEL THRESHOLD 

Commercial expediency has brought about the implementation of 

accept/reject criteria using a single level threshold approach. MFE 

Enterprises, as a manufacturer of Magnetic Flux Leakage equipment, does 

not support this approach. As previously stated, the amplitude of signals 

alone is not a reliable indicator of remaining wall thickness. Significant 

indications can be completely missed especially in cases where the 

equipment does not incorporate some form of real time on line display. In 

order to carry out a reliable MFL inspection, the operator must have as much 

information as possible available to him in the form of an easy-to-interpret real 

time display. The use of a blind single threshold is absolutely indefensible in 

this application. 


MFL OPERATOR TRAINING AND QUALIFICATION 

REQUIREMENTS 

Currently, there is limited training available to users of the MFL equipment in 

regard to this application. MFE Enterprises Inc. recognizes this fact and offers 

initial basic training in magnetic flux leakage and the use of MFL inspection 

equipment on delivery of the scanner. This is obviously geared to our 

equipment and is quite specific. The ultrasonic prove up necessary must be 

carried out by personnel who are adequately trained and qualified. This is not 

just a “thickness measurement,” but rather a corrosion evaluation and the 

technician must have a full understanding of the technique that should be 

applied. 





Magnetic Flux Leakage Testing 

for Back-side Defects Using a Tunnel 

Magnetoresistive Device 


Abstract— Magnetic non-destructive testing is limited to surface inspection, 

however demand for the detection of deep defects is increasing. Therefore, 

we developed a magnetic flux leakage (MFL) system using a tunnel 

magnetoresistive (TMR) device that has high sensitivity and wide frequency 

range in order to detect deep defects. Using the developed system, back-side 

pits of steel plates having different depth and diameter were measured and 

2D images were created. Moreover, we analyzed the detected vector signal 

with optimized phase data. As a result, the developed MFL system can detect 

a defect that has a wall thinning rate of more than 56 % of 8.6 mm thick steel 

plates. Furthermore, the defect’s diameter size was estimated by spatial 

signal change. 

Keywords-MFL; magnetic imaging; TMR device; Low-Freaquency field; backside 

pit. 


Tunnel magnetoresistance (TMR) is a magnetoresistive effect that 

occurs in a magnetic tunnel junction (MTJ), which is a component consisting 

of two ferromagnets separated by a thin insulator. If the insulating layer is thin 

enough (typically a few nanometers), electrons can tunnel from one 

ferromagnet into the other. Since this process is forbidden in classical physics, 

the tunnel magnetoresistance is a strictly quantum mechanical phenomenon. 

Magnetic tunnel junctions are manufactured in thin film technology. On an 

industrial scale the film deposition is done by magnetron sputter deposition; 

on a laboratory scale molecular beam epitaxy, pulsed laser deposition and 

electron beam physical vapor deposition are also utilized. The junctions are 

prepared by photolithography. 


https://en.wikipedia.org/wiki/Tunnel_magnetoresistance

History 

The effect was originally discovered in 1975 by M. Jullière (University of 

Rennes, France) in Fe/Ge-O/Co-junctions at 4.2 K. The relative change of 

resistance was around 14%, and did not attract much attention.[1] In 1991 

Terunobu Miyazaki (Tohoku University, Japan) found an effect of 2.7% at 

room temperature. Later, in 1994, Miyazaki found 18% in junctions of iron 

separated by an amorphous aluminum oxide insulator [2] and Jagadeesh 

Moodera found 11.8% in junctions with electrodes of CoFe and Co.[3] The 

highest effects observed to date with aluminum oxide insulators are around 

70% at room temperature. 

Since the year 2000, tunnel barriers of crystalline magnesium oxide (MgO) 

have been under development. In 2001 Butler and Mathon independently 

made the theoretical prediction that using iron as the ferromagnet and MgO 

as the insulator, the tunnel magnetoresistance can reach several thousand 

percent.[4][5] 



The same year, Bowen et al. were the first to report experiments showing a 

significant TMR in a MgO based magnetic tunnel junction 

[Fe/MgO/FeCo(001)].[6] In 2004, Parkin and Yuasa were able to make 

Fe/MgO/Fe junctions that reach over 200% TMR at room temperature.[7][8] In 

2008, effects of up to 600% at room temperature and more than 1100% at 4.2 

K were observed in junctions of CoFeB/MgO/CoFeB.[9] 

Applications 

The read-heads of modern hard disk drives work on the basis of magnetic 

tunnel junctions. TMR, or more specifically the magnetic tunnel junction, is 

also the basis of MRAM, a new type of non-volatile memory. The 1st 

generation technologies relied on creating cross-point magnetic fields on 

each bit to write the data on it, although this approach has a scaling limit at 

around 90–130 nm.[10] There are two 2nd generation techniques currently 

being developed: Thermal Assisted Switching (TAS)[10] and Spin Torque 

Transfer (STT). Magnetic tunnel junctions are also used for sensing 

applications. For example, a TMR-Sensor can measure angles in modern 

high precision wind vanes, used in the wind power industry. 



I. INTRODUCTION 

Accidents due to defects in steel structures such as power plants or pipe line 

cause serious injuries to humans and harm to the natural environment. 

Therefore, it is important to use non-destructive testing for detecting defects 

at an early stage. In many cases, it is difficult to find defects in the interior or 

on the back side, and thus a detection method for deep defects is desired. 

There are many non-destructive testing methods such as radiographic testing, 

ultrasonic testing, magnetic flux leakage (MFL) testing, and eddy current 

testing. Among them, MFL is commonly used for ferromagnetic material such 

as steel and it is a method for detecting flux with bypass defects due to 

differences in permeability and leakage from the sample’s surface when an 

external field is applied to the sample. 

MFL for deep defects needs to be operated at low frequency because the 

penetration of the applied external field becomes deeper with decreasing 

frequency. However, the conventional MFL method, which uses a detection 

coil as a magnetic sensor, cannot be operated at low frequency because it 

has low sensitivity at low frequency due to Faraday’s law of induction. 

Therefore, it can detect only surface defects near the detection coil. 

(I =dΦ/dt?) 


Moreover, the detection of deep defects also requires a high magnetic 

resolution because the change of flux generated by the deep defect is very 

small. The other problem of MFL is that the magnetic field intensity of MFL 

needs to be operated at the saturation region of the B-H curve in order to 

obtain measurable large magnetic flux leakage. However, a measurement 

system that gives such large magnetic field intensity is costly because a high 

power current source is necessary. 

One way to solve these problems is to use a high sensitivity magnetic sensor 

that can detect a low magnetic intensity field at low frequency such as a 

magnetoresistive (MR) sensor. If such a sensor were installed, we could 

operate MFL at extra low frequency, which would give deep skin depth and 

detect small magnetic flux leakage caused by a low power source. 

We reported a MFL system using an anisotropic magnetoresistive (AMR) 

sensor [11]. Recently, the tunnel magnetoresistive (TMR) sensor has 

progressed because it has a larger MR ratio than other MR devices with a 

wide frequency range. 


In this study, we developed the MFL system using a TMR device having high 

sensitivity at extreme-low frequency in order to enable us to detect defects 

deeper and more clearly than the AMR sensor and other magnetic sensors. 

Moreover, we investigated the performance of the developed system using 

samples having various back-side pits. 

δ= √(2/ωσμ) = (πfσμ) -½ 


II. TMR DEVICE 

A TMR device is a kind of MR device and is usually applied in the magnetic 

head of a hard disk. It has a larger MR ratio than other MR devices. A 

common TMR device shows a step response to magnetic fields and has 

hysteresis. The TMR device used in this study was designed for sensor 

application [12]-[14]. It was annealed at different temperatures and directions 

two times in order to make easy directions of the pin layer and the free layer 

orthogonal. In this structure, the output is linear with respect to the magnetic 

field. In addition, it has a large MR ratio because of magnetic coupling of the 

free layer and the soft magnetic material layer. Figure 1 shows the TMR 

resistance as a function of an applied field. The range from -400μT to 400μT, 

which is treated in MFL, can be applicable to the sensor application. 


Figure 1. Resistance of the TMR device to an applied field. 


III. EXPERIMENTAL 

The developed MFL system (Figure 2) consists of a sensor probe with a TMR 

device, a lock-in amplifier, a current source, an oscillator, two excitation coils, 

a half shaped ferrite yoke, a sample stage, and a PC. Two excitation coils 

with 30 turns were connected to both ends of the yoke and an AC field was 

induced in the sample between both ends. The sensor probe was installed 

between the ends of the yoke and they were 1 mm away from the sample’s 

surface. The TMR device measured magnetic flux leakage bypassing defects. 

In this study, the TMR device had sensitivity to the direction parallel to both 

ends of the yoke in order to obtain a larger output [11]. The excitation coils 

were operated by a sine wave of 1.2 App and 5 Hz or 10 Hz from the current 

source controlled by the oscillator. The effect of the eddy current can be 

ignored in such an extreme-low frequency field. The output signal from the 

TMR device was detected by the lock-in amplifier, which is synchronized with 

the current source in order to obtain a high signal-to-noise ratio. 

Comments: How eddy current density J o of J= J o e -x/δ calculated? 

How does the frequency affect J o ? 


Faraday Law 

E(emf) = - N ∆(BA)/∆t = -N (dФ/dt) 

∆Ф = B ┴ ∆A = E ∆A = A ∆E 


Figure 2. Schematic diagram of the developed MFL system. 


The signal from the lock-in amplifier contains the signal intensity R and the 

phase θ. In this measurement system, magnetic flux leakage is very small so 

that it is strongly affected by the phase shift of the entire measurement 

system. Therefore, we calculated the imaginary part of the signal intensity 

with the common phase φ [11]. 

R’ = R sin(θ+φ) 

Here, φ is a common phase adjusting the phase shift of the entire 

measurement system. 

The samples used in this study were two steel plates (SPHC) with four backside 

pits as shown in Figure 3. Both samples were 8.6 mm thick. The pits of 

Sample (a) are of the same diameter (6 mm) and different wall thinning rates 

(23, 57, 70, 93 %). Sample (b) has the same wall thinning rate (70 %) and 

different diameters (4, 6, 8. 10 mm). Multipoint measurement was carried out 

in the range of 20 mm × 20 mm around a pit from front surface with an 

interval of 1 mm for 21 × 21 steps as shown in Figure4. 


Figure 3. Schematic diagram of the test plates with pits. 


Figure 4. Measuring points for back-side pits. 


We investigated the common phase φ in this measurement system. The 

measurement was carried out around a pit that has a wall thinning rate of 

70% and a diameter of 4 mm. The excitation coils were operated by sine 

wave of 1.2 App and 10 Hz or 5 Hz from the current source. The 

measurement results show as contour maps of calculated intensity (mV) with 

different common phases. 

Figure 5 shows magnetic images with a frequency of 10 Hz and different 

common phases and Figure 6 shows that with 5 Hz and different common 

phases. Magnetic images with a common phase φ of 130 ° show the 

emphasis of the intensity change due to the pit in the center of the scanning 

range. The magnetic image with a frequency of 5 Hz shows the presence of 

the back-side pit more clearly than that of 10 Hz because the skin depth 

becomes deeper with decreasing frequency. Therefore, the frequency was 5 

Hz and the optimized common phase φ was 130 ° for the measurement 

system. 


Figure 7 shows the power spectrum of the developed system when the 

magnetic field was not applied and the sine field was applied at 100μT and 5 

Hz in the unshielded environment. The sensitivity at 5 Hz of the developed 

system is 2.44 mV/μT. We estimated the magnetic noise without an applied 

field that corresponds to the minimum magnetic field resolution at 5 Hz. As a 

result, the magnetic field resolution was 1.08 nT. 

To evaluate the performance of the developed MFL system, we analyzed the 

magnetic image change of a steel plate having different pit wall thinning rates 

and diameters under optimum conditions. The excitation coils were operated 

by a sine wave of 5 Hz and 1.2 App from the current source. We calculated 

the signal vector with the optimized phase φ = 130°. The aforementioned 

Sample (a) and Sample (b) were measured and we made contour maps of 

the calculated signal vector. 


Figure 5. Magnetic images with 10 Hz and different phase. 








Figure 7. Power spectrum of the developed system. 


IV. RESULTS AND DISCUSSION 

First, we used Sample (a) and investigated the change of magnetic images of 

the steel plates with different wall thinning rates. The map showed the 

existence of the pit and it becomes clear with increasing the actual pit’s wall 

thinning rate (Figure 8). However, the magnetic image of a pit that has a wall 

thinning rate of 23 % is unclear. This was caused by the weak magnetic flux 

leakage from the small thinning rate of the wall. The detection limit was a 

thinning rate 57 % corresponding to a wall thickness of 4.6 mm. Next, we 

used Sample (b) and investigated the changes of the magnetic images by 

changing the diameter (Figure 9). Apparent differences were observed in 

each figure. The contour map change became large according to the 

increment of the diameter. 


Moreover, we quantitatively evaluated the magnetic field intensity change and 

examined the relationship of the defect’s characteristics and the calculated 

intensity. The center line of the contour of the magnetic image was extracted 

as shown in Figure 10 and ΔB was defined as the value obtained by 

subtracting the minimum value from the maximum value as shown in Figure 

10. Figure 11 shows the relationship of ΔB and the wall thinning rate and 

Figure 12 shows that of ΔB and the diameter. ΔB was increased with the 

increment of the wall thinning rate and the diameter. Therefore, we can 

estimate the defect’s characteristics using the magnetic image and ΔB. 


Figure8. Magnetic images of pits with different wall thinning rate. 


Figure 9. Magnetic images of pits with different diameter. 


Figure 10. Example of the extracted line and the definition of ΔB. 


Figure 11. Relationship of the defect’s wall thinning rate and ΔB. 


Figure 12. Relationship of the defect’s diameter and ΔB. 


V. CONCLUSIONS 

We developed a magnetic flux leakage (MFL) testing system using TMR for 

back-side defects. Analysis using the signal vector with optimized phase was 

effective for magnetic imaging of the back-side pits. The magnetic images 

reflected the actual defect’s characteristics and were able to detect more than 

the wall thinning rate of 57%. The developed MFL system does not require a 

high power current source so that this measurement system is expected to be 

applicable to field testing. 





Chapter Nine: 

Magnetic Flux Leakage Testing 


9.1 PART 1. Introduction to Magnetic Flux Leakage 

Testing MFLT 

9.1.0 Introduction 

Magnetic flux leakage testing is part of the widely used family of 

electromagnetic nondestructive techniques. Magnetic particle testing is a 

variation of flux leakage testing that uses particles to show indications. When 

used with other methods, magnetic tests can provide a quick and relatively 

inexpensive assessment of the integrity of ferromagnetic materials. The 

theory and practice of electromagnetic techniques are discussed elsewhere in 

this volume. The origins of magnetic particle testing are described in the 

literature1 and information that the practicing magnetic test engineer might 

require is available from a variety of manuals and journal articles. The 

magnetic circuit and the means for producing the magnetizing force that 

causes magnetic flux leakage are described below. Theories developed for 

surface and subsurface discontinuities are outlined along with some results 

that can be expected. 


9.1.1 Industrial Uses 

Magnetic flux leakage testing is used in many industries to find a wide variety 

of discontinuities. Much of the world’s production of ferromagnetic steel is 

tested by magnetic or electromagnetic techniques. Steel is tested many times 

before it is used and some steel products are tested during use for safety and 

reliability and to maximize their length of service. 

9.1.1.1 Production Testing 

Typical applications of magnetic flux leakage testing are by the steel producer, 

where blooms, billets, rods, bars, tubes and ropes are tested to establish the 

integrity of the final product. In many instances, the end user will not accept 

delivery of steel product without testing by the mill and independent agencies. 


9.1.1.2 Receiving Testing 

The end user often uses magnetic flux leakage tests before fabrication. This 

test ensures the manufacturer’s claim that the product is within agreed 

specifications. Such tests are frequently performed by independent testing 

companies or the end user’s quality assurance department. Oil field tubular 

goods are often tested at this stage. 

9.1.1.3 In-service Testing 

Good examples of in-service applications are the testing of used wire rope, 

installed tubing, or retrieved oil field tubular goods by independent facilities. 

Many laboratories also use magnetic techniques (along with metallurgical 

sectioning and other techniques) for the assessment of steel products and 

prediction of failure modes. 


9.1.2 Discontinuities 

Discontinuities can be divided into two general categories: those caused 

during manufacture in new materials and those caused after manufacture in 

used materials. Discontinuities caused during manufacture include cracks, 

seams, forging laps, laminations and inclusions. 

1. Cracking occurs when quenched steel cools too rapidly. 

2. Seams occur in several ways, depending on when they originate during 

fabrication. 

3. Discontinuities such as piping or inclusions within a bloom or billet can be 

elongated until they emerge as long tight seams or gouges during initial 

forming processes. They may later be closed with additional forming. 

4. Their metallurgical structures are often different but the origin of 

manufactured discontinuities is not usually taken into account when 

rejecting a part. 


5. Forging laps occur when gouges or fins created in one metal working 

process are rolled over at an angle to the surface in subsequent 

processes. 

6. Inclusions are pieces of nonmagnetic or nonmetallic materials embedded 

inside the metal during cooling. Inclusions are not necessarily detrimental 

to the use of the material. 

7. The pouring and cooling processes can also result in lack of fusion within 

the steel. Such regions may be worked into internal laminations. 

Discontinuities in used materials include fatigue cracks, pitting corrosion, 

erosion and abrasive wear. 


Much steel is acceptable to the producer’s quality assurance department if no 

discontinuities are found or if discontinuities are considered to be of a depth 

or size less than some prescribed maximum. Specifications exist for the 

acceptance or rejection of such materials and such specifications sometimes 

lead to debate between the producer and the end user. Discontinuities can 

either remain benign or can grow and cause premature failure of the part. 

Abrasive wear can turn benign subsurface discontinuities into detrimental 

surface breaking discontinuities. 


For used materials, fatigue cracking commonly occurs as the material is 

cyclically stressed. Fatigue cracks grow rapidly under stress or in the 

presence of corrosive materials such as hydrogen sulfide, chlorides, carbon 

dioxide and water. For example, drill pipe failure from fatigue often initiates at 

the bases of pits, at tong marks or in regions where the tube has been worn 

by abrasion. Pitting is caused by corrosion and erosion between the steel and 

a surrounding or containing fluid. Abrasive wear occurs in many steel 

structures. 

Good examples are: 

(1) the wear on drill pipe caused by hard formations when drilling crooked 

holes or 

(2) the wear on both the sucker rod and the producing tubing in rod pumping 

oil wells. 

Specifications exist for the maximum permitted wear under these and other 

circumstances. In many instances, such induced damage is first found by 

automated magnetic techniques. 


9.1.3 Steps in Magnetic Flux Leakage Testing 

There are four steps in magnetic flux leakage testing: 

(1) magnetize the test object so that discontinuities perturb the flux, 

(2) scan the surface of the test object with a magnetic flux sensitive detector, 

(3) process the raw data from these detectors in a manner that best 

accentuates discontinuity signals and 

(4) present the test results clearly for interpretation. 

The next section discussion deals with the first step, producing the magnetizing 

force. 


Steel Mill – Expert at Works 


Steel Mill – Expert at Works 


Steel Mill 


Steel Mill 


http://jyhengrun.en.made-in-china.com/productimage/LXxJqIKWhmhC- 

2f1j00bScaFVoCEUql/China-Rolling-Ring-Forging-Stainless-Steel-Flange.html

Seams & Laps 


http://azterlan.blogspot.com/2013/09/sensitivity-in-magnetic-particle.html

9.2 PART 2. Magnetization Techniques 


Successful testing requires the test object to be magnetized properly. The 

magnetization can be accomplished using one of several approaches: 

(1) permanent magnets, 

(2) electromagnets and 

(3) electric currents used to induce the required magnetic field. 

Excitation systems that use permanent magnets offer the least flexibility. 

Such systems use high energy product permanent magnet materials such as 

neodymium iron boron, samarium cobalt and aluminum nickel. The major 

disadvantage with such systems lies in the fact that the excitation cannot be 

switched off. Because the magnetization is always turned on, it is difficult to 

insert and remove the test object from the test rig. Although the magnetization 

level can be adjusted using appropriate magnetic shunts, it is awkward to do 

so. Consequently, permanent magnets are very rarely used for magnetization. 


Keywords: 

• Excitation systems. 

• Neodymium iron boron, samarium cobalt and aluminum nickel. 

• Appropriate magnetic shunts. 


http://en.wikipedia.org/wiki/Neodymium_magnet

Electromagnets, as well as electric currents, are used extensively to 

magnetize the test object. Figure 1 shows an excitation system where the test 

object is part of a magnetic circuit energized by current passing through an 

excitation coil. The magnetic circuit passes through a yoke made of a soft 

magnetic material and through a test object placed between the poles of the 

yoke. When the coil wound on the yoke carries current, the resulting 

magnetomotive force drives magnetic flux through the yoke and the test 

object. The total magnetic flux Ф (phi) ( in weber) is given by: 

(1) Ф = N I / S = ampere/(ampere/weber) 

where I is the current (ampere) in the coil, N is the number of turns in the coil 

and S is the reluctance (ampere per weber) of the magnetic circuit. 

Keywords: 

Reluctance (ampere per weber) 


FIGURE 1. Electromagnetic yoke for magnetizing of test object. 


Yangtze River China - 水落石出 


http://en.wikipedia.org/wiki/Magnetic_field

Reluctance. 

Magnetic reluctance, or magnetic resistance, is a concept used in the analysis of magnetic 

circuits. It is analogous to resistance in an electrical circuit, but rather than dissipating electric 

energy it stores magnetic energy. In likeness to the way an electric field causes an electric 

current to follow the path of least resistance, a magnetic field causes magnetic flux to follow the 

path of least magnetic reluctance. It is a scalar, extensive quantity, akin to electrical resistance. 

The unit for magnetic reluctance is inverse henry, H -1 . 

In a DC field, the reluctance is the ratio of the "magnetomotive force” (MMF) in a magnetic circuit 

to the magnetic flux in this circuit. In a pulsating DC or AC field, the reluctance is the ratio of the 

amplitude of the "magnetomotive force” (MMF) in a magnetic circuit to the amplitude of the 

magnetic flux in this circuit. (see phasors) 

S = N I / Ф, F =NI 

where 

S is the reluctance in ampere-turns per weber (a unit that is equivalent to turns per henry). 

F is the magnetomotive force (MMF) in ampere-turns 

Ф is the magnetic flux in webers. 

"Turns" refers to the winding number of an electrical conductor comprising an inductor 

1/Henry = ampere/ weber, Henry= Weber/ampere? 


http://en.wikipedia.org/wiki/Magnetic_reluctance

• Henry, unit of either self-inductance or mutual inductance, abbreviated h, 

and named for the American physicist Joseph Henry. One henry is the 

value of self-inductance in a closed circuit or coil in which one volt is 

produced by a variation of the inducing current of one ampere per second. 

One henry is also the value of the mutual inductance of two coils arranged 

such that an electromotive force of one volt is induced in one if the current 

in the other is changing at a rate of one ampere per second. 

• Weber, unit of magnetic flux in the International System of Units (SI), 

defined as the amount of flux that, linking an electrical circuit of one turn 

(one loop of wire, N=1), produces in it an electromotive force (E) of one 

volt as the flux is reduced to zero at a uniform rate in one second. 

• Tesla, a flux density of one Wb/m 2 (one weber per square metre) is one 

tesla. 


http://global.britannica.com/EBchecked/topic/261372/henry

L, Henry – H, The inductance of an electric circuit is one henry when an 

electric current that is changing at one ampere per second results in an 

electromotive force across the inductor of one volt. 

Ф, Weber – Wb, magnetic flux 

B, Tesla – T, magnetic flux density (1 weber / m2) = 10000 Gauss 

S, Reluctance – Ampere Turn/ weber NI/Ф 

F, Magnetomotive force – Ampere Turn 

H, Magnetic field intensity – Amperes per meter (symbol: A·m-1 or A/m) 

μ, permeability – B/H, henries per meter (H·m-1), or newtons per ampere 

squared (N·A-2). 



A magnetic field is the magnetic effect of electric currents and magnetic 

materials. The magnetic field at any given point is specified by both a 

direction and a magnitude (or strength); as such it is a vector field. The term 

is used for two distinct but closely related fields denoted by the symbols B 

and H, where 

■ H is measured in units of amperes per meter (symbol: A·m -1 or A/m) in 

the SI. 

■ B is measured in teslas (symbol:T) and newtons per meter per ampere 

[symbol: N·m -1·A-1 or N/(m·A)] in the SI. (1 teslas = 10000 Gauss) 

B is most commonly defined in terms of the Lorentz force it exerts on moving 

electric charges. Magnetic fields can be produced by moving electric charges 

and the intrinsic magnetic moments of elementary particles associated with a 

fundamental quantum property, their spin. In special relativity, electric and 

magnetic fields are two interrelated aspects of a single object, called the 

electromagnetic tensor; the split of this tensor into electric and magnetic fields 

depends on the relative velocity of the observer and charge. In 

quantum physics, the electromagnetic field is quantized and 

electromagnetic interactions result from the exchange of photons. 


https://en.wikipedia.org/wiki/Magnetic_field

Weber (Magnetic Flux Ф) 

In physics, specifically electromagnetism, the magnetic flux (often denoted Φ 

or Φ B ) through a surface is the surface integral of the normal component of 

the magnetic field B passing through that surface. 

■ The SI unit of magnetic flux is the weber (Wb) (in derived units: voltseconds), 

and the CGS unit is the maxwell. 

Magnetic flux is usually measured with a fluxmeter, which contains measuring 

coils and electronics, that evaluates the change of voltage in the measuring 

coils to calculate the magnetic flux. 


https://en.wikipedia.org/wiki/Magnetic_flux

Gauss (Magnetic Flux Density) 

gauss, unit of magnetic induction in the centimetre-gram-second CGS 

system of physical units. One gauss corresponds to the magnetic flux density 

that will induce an electromotive force of one abvolt (10 -8 volt) in each linear 

centimetre of a wire moving laterally at one centimetre per second at right 

angles to a magnetic flux. One gauss corresponds to 10 -4 tesla (T), the 

International System Unit. The gauss is equal to 1 maxwell per square 

centimetre, or 10 -4 weber per square metre. Magnets are rated in gauss. The 

gauss was named for the German scientist Carl Friedrich Gauss. 


http://global.britannica.com/science/gauss

Electrical Inductance - Henry 

The henry (symbol H) is the unit of electrical inductance in the International 

System of Units. The unit is named after Joseph Henry (1797–1878), the 

American scientist who discovered electromagnetic induction independently 

of and at about the same time as Michael Faraday (1791–1867) in England. 

The magnetic permeability of a vacuum μ o is 4π×10 -7 H m -1 (henries per 

metre). 

The National Institute of Standards and Technology provides guidance for 

American users of SI to write the plural as "henries". The inductance of an 

electric circuit is one henry when an electric current that is changing at one 

ampere per second results in an electromotive force across the inductor of 

one volt: 

v(t) = L di/dt 

where v(t) denotes the resulting voltage across the circuit, i(t) is the current 

through the circuit, and L is the inductance of the circuit. 


https://en.wikipedia.org/wiki/Henry_(unit)

Magnetic Permeability (B/H) 

In electromagnetism, permeability is the measure of the ability of a material to 

support the formation of a magnetic field within itself. Hence, it is the degree 

of magnetization that a material obtains in response to an applied magnetic 

field. Magnetic permeability is typically represented by the Greek letter μ. The 

term was coined in September 1885 by Oliver Heaviside. The reciprocal of 

magnetic permeability is magnetic reluctivity. 

In SI units, permeability is measured in henries per meter (H·m -1 ), or newtons 

per ampere squared (N·A -2 ). The permeability constant (μ 0 ), also known as 

the magnetic constant or the permeability of free space, is a measure of the 

amount of resistance encountered when forming a magnetic field in a 

classical vacuum. The magnetic constant has the exact (defined) value µ 0 = 

4π×10 -7 H·m -1 ≈ 1.2566370614…×10−6 H·m-1 or N·A -2 ). 

A closely related property of materials is magnetic susceptibility, which is a 

dimensionless proportionality factor that indicates the degree of 

magnetization of a material in response to an applied magnetic field. 


https://en.wikipedia.org/wiki/Permeability_(electromagnetism)

More Reading on: Magnetic field 

A magnetic field is the magnetic influence of electric currents and magnetic 

materials. The magnetic field at any given point is specified by both a 

direction and a magnitude (or strength); as such it is a vector field. The term 

is used for two distinct but closely related fields denoted by the symbols B 

and H, 

Where: 

H: (magnetic field intensity) is measured in units of amperes per meter 

(symbol: A·m -1 or A/m) in the SI. 

B: (magnetic flux density) is measured in teslas (symbol: T) and newtons per 

meter per ampere (symbol: N·m -1·A -1 or Newtons per Ampere meter N/(m·A)) 

or weber/m 2 in the SI. 

B is most commonly defined in terms of the Lorentz force it exerts on moving 

electric charges. 



• Henry, unit of either self-inductance or mutual inductance, abbreviated h, 

and named for the American physicist Joseph Henry. One henry is the 

value of self-inductance in a closed circuit or coil in which one volt is 

produced by a variation of the inducing current of one ampere per second. 

One henry is also the value of the mutual inductance of two coils arranged 

such that an electromotive force of one volt is induced in one if the current 

in the other is changing at a rate of one ampere per second. 

• Weber, unit of magnetic flux in the International System of Units (SI), 

defined as the amount of flux that, linking an electrical circuit of one turn 

(one loop of wire, N=1), produces in it an electromotive force (E) of one 

volt as the flux is reduced to zero at a uniform rate in one second. 

• Tesla, (B: magnetic flux density) a flux density of one Wb/m 2 (one weber 

per square metre) is one tesla. 

• H: (magnetic field intensity) is measured in units of amperes per meter 

(symbol: A·m -1 or A/m) in the SI. 



The weber 

The weber may be defined in terms of Faraday's law, which relates a 

changing magnetic flux through a loop to the electric field around the loop. A 

change in flux of one weber per second will induce an electromotive force of 

one volt (produce an electric potential difference of one volt across two opencircuited 

terminals). 

Officially, 

Weber (unit of magnetic flux) - The weber is the magnetic flux which, linking a 

circuit of one turn, would produce in it an electromotive force of 1 volt if it were 

reduced to zero at a uniform rate in 1 second 


https://en.wikipedia.org/wiki/Weber_(unit)

Magnetic fields are produced by moving electric charges and the intrinsic 

magnetic moments of elementary particles associated with a fundamental 

quantum property, their spin. In special relativity, electric and magnetic fields 

are two interrelated aspects of a single object, called the electromagnetic 

tensor; the split of this tensor into electric and magnetic fields depends on the 

relative velocity of the observer and charge. In quantum physics, the 

electromagnetic field is quantized and electromagnetic interactions result 

from the exchange of photons.(?) 

In everyday life, magnetic fields are most often encountered as a force 

created by permanent magnets, which pull on ferromagnetic materials such 

as iron, cobalt, or nickel and attract or repel other magnets. Magnetic fields 

are widely used throughout modern technology, particularly in electrical 

engineering and electromechanics. The Earth produces its own magnetic field, 

which is important in navigation, and it guards Earth's atmosphere from solar 

wind. Rotating magnetic fields are used in both electric motors and 

generators. Magnetic forces give information about the charge carriers in a 

material through the Hall effect. The interaction of magnetic fields in electric 

devices such as transformers is studied in the discipline of magnetic circuits. 



History: 

Although magnets and magnetism were known much earlier, the study of 

magnetic fields began in 1269 when French scholar Petrus Peregrinus de 

Maricourt mapped out the magnetic field on the surface of a spherical 

magnet using iron needles Noting that the resulting field lines crossed at two 

points he named those points 'poles' in analogy to Earth's poles. He also 

clearly articulated the principle that magnets always have both a north and 

south pole, no matter how finely one slices them. 

Almost three centuries later, William Gilbert of Colchester replicated Petrus 

Peregrinus' work and was the first to state explicitly that Earth is a magnet 

Published in 1600, Gilbert's work, De Magnete, helped to establish 

magnetism as a science. 



In 1750, John Michell stated that magnetic poles attract and repel in 

accordance with an inverse square law. Charles-Augustin de Coulomb 

experimentally verified this in 1785 and stated explicitly that the north and 

south poles cannot be separated (dipoles) . Building on this force between 

poles, Siméon Denis Poisson (1781–1840) created the first successful model 

of the magnetic field, which he presented in 1824. In this model, a magnetic 

H-field is produced by 'magnetic poles' and magnetism is due to small pairs 

of north/south magnetic poles. 

Comment: 

H-field – magnetic field intensity? Relates to A∙m -1 



Three discoveries challenged this foundation of magnetism, though. 

First, in 1819, Hans Christian Oersted discovered that an electric current 

generates a magnetic field encircling it. Then in1820, André-Marie Ampère 

showed that parallel wires having currents in the same direction attract one 

another. Finally, Jean-Baptiste Biot and Félix Savart discovered the Biot- 

Savart law in 1820, which correctly predicts the magnetic field around any 

current- carrying wire. 

Extending these experiments, Ampère published his own successful model of 

magnetism in 1825. In it, he showed the equivalence of electrical currents to 

magnets and proposed that magnetism is due to perpetually flowing loops of 

current instead of the dipoles of magnetic charge in Poisson's model. This 

has the additional benefit of explaining why magnetic charge can not be 

isolated. Further, Ampère derived both Ampère's force law describing the 

force between two currents and Ampère's law, which, like the Biot–Savart law, 

correctly described the magnetic field generated by a steady current. Also in 

this work, Ampère introduced the term electrodynamics to describe the 

relationship between electricity and magnetism. 



In 1831, Michael Faraday discovered electromagnetic induction when he 

found that a changing magnetic field generates an encircling electric field. He 

described this phenomenon in what is known as Faraday's law of induction. 

Later, Franz Ernst Neumann proved that, for a moving conductor in a 

magnetic field, induction is a consequence of Ampère's force law. In the 

process he introduced the magnetic vector potential, which was later shown 

to be equivalent to the underlying mechanism proposed by Faraday. 

In 1850, Lord Kelvin, then known as William Thomson, distinguished between 

two magnetic fields now denoted H and B. The former applied to Poisson's 

model and the latter to Ampère's model and induction. Further, he derived 

how H and B relate to each other. 



Between 1861 and 1865, James Clerk Maxwell developed and published 

Maxwell's equations, which explained and united all of classical electricity and 

magnetism. The first set of these equations was published in a paper entitled 

On Physical Lines of Force in 1861. These equations were valid although 

incomplete. Maxwell completed his set of equations in his later 1865 paper A 

Dynamical Theory of the Electromagnetic Field and demonstrated the fact 

that light is an electromagnetic wave. Heinrich Hertz experimentally 

confirmed this fact in 1887. 

The twentieth century extended electrodynamics to include relativity and 

quantum mechanics. Albert Einstein, in his paper of 1905 that established 

relativity, showed that both the electric and magnetic fields are part of the 

same phenomena viewed from different reference frames. (See moving 

magnet and conductor problem for details about the thought experiment that 

eventually helped Albert Einstein to develop special relativity.) Finally, the 

emergent field of quantum mechanics was merged with electrodynamics to 

form quantum electrodynamics (QED). 



The B-field: 

The magnetic field can be defined in several equivalent ways based on the 

effects it has on its environment. 

Often the magnetic field is defined by the force it exerts on a moving charged 

particle. It is known from experiments in electrostatics that a particle of charge 

q in an electric field E experiences a force 

F = q·E. 

However, in other situations, such as when a charged particle moves in the 

vicinity of a current-carrying wire, the force also depends on the velocity of 

that particle. Fortunately, the velocity dependent portion can be separated 

out such that the force on the particle satisfies the Lorentz force law, 

F = q·(E + v × B) 

Here v is the particle's velocity and × denotes the cross product. The vector B 

is termed the magnetic field, and it is defined as the vector field necessary to 

make the Lorentz force law correctly describe the motion of a charged 

particle. 



Lorentz Force Law 

Both the electric field and magnetic field can be defined from the Lorentz 

force law: The electric force is straightforward, being in the direction of the 

electric field if the charge q is positive, but the direction of the magnetic part of 

the force is given by the right hand rule. 


http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/magfor.html

Electric Force 

F = q·E 

The magnetic field B is defined from the Lorentz Force Law, and specifically 

from the magnetic force on a moving charge: 

The implications of this expression include: 

1. The force is perpendicular to both the velocity v of the charge q and the 

magnetic field B. 

2. The magnitude of the force is F = q∙v∙Bsinϴ where ϴ is the angle < 180 

degrees between the velocity and the magnetic field. This implies that the 

magnetic force on a stationary charge or a charge moving parallel to the 

magnetic field is zero. 

3. The direction of the force is given by the right hand rule. The force 

relationship above is in the form of a vector product. 



From the force relationship above it can be deduced that the units of 

magnetic are Newton seconds /(Coulomb meter) or Newtons per Ampere 

meter. This unit is named the Tesla. It is a large unit, and the smaller unit 

Gauss is used for small fields like the Earth's magnetic field. A Tesla is 

10,000 Gauss. The Earth's magnetic field at the surface is on the order of 

half a Gauss. 

Keywords: 

F magnetic = q∙v∙B sinϴ 

The magnitude of the force is F = q∙v∙B sinϴ where ϴ is the angle < 180 

degrees between the velocity and the magnetic field. This implies that the 

magnetic force on a stationary charge or a charge moving parallel to the 

magnetic field is zero. 



The H-field 

In addition to B, there is a quantity H, which is also sometimes called the 

magnetic field. In a vacuum, B and H are proportional to each other, with the 

multiplicative constant depending on the physical units. 

Inside a material they are different (see H and B inside and outside of 

magnetic materials). The term "magnetic field" is historically reserved for H 

while using other terms for B. Informally, though, and formally for some 

recent textbooks mostly in physics, the term 'magnetic field' is used to 

describe B as well as or in place of H. There are many alternative names for 

both 

Comments: 

H: (magnetic field intensity) is measured in units of amperes per meter (symbol: A·m -1 or A/m) in 

the SI. 

B: (magnetic flux density) is measured in teslas (symbol: T) and newtons per meter per ampere 

(symbol: N·m -1·A -1 or Newtons per Ampere meter N/(m·A)) or weber/m 2 in the SI. 



Alternative names for B 

• Magnetic flux density 

• Magnetic induction 

• Magnetic field 

Alternative names for H 

• Magnetic field intensity 

• Magnetic field strength 

• Magnetic field 

• Magnetizing field 



Fleming Right Hand Rule 




http://www.encyclopedia-magnetica.com/doku.php/magnetism


http://www.electronenergy.com/magnetic-design/magnetic-design.htm

Reluctance S is the sum of the reluctance S g of air gaps (between the test 

object and the yoke), test object reluctance S s and yoke reluctance S y . The 

reluctance values of the air gaps, test object and yoke are given by Eq. 2 to 4: 

(2) (3) 

(4) 


(2) (3) 

(4) 

Where: 

• a x is the cross sectional area (square meter) of the air gaps, test object or 

yoke; 

• L x is the length (meter) of the air gaps, test object or yoke; μ 0 is the 

permeability of free space (μ 0 = 4π.10 –7 H·m –1 ); μ r is relative permeability; 

and subscripts g, s and y denote the air gaps, test object and yoke, 

respectively. 

Note that the magnetic circuit consists of two air gaps, one at each end of the 

test object. Both air gaps need to be taken into account in calculating the total 

reluctance of the magnetic circuit. 


Reluctance S 

Reluctance, S = Length L / (cross sectional area a ∙ permeability μ) 


To obtain maximum sensitivity, it is necessary to ensure that the magnetic 

flux is perpendicular to the discontinuity. This direction is in contrast to the 

orientation in techniques that use an electric current for inspection of a test 

object, where it may be more advantageous to orient the direction of current 

so that a discontinuity would impede the current as much as possible. 

Because the orientation of the discontinuity is unknown, it is necessary to test 

twice with the yoke, in two directions perpendicular to each other. A grid is 

usually drawn on the test object to facilitate the tests. 


9.2.1 Magnetizing Coil 

A commonly used encircling coil is shown in Fig. 2. The field direction follows 

the right hand rule. (The right hand rule states that, if someone grips a rod, 

holds it out and imagines an electric current flowing down the thumb, the 

induced circular field in the rod would flow in the direction that the fingers 

point.) With no test object present, the field lines form closed loops that 

encircle the current carrying conductors. The value of the field at any point 

has been established for a great many coil configurations. The value depends 

on the current in the coils, the number of turns N and a geometrical factor. 

Calculation of the field from first principles is generally unnecessary for 

nondestructive testing; a hall element tesla meter will measure this field. 


FIGURE 2. Encircling coil using direct current to produce magnetizing force. 

Legend 

I = electric current 

P, Q = points of discontinuities in 

example 

R = point at which magnetic field 

intensity H is measured 

S = point at which magnetic flux 

density B is measured 


Introduction of the test object into the field of the coil changes the field. The 

metal becomes part of the magnetic circuit, with the result that, close to the 

surface of the test object, magnetic field intensity H is lower than it would be if 

the test object were removed. Again, a hall element tesla meter will show the 

field intensity at the test object. This reduces the need for semi-empirical 

formulas. With the test object inserted, the flux density changes and the flux 

lines get concentrated within the test object. Thus, the fields inside and 

outside the test object are not the same. However, two boundary conditions 

allow assessment of the magnetic state of the test object. The fact that the 

tangential field is continuous across the air-to-metal interface allows 

measurement of H at the point R to yield the value of the tangential field at 

the test surface. In addition, because the normal component of magnetic flux 

density B is continuous, a tesla meter at point S will yield B inside the test 

object at that point. Two totally different situations, common in magnetic flux 

leakage testing, are described below. 


9.2.1.1 Testing in Active Field 

In this technique, the test object is scanned by probes near position R in Fig. 

2, in the presence of an active field. Air fields of 16 to 24 kA·m –1 (200 to 300 

Oe) are commonly used. In this situation, application of small fields is 

sufficient to cause magnetic flux leakage from transversely oriented surface 

breaking discontinuities. For subsurface discontinuities or those on the inside 

surface of tubes, larger fields are required. The inspector must experiment to 

optimize the applied field for the particular discontinuity. 


9.2.1.2 Testing in Residual Field 

Test objects are first passed through the coil field and then tested in the 

resulting residual field. Elongating the coil and placing the test object next to 

the inside surface of the coil will expose the test object to the largest field that 

the coil can produce. This technique is often used in magnetic particle testing. 

The main problem to avoid is the induction of so much magnetic flux in the 

test object that the magnetic particles stand out like fur along the field lines 

that enter and leave the test object, especially close to its ends. Optimum 

conditions require that the test object be somewhat less than saturated. The 

inspector should experiment to optimize the coil field requirements for the test 

object because this field depends on test object geometry. 


9.2.2 Applied Direct Current 

If an electric current is used to magnetize the test object, it may be more 

advantageous to orient the direction of current in a manner where the 

presence of a discontinuity impedes the current flow as much as possible. 

Bars, billets and tubes are often magnetized by application of a direct current 

I to their ends (Fig. 3). Figure 4 shows a system where the current I is passed 

directly through a tubular test object to magnetize the test object circularly. 

Figure 5 shows a central conductor energized by a current source I, again, to 

establish a circular magnetic field intensity H (ampere per square meter) in a 

tubular test object: 

(5) 

where a is area (square meter). 

Question: a or r? 


FIGURE 3. Circumferential magnetization by application of direct current: (a) 

rectilinear bar; (b) round bar; (c) tube. 

Legend 

H = magnetic field intensity 



FIGURE 4. Current carrying clamp electrodes used for testing ferromagnetic 

tubular objects with small diameters. 


FIGURE 5. Simple technique for circumferential magnetization of 

ferromagnetic tube. 

Legend 

H = magnetic field intensity 


r = tube radius 


9.2.2.1 Capacitor Discharge Devices 

For the circular magnetization of tubes or the longitudinal magnetization of the 

ends of elongated test objects, a capacitor discharge device is sometimes 

used. The capacitor discharge unit represents a practical advance over 

battery packs and consists of a capacitor bank charged to a voltage V and 

then discharged through a rod, a cable and a silicon controlled rectifier of total 

resistance R. The full system, considered mathematically, also contains a 

variable amount of inductance, so that if the current I c were allowed to 

oscillate, it would do so according to the theory of LCR circuits (that is, circuits 

described by inductance L, capacitance C and resistance R). The theory is 

complicated by the time required to magnetize the material and to induce an 

eddy current in the test object. 


Typical configurations shown in Fig. 6 illustrate the complexity of the situation. 

In the case of the magnetization of a tube, the current I c first rises rapidly, 

inducing magnetic flux in the tube. This time varying flux changes rapidly and 

induces an electromotive force in the tube, as dictated by Faraday’s law, the 

result being that an eddy current I e flows around the tube as shown in Fig. 6a, 

where the dashed line is the inner surface eddy current and the solid line is 

the outer surface current. 


FIGURE 6. Capacitor discharge configurations causing magnetization 

perpendicular to current direction: (a) conductor internal to test object 

creates circular field; (b) flexible cable around test object creates 

longitudinal field. 

Legend 

C = capacitor 

I c = capacitor discharge current 

I e = eddy current 

SCR = silicon controlled rectifier 


FIGURE 6. Capacitor discharge configurations causing magnetization 

perpendicular to current direction: (a) conductor internal to test object creates 

circular field; (b) flexible cable around test object creates longitudinal 

field. 

Legend 

C = capacitor 

I c = capacitor discharge current 

I e = eddy current 

SCR = silicon controlled rectifier 


The net result is a lack of penetration of the field caused by the capacitor 

discharge current I c . For a centered rod, in effect, the magnetic field intensity 

in the test object at radius r is given not by H = Ic·(2πr) –1 but rather by Eq. 6: 

(6) 

Here I e is the amount of eddy current (ampere) contained within the cylinder 

of radius r (meter). Investigation of the effect of the eddy current is 

theoretically quite complicated because of its effect on the inductance, which 

in turn affects I c . In practice, however, measurement of the magnetic flux 

density B in the material will yield the final degree of magnetization of that 

material. 


A good rule is that, if H(r) in Eq. 6 can be maintained at about 3.2 kA·m –1 

(40 Oe), the material will be magnetized almost to saturation and can be 

tested for both surface and subsurface discontinuities. 

Several other practical conclusions can be drawn from the above discussion. 

• Pulse duration plays a greater role than pulse amplitude I c(max) in 

determining the amount of flux induced in a test object. This is intuitively 

seen in direct current tests. 

• It is not possible to give simple rules that relate I c(max) to magnetization 

requirements. This relationship can be shown with a magnetic flux meter. 

• The eddy currents induced during pulse magnetization play an important 

role in the result. They can shield midwall regions from magnetization. 

• Larger capacitances at lower voltages provide better magnetization than 

smaller capacitances at higher voltages because larger capacitances at 

lower voltages lead to longer duration pulses and therefore to lower eddy 

currents. The lower voltage is an essential safety feature for outdoor use. 

A maximum of 50 V is recommended. 


9.2.3 Magnitudes of Magnetic Flux Leakage Fields 

The magnitude of the magnetic flux leakage field under active direct current 

excitation naturally depends on the applied field. An applied field of 3.2 to 4.0 

kA·m –1 (40 to 50 Oe) inside the material can cause leakage fields with peak 

values of tens of millitesla (hundreds of gauss). However, in the case of 

residual induction, the magnetic flux leakage fields may be only a few 

hundred microtesla (a few gauss). Furthermore, with residual field excitation, 

an interesting field reversal may occur, depending on the value of the initial 

active field excitation and the dimensions of the discontinuity. 


9.2.4 Optimal Operating Point 

Consider raising the magnetization level in a block of steel containing a 

discontinuity (Fig. 7). At low flux density levels, the field lines tend to crowd 

together in the steel around the discontinuity rather than go through the 

nonmagnetic region of the discontinuity. The field lines are therefore more 

crowded above and below the discontinuity than they are on the left or right. 

The material can hold more flux as the permeability rises, so there is no 

significant leakage flux at the surfaces (Fig. 7a). However, an increase in the 

number of lines causes ΔB·(ΔH) –1 to fall — the material is becoming less 

permeable. At about this point, magnetic flux leakage is first noticed at the 

surfaces. Although the lines are now closer together, representing a higher 

magnetic flux density, they do not have the ability to crowd closer together 

around the discontinuity where the permeability is low. 


FIGURE 7. Effects of induction on magnetic flux lines at discontinuity: (a) no 

surface flux leakage occurs where magnetic flux lines are compressed at low 

levels of induction around discontinuity; (b) lack of compression at high 

magnetization results in surface magnetic flux leakage. 


At higher and higher values of applied field, the permeability falls. It is, 

however, still large compared to the permeability of air, so the reluctance of 

the path through the discontinuity is still larger than through the metal. As a 

result, magnetic flux leakage at the outside surface helps provide a 

sufficiently high flux density in the material for the leakage of magnetic flux 

from discontinuities (Fig. 7b) while partially suppressing long range surface 

noise. For residual field testing, it is best to ensure that the material is 

saturated. The magnetic field starts to decay as soon as the energizing 

current is removed. 


The Great Rationalizer 


http://www.heitu5.com/kehuan/mojingxianzong/player-0-0.html

9.3 PART 3. Magnetic Flux Leakage Test Results 


Magnetic flux leakage testing continues to be one of the most popular 

nondestructive test techniques in industry. A number of factors, including low 

cost and simplicity of the data interpretation process, contribute to this 

popularity. The underlying principles and modeling techniques are described 

elsewhere in this volume. The discussion below focuses on probes and 

excitation schemes to detect and measure magnetic leakage fields. 


9.3.1 Magnetic Flux Leakage Probes 

The purpose of probes for magnetic testing is to detect and possibly quantify 

the magnetic flux leakage field generated by heterogeneities in the test object. 

The leakage fields tend to be local and concentrated near the discontinuities. 

The leakage field can be divided into three orthogonal components: normal 

(vertical), tangential (horizontal) and axial directions. Probes are usually 

either designed or oriented to measure one of these components. Typical 

plots of these components near discontinuities are shown in this volume’s 

chapter on probes. A variety of probes (or transducers) are used in industry 

for detecting and measuring leakage fields. 


Magnetic Flux Leakage fields. 




BS EN 10246-5:2000 MFLT Set-up 

1: Transducer 2: Tube 3: Rotating Magnet & Transducer 


BS EN 10246-5:2000



http://www.railwaystrategies.co.uk/article-page.php?contentid=9524&issueid=303

MFLT- Expert at Work 


MFLT- Expert at Works 




http://www.puretechltd.com/articles/newsletter/2012/03/California_MFL.shtml



http://www.puretechltd.com/articles/newsletter/2012/03/California_MFL.shtml

The most commonly used in-service inspection tools utilize the Magnetic Flux 

Leakage (MFL) technique in order to detect internal or external corrosion. The 

MFL inspection pig uses a circumferential array of MFL detectors embodying 

strong permanent magnets to magnetize the pipe wall to near saturation flux 

density. Abnormalities in the pipe wall, such as corrosion pits, result in 

magnetic flux leakage near the pipe's surface. These leakage fluxes are 

detected by Hall probes or induction coils moving with the MFL detector. The 

demands now being placed on magnetic inspection tools are shifting from the 

mere detection, location and classification of pipeline defects, to the accurate 

measurements of defect size and geometry. Modern, high-resolution MFL 

inspection tools are capable of giving very detailed signals. However, 

converting these signals to accurate estimates of size requires considerable 

expertise, as well as a detailed understanding of the effects of inspection 

conditions and the magnetic behaviour of the type of pipeline steel used. 


http://www.physics.queensu.ca/~amg/expertise/inline.html







Intelligence Pigging with MFLT 



Intelligence Piggy 



9.3.1.1 Pickup Coils 

One of the simplest and most popular means for detecting leakage fields is to 

use a pickup coil.6 Pickup coils consist of very small coils that are either air 

cored or use a small ferrite core. The voltage induced in the coil is given by 

the rate of change of flux linkages associated with the pickup coil. 

(7) 

Where: 

N is the number of turns in the coil, 

V is the voltage induced in the coil and 

Ф is the magnetic flux (weber) linking the coil. 

It must be mentioned that only the component of the flux parallel to the axis of 

the coil (or alternately perpendicular to the plane of the coil) is instrumental in 

inducing the voltage. 


This induction direction makes it possible to orient the pickup coil so as to 

measure any of the three leakage field components selectively. Thus, a coil A 

whose axis is perpendicular to the surface of the test object (Fig. 8a), is 

sensitive only to the normal component. In contrast, the coil in Fig. 8b is 

sensitive only to the tangential component. Consider the case where the 

pickup coil is moving over the test object in the X direction. Making use of the 

fact that Ф = B·A, where B is the magnetic flux density (tesla) and A is the 

cross sectional area (square meter) of the pickup coil, Eq. 7 can be rewritten: 

(8) 


FIGURE 8. Effect of pickup coil orientation on sensitivity to components of 

magnetic flux density: (a) coil sensitive to normal component; (b) coil sensitive 

to tangential component. 

(a) 

(b) 





(a) 

axis of the coil 

(b) 

plane of the coil 

plane of the coil 

axis of the coil 





(a) 

flux 

(b) 

flux 


This equation indicates that the output of the pickup coil is proportional to the 

spatial gradient of the flux along the direction of the coil movement as well as 

the velocity of the coil. Two issues arise as a result. 

1. It is essential that the probe scan velocity (relative to the test object) 

should be constant to avoid introducing artifacts into the signal through 

probe velocity variations. 

2. The output is proportional to the spatial gradient of the flux in the direction 

of the coil. The output of the pickup coil can be integrated for 

measurement of the leakage flux density rather than of its gradient. 


Figure 9 shows the output of a pickup coil and the signal obtained after 

integrating the output. The coil is used to measure, in units of tesla (or gauss), 

the magnetic flux density B leaking from a rectangular slot. The sensitivity of 

the pickup coil can be improved by using a ferrite core. Tools for designing 

pickup coils, as well as predicting their performance, are described elsewhere 

in this volume. 


FIGURE 9. Pickup coil and signal integrator (magnetic flux leakage) output for 

rectangular discontinuity. 


Signal and Magnetic Disturbances 


Magnetic Flux Leakage & Signals 






9.3.1.2 Magnetodiodes 

The magnetodiode is suitable for sensing leakage fields from discontinuities 

because of its small size and its high sensitivity. Because the coil probe is 

usually larger than the magnetodiode, it is less sensitive to longitudinally 

angled discontinuities than the magnetodiode is. However, the coil probe is 

better than the magnetodiode for large discontinuities, such as cavities. 


Magnetodiodes 


http://www.craft-3.com/Semiconductor/SONY_Transistor/sony_diode.html

9.3.1.3 Hall Effect Detectors 

Hall effect detector probes are used extensively in industry for measuring 

magnetic flux leakage fields in units of tesla (or gauss). Hall effect detector 

probes are described in this volume’s chapter on probes for electromagnetic 

testing. 


Hall Effect Detectors 


Hall Effect Detectors 


http://movableparts.org/rear-wheel-tachometer/

9.3.1.4 Giant Magnetoresistive Probes 

Magnetic field sensitive devices called giant magnetoresistive probes, at the 

most basic level, consist of a nonmagnetic layer sandwiched between two 

magnetic layers. The apparent resistivity of the structure varies depending on 

whether the direction of the electron spin is parallel or antiparallel to the 

moments of the magnetic layers. When the moments associated with the 

magnetic layers are aligned antiparallel, the electrons with spin in one 

direction (up) that are not scattered in one layer will be scattered in the other 

layer. This increases the resistance of the device. 

This is in contrast to the situation when the magnetic moments associated 

with the layers are parallel where the electrons that are not scattered in one 

layer are not scattered in the other layer, either. Giant magnetoresistive 

probes use a biasing current to push the magnetic layers into an antiparallel 

moment state and the external field is used to overcome the effect of the bias. 

The resistance of the device, therefore, decreases with increasing field 

intensity values. 

Figure 10 shows a typical response of a giant magnetoresistive probe. 


Keywords: 

The resistance of the device, therefore, decreases with increasing field 

intensity values. 


FIGURE 10. Resistance versus applied field for 2 μm (8 . 10–5 in.) wide strip 

of anti-ferromagnetically coupled, multilayer test object composed of 14 

percent giant magnetoresistive material. 


More Reading on: Giant Magnetoresistive Probes 

There are better alternatives to detect pneumatic cylinder end of stroke 

position than reed switches or proximity switches. By better, I mean they are 

faster and easier to implement into your control system. In addition, you can 

realize other benefits such as commonality of spare sensors and lower longterm 

costs. So what are the better solutions? Three types of sensor 

technologies lead the way to better alternatives. First, there is the Hall Effect 

magnetic field sensor, see figure 1. 

The benefit of Hall Effect 

sensors is speed; they 

are electronic so there 

are no moving parts and 

nothing to wear out. 

They are not affected by 

shock and vibration 

unlike the reed switch. 

figure 1 


https://sensortech.wordpress.com/2010/06/25/better-alternatives-to-pneumatic-cylinder-end-of-stroke-detection/

However, there are some disadvantages of Hall Effects such as they typically 

require fairly high magnetic gauss strength and they require a radially 

magnetized magnet. Typically, a Hall Effect will not work as a replacement of 

a reed switch or if it does operate, it may produce double switch points. A Hall 

Effect sensor is looking for a single magnetic pole, so if it is used with an 

axially magnetized magnet, it will switch when it sees the north pole and then 

again with the south pole, thus causing the double switch points. 

The second and newer technology is the magnetoresistive sensor shown in 

figure 2 or sometimes referred to as AMR (Anisotropic magnetoresistance). 

Unlike the Hall Effect sensor that uses a change in voltage the AMR is based 

off a change in resistance. This change in resistance is more sensitive thus; a 

lower strength magnet can be utilized. The best advantage of the AMR 

sensor is that it will work with the axially magnetized magnet and in most 

cases the radially magnetized magnet. Like the Hall Effect, the AMR has no 

moving parts and nothing to wear out and is fast therefore it is a good solution 

for high-speed applications. The magnetoresistive sensor does not suffer 

from double switch points and has a much better noise immunity than Hall 

Effects. 


Figure 2: 



Giant Magnetoresistive or GMR sensors shown in figure 3 are technologically 

the newer of the magnetic field sensors. They operate on a change in 

resistance, as does the AMR, however; the magnetic field causes a larger or 

giant change in resistance. Although you would think the GMR sensors are 

physically larger than the AMR, they are actually smaller. Major advantages 

of the GMR sensor are they are more sensitive, are more precise and have a 

better hysteresis than the AMR. 



Giant Magnetoresistive Probes 



Okay so the AMR and GMR sensors seem to be the better or even the best 

solution. Are there other advantages to them? Higher quality sensor 

manufacturers offer better output circuitry that includes reverse polarity 

protection, overload protection and short circuit protection. Couple that with 

lifetime warranty offered on some manufacturer’s sensors and you end up 

with a better alternative to the pneumatic cylinder end of stroke sensor. 

I know what you are thinking there must be some negatives. The initial cost of 

the AMR or GMR sensor may be slightly more than the reed sensor however 

this cost is becoming less and less and it is even less once you figure the cost 

of down time after your reed switch fails or the proximity flag is moved. In 

addition, the AMR and GMR sensors are 3-wire devices unlike the 2-wire 

reed switch. However, in the end the AMR and GMR sensors are still the 

better solution. 



9.3.1.5 Magnetic Tape 

For the testing of flat surfaces, magnetic tape can be used. The tape is 

pressed to the surface of the magnetized billet and then scanned by small 

probes before being erased. This technique is sometimes called 

magnetography. In automated systems, magnetic tape can be fed from a 

spool. The signals can be read and the tape can be erased and reused. 

Unfortunately, the tangential leakage field intensity at the surface of the 

material is not constant. To optimize the response, the amplification of the 

signals can be varied. Scabs or slivers projecting from the test surface can 

easily tear the tape 


9.3.2 Magnetic Particles 

Magnetic particles are one of the most popular means used in industry for 

detecting magnetic fields. Indeed, magnetic particle testing is so popular that 

an entire volume of the Nondestructive Testing Handbook is devoted to the 

subject. The descriptions below are therefore cursory 粗略的 . Magnetic 

particle testing involves the application of magnetic particles to the test object 

after it is magnetized by using an appropriate technique. The ferromagnetic 

particles preferentially adhere to the surface of the test object in areas where 

the flux is diverted, or leaks out. The magnetic flux leakage near 

discontinuities causes the magnetic particles to accumulate in the region and 

in some cases form an outline of the discontinuity. Heterogeneities can 

therefore be detected by looking for indications of magnetic particle 

accumulations on the surface of the test object either with the naked eye or 

through a camera. The indications are easier to see if the particles are bright 

and reflective. Alternately, particles that fluoresce under ultraviolet or visible 

radiation may be used. The test object has to be viewed under appropriate 

levels of illumination with radiation of appropriate wavelength (visible, 

ultraviolet or other). 


9.3.2.1 Application Techniques 

Magnetic particles are applied to the surface by two different techniques in 

industry. 

(A) Dry Testing. 

Dry techniques use particles applied in the form of a fine stream or an aerosol. 

They consist of high permeability ferromagnetic particles coated with either 

reflective or fluorescent pigments. The particle size is chosen according to the 

dimensions of the discontinuity sought. Particle diameters range from ≤50μm 

to 180 μm (≤0.002 to 0.007 in.). Finer particles are used for detecting smaller 

discontinuities where the leakage intensity is low. Dry techniques are used 

extensively for testing welds and castings where heterogeneities of interest 

are relatively large. 


(B) Wet Testing. 

Wet techniques are used for detecting relatively fine cracks. The magnetic 

particles are suspended in a liquid (usually oil or water) usually sprayed on 

the test object. Particle sizes are significantly smaller than those used with dry 

techniques and vary in size within a normal distribution, with most particles 

measuring from 5 to 20μm (2·10 –4 to 8·10 –4 in.). As in the case of dry 

powders, the ferromagnetic particles are coated with either reflective or 

fluorescent pigments. More information on this subject is available elsewhere. 


MPI 


9.3.2.2 Imaging of Magnetic Particle Indications 

The magnetic particle distribution can be examined visually after illuminating 

the surface or the surface can be scanned with a flying spot system or 

imaged with a charge coupled device camera. 

(A) Flying Spot Scanners. 

To illuminate the test object (Fig. 11), flying spot scanners use a narrow beam 

of radiation - visible light for non-fluorescent particles and ultraviolet radiation 

for fluorescent ones. The source of the beam is usually a laser. The 

wavelength of the beam is chosen carefully to excite the pigment of the 

magnetic particles. The incidence of the radiation beam on the test object can 

be varied by moving the scanning mirror. The photocell does not sense any 

light when the test object is scanned by the narrow radiation beam until the 

beam is directly incident on the magnetic particles adhering to the test object 

near a discontinuity. When this occurs, a large amount of light is emitted, 

called fluorescence if excited by ultraviolet radiation. The fluorescence is 

detected by a single phototube equipped with a filter that renders the system 

blind to the radiation from the irradiating source. The output of the photocell is 

suitably amplified, digitized and processed by a computer. 


FIGURE 11. Flying spot scanner for automated magnetic particle testing. 


FIGURE 11. Flying spot scanner for automated magnetic particle testing. 

filter that renders the system 

blind to the radiation from 

the irradiating source 


(B) Charge Coupled Devices CCD. 

An alternative approach is to flood the test object with radiation whose 

wavelength is carefully chosen to excite the pigment of the magnetic particles. 

Charge coupled device cameras, equipped with optical filters that render the 

camera blind to radiation from the source but are transparent to light emitted 

by the magnetic particles, can be used to image the surface very rapidly. In 

very simple terms, charge coupled devices each consist of a two dimensional 

array of tiny pixels that each accumulates a charge corresponding to the 

number of photons incident on it. When a readout pulse is applied to the 

device, the accumulated charge is transferred from the pixel to a holding or 

charge transfer cell. The charge transfer cells are connected in a manner that 

allows them to function as a bucket brigade or shift register. The charges can, 

therefore, be serially clocked out through a charge-to-voltage amplifier that 

produces a video signal. 


In practice, charge coupled device cameras can be interfaced to a personal 

computer through frame grabbers, which are commercially available. Vendors 

of frame grabbers usually provide software that can be executed on the 

personal computer to process the image. Image processing software can be 

used for example to improve contrast, highlight the edges of discontinuity or 

to minimize noise in the image. 


CCD 


http://oneslidephotography.com/ccd-vs-cmos-dslr-camera-wich-one-is-better/

CCD 


CCD 


http://www.smartinfoblog.com/cmos-vs-ccd-sensor/

CCD 


http://www.rocketroberts.com/astro/ccd_fundamentals.htm

9.3.3 Test Calculations 

In determining the magnetic flux leakage from a discontinuity, certain 

conditions must be known: 

1. the discontinuity’s location with respect to the surfaces from which 

measurements are made, 

2. the relative permeability of the material containing the discontinuity and 

3. the levels of magnetic field intensity H and magnetic flux density B in the 

vicinity of the discontinuity. 

Even with this knowledge, the solution of the applicable field equations 

(derived from Maxwell’s equations of electromagnetism) is difficult and is 

generally impossible in closed algebraic form. Under certain circumstances, 

such as those of discontinuity shapes that are easy to handle mathematically, 

relatively simple equations can be derived for the magnetic flux leakage if 

simplifying assumptions are made. This simplification does not apply to 

subsurface inclusions. 


9.3.3.1 Finite Element Techniques 

An advance in magnetic theory since 1980 has been the introduction of finite 

element computer codes to the solution of magnetostatic problems. Such 

codes came originally from a desire to minimize electrical losses from 

electromagnetic machinery but soon found application in magnetic flux 

leakage theory. The advantage of such codes is that, once set up, 

discontinuity leakage fields can be calculated by computer for any size and 

shape of discontinuity, under any magnetization condition, so long as the B,H 

curve for the material is known. In the models of magnetic flux leakage 

discussed so far, the implicit assumptions are (1) that the field within a 

discontinuity is uniform and (2) that the nonlinear magnetization characteristic 

(B,H curve) of the tested material can be ignored. Much of the early 

pioneering work in magnetic flux leakage modeling used these assumptions 

to obtain closed form solutions for leakage fields. 


Nonlinear magnetization characteristic (B,H curve) of the tested material 


http://www.electronics-tutorials.ws/electromagnetism/magnetic-hysteresis.html

The solutions of classical problems in electrostatics have been well known to 

physicists for almost a century and their magnetostatic analogs were used to 

approximate discontinuity leakage fields. Such techniques work reasonably 

well when the permeability around a discontinuity is constant or when 

nonlinear permeability effects can be ignored. The major problem that 

remains is how to deal with real discontinuity shapes often impossible to 

handle by classical techniques. Such deficiencies are overcome by the use of 

computer programs written to allow for nonlinear permeability effects around 

oddly shaped discontinuities. Specifically, computerized finite element 

techniques, originally developed for studying magnetic flux distributions in 

electromagnetic machinery, have also been developed for nondestructive 

testing. Both active and residual excitation are discussed above. The 

extension of the technique to include eddy currents is detailed elsewhere in 

this volume. 


Further Reading: 

Understanding Magnetic Flux Leakage Signals from Mechanical 

Damage in Pipelines 

In-line inspection using the Magnetic Flux Leakage (MFL) technique is 

sensitive both to pipe wall geometry and pipe wall stresses. Therefore, MFL 

inspection tools have the potential to locate and characterize mechanical 

damage in pipelines. However, the combined influence of stress and 

geometry make MFL signals from dents and gouges difficult to interpret. 

Accurate magnetic models that can incorporate both stress and geometry 

effects are essential to improve the current understanding of MFL signals 

from mechanical damage. MFL signals from dents include a geometry 

component in addition to a component due to residual stresses. If gouging is 

present, then there may also be an additional magnetic contribution from the 

heavily worked material at the gouge surface. The relative contribution of 

each of these components to the MFL signal depends on the size and shape 

of the dent in addition to other effects such as metal loss, wall thinning, 

corrosion, etc. 


http://prci.org/index.php/site/projects_single/understanding_magnetic_flux_leakage_signals_from_mechanical_damage_in_pipel/

FEA Model 



Key Results 

Magnetic Finite Element Analysis (FEA) can be applied to model MFL signals 

from mechanical damage defects having various sizes, shapes, and 

configurations. These models included geometry effects, contributions due to 

elastic strain (either residual strain or strain due to in-service loading), and 

also magnetic behavior changes due to severe deformation. The modeled 

results were then compared with experimental MFL signal measurements on 

dents and gouges produced in the laboratory as well under “field” 

conditions. Magnetic FEA models were produced of circular dents as well as 

dents elongated in the pipe axial and pipe hoop directions. Residual stress 

patterns were predicted in and around the dent using stress FEA 

modeling. The magnetic effects of these predicted residual stresses were 

incorporated into the magnetic FEA model by modifying the magnetic 

permeability in stressed regions in and around the dent. The modeled stress 

and geometry contributions to the MFL signal were examined separately, and 

also combined for comparison with experimental MFL results. Agreement 

between modeled and measured MFL signals was generally good, and the 

measured MFL signals were used to validate and refine the models. 



Other Reading: 

Leakage signals due the two defects. Field shown in (a) corresponds to the 

deeper defect and field shown in (b) to the shallow one. 



9.4 PART 4. Applications of Magnetic Flux Leakage 

Testing 


Magnetic flux leakage testing is a commonly used technique. Signals from 

probes are processed electronically and presented in a manner that indicates 

the presence of discontinuities. Although some techniques of magnetic flux 

leakage testing may not be as sophisticated as others, it is probable that 

more ferromagnetic material is tested with magnetic flux leakage than with 

any other technique. Magnetizing techniques have evolved to suit the 

geometry of the test objects. The techniques include yokes, coils, the 

application of current to the test object and conductors that carry current 

through hollow test objects. Many situations exist in which current cannot be 

applied directly to the test object because of the possibility of arc burns. 


Design considerations for magnetization of test objects often require 

minimizing the reluctance of the magnetic circuit, consisting of 

(1) the test object, 

(2) the magnetizing system and 

(3) any air gaps that might be present. 


Reluctance S 

Reluctance, S = Length L / (cross sectional area a ∙ permeability μ) 


9.4.1 Test Object Configurations 

9.4.1.1 Short Asymmetrical Objects 

A short test object with little or no symmetry may be magnetized to saturation 

by passing current through it or by placing it in an encircling coil. If hollow, a 

conductor can be passed through the test object and magnetization achieved 

by any of the standard techniques (these include half-wave and full-wave 

rectified alternating current, pure direct current from battery packs or pulses 

from capacitor discharge systems). For irregularly shaped test objects, testing 

by wet or dry magnetic particles is often performed, especially if specifications 

require that only surface breaking discontinuities be found. 


9.4.1.2 Elongated Objects 

The cylindrical symmetry of elongated test objects such as wire rope permits 

the use of a relatively simple flux loop to magnetize a relatively short section 

of the rope. Encircling probes are placed at some distance from the rope to 

permit the passage of splices. Such systems are also suited for pumping well 

sucker rods and other elongated oil field test objects. After a well is drilled, the 

sides of the well are lined with a relatively thin steel casing material, which is 

then cemented in. This casing can be tested only from the inside surface. The 

cylindrical geometry of the casing permits the flux loop to be easily calculated 

so that magnetic saturation of the well casing is achieved. As with in-service 

well casing, buried pipelines are accessible only from the inside surface. The 

magnetic flux loop is the same as for the well casing test system. In this case, 

a drive mechanism must be provided to propel the test system through the 

pipeline. 


Elongated Objects- Pump Jack 


9.4.1.3 Threaded Regions of Pipe 

An area that requires special attention during the inservice testing of drill pipe 

is the threaded region of the pin and box connections. Common problems that 

occur in these regions include fatigue cracking at the roots of the threads and 

stretching of the thread metal. Automated systems that use both active and 

residual magnetic flux techniques can be used for detecting such 

discontinuities. 


Threaded Regions of Sucker Rod 


9.4.1.4 Ball Bearings and Races 

Systems have been built for the magnetization of both steel ball bearings 

and their races. One such system uses specially fabricated hall elements as 

detectors. 

9.4.1.5 Relatively Flat Surfaces 

The testing of welded regions between flat or curved plates is often performed 

using a magnetizing yoke. Probe systems include coils, hall effect detectors, 

magnetic particles and magnetic tape. 


9.4.1.4 Ball Bearings and Races 

Systems have been built for the magnetization of both steel ball bearings 

and their races. One such system uses specially fabricated hall elements as 

detectors. 

9.4.1.5 Relatively Flat Surfaces 

The testing of welded regions between flat or curved plates is often performed 

using a magnetizing yoke. Probe systems include coils, hall effect detectors, 

magnetic particles and magnetic tape. 


Relatively Flat Surfaces 


Relatively Flat Surfaces 


9.4.2 Discontinuity Mechanisms 

In the metal forming industry, discontinuities commonly found by magnetic 

flux leakage techniques include overlaps, seams, quench cracks, gouges, 

rolled-in slugs and subsurface inclusions. In the case of tubular goods, 

internal mandrel marks (plug scores) can also be identified when they result 

in remaining wall thicknesses below some specified minimum. Small marks of 

the same type can also act as stress raisers and cracking can originate from 

them during quench and temper procedures. Depending on the use to which 

the material is put, subsurface discontinuities such as porosity and 

laminations may also be considered detrimental. These types of 

discontinuities may be acceptable in welds where there are no cyclic stresses 

but may cause injurious cracking when such stresses are present. 


In the metal processing industries, grinding especially can lead to surface 

cracking and to some changes in surface metallurgy. Such discontinuities as 

cracking have traditionally been found by magnetic flux leakage techniques, 

especially wet magnetic particle testing. Service induced discontinuities 

include cracks, corrosion pitting, stress induced metallurgy changes and 

erosion from turbulent fluid flow or metal-to-metal contact. In those materials 

placed in tension and under torque, fatigue cracking is likely to occur. A 

discontinuity that arises from metal-to-metal wear is sucker rod wear in tubing 

from producing oil wells. Here, the pumping rod can rub against the inner 

surface of the tube and both the rod and tube wear thin. In wire rope, the 

outer strands will break after wearing thin and inner strands sometimes break 

at discontinuities present when the rope was made. Railroad rails are subject 

to cyclic stresses that can cause cracking to originate from otherwise benign 

internal discontinuities. 


Loss of metal caused by a conducting fluid near two slightly dissimilar metals 

is a very common form of corrosion. The dissimilarity can be quite small, as 

for example, at the heat treated end of a rod or tube. The result is preferential 

corrosion by electrolytic processes, compounded by erosion from a contained 

flowing fluid. Such loss mechanisms are common in subterranean pipelines, 

installed petroleum well casing and in refinery and chemical plant tubing. The 

stretching and cracking of threads is a common problem. For example, when 

tubing, casing and drill pipe are overtorqued at the coupling, the threads exist 

in their plastic region. This causes metallurgical changes in the metal and can 

create regions where stress corrosion cracking takes place in highly stressed 

areas at a faster rate than in areas of less stress. Couplings between tubes 

are a good example of places where material may be highly stressed. Drill 

pipe threads are a good example of places where such stress causes plastic 

deformation and thread root cracking. 


9.4.3 Typical Magnetic Flux Leakage Techniques 

9.4.3.1 Short Parts 

For many short test objects, the most convenient probe to use is the magnetic 

particle. The test object can be inspected for surface breaking discontinuities 

during or after it has been magnetized to saturation. For active field testing, 

the test object can be placed in a coil carrying alternating current and sprayed 

with magnetic particles. Or it can be magnetized to saturation by a direct 

current coil and the resulting residual induction can be shown with magnetic 

particles. In the latter case, the induction in the test object can be measured 

with a flux meter. Wet particles perform better than dry ones because there is 

less tendency for the wet particles to fur (that is, to stand up like short hairs) 

along the field lines that leave the test object. These techniques will detect 

transversely oriented, tight discontinuities. 


The magnetic flux leakage field intensity from a tight crack is roughly 

proportional to the magnetic field intensity H g across the crack, multiplied by 

crack width L g . If the test is performed in residual induction, the value of Hg 

(which depends on the local value of the demagnetization field in the test 

object) will vary along the test object. Thus, the sensitivity of the technique to 

discontinuities of the same geometry varies along the length of the test object. 

For longitudinally oriented discontinuities, the test object must be magnetized 

circumferentially. If the test object is solid, then current can be passed 

through the test object, the surface field intensity being given by Ampere’s law: 

(9) 

Where: 

dl is an element of length (meter), H is the magnetic field intensity (ampere 

per meter) and I is the current (ampere) in the test object. 


Ampere's Law 

The magnetic field in space around an electric current is proportional to the 

electric current which serves as its source, just as the electric field in space is 

proportional to the charge which serves as its source. 

Ampere's Law states that for any closed loop path, the sum of the length 

elements times the magnetic field in the direction of the length element is 

equal to the permeability times the electric current enclosed in the loop. 

In the electric case, the relation of field to source is quantified in Gauss's Law 

which is a very powerful tool for calculating electric fields 


http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/amplaw.html

If the test object is a cylindrical bar, the symmetry of the situation allows H to 

be constant around the circumference, so the closed integral reduces: 

(10) 

(11) 

Where: 

R is the radius (meter) of the cylindrical test object. A surface field intensity 

that creates an acceptable magnetic flux leakage field from the minimum 

sized discontinuity must be used. Such fields are often created by specifying 

the amperage per meter of the test object’s outside diameter. 


9.4.3.2 Transverse Discontinuities 

Because of the demagnetizing effect at the end of a tube, automated 

magnetic flux leakage test systems do not generally perform well when 

scanning for transverse discontinuities at the ends of tubes. The normal 

component H y of the field outside the tube is large and can obscure 

discontinuity signals. Test specifications for such regions often include the 

requirement of additional longitudinal magnetization at the tube ends and 

subsequent magnetic particle tests during residual induction. This situation is 

equivalent to the magnetization and testing of short test objects as outlined 

above. 


The flux lines must be continuous and must therefore have a relatively short 

path in the metal. Large values of the magnetizing force at the center of the 

coil are usually specified. Such values depend on the weight per unit length of 

the test object because this quantity affects the ratio of length L to diameter D. 

Where the test object is a tube, the L·D –1 ratio is given by the length between 

the poles divided by twice the wall thickness of the tube. (The distance L from 

pole to pole can be longer or shorter than the actual length of the test object 

and must be estimated by the operator.) 

As a rough example, with L = 460 mm (18 in.) and D = 19 (0.75 in.), the L·D –1 

ratio is 24. 


The effective permeability of the metal under test is small because of the 

large demagnetization field created in the test object by the physical end of 

the test object. An empirical formula is often used to calculate approximately 

the effective permeability μ: 

(12) 

so effective permeability μ = 139 in the above example. For wet magnetic 

particle testing, the surface tension of the fluids that carry the particles is large 

enough to confine the particles to the surface of the test object. This is not the 

case with dry particles, which have the tendency to stand up like fur along 

lines of magnetizing force. In many instances, it may be better to use some 

other test technique for transverse discontinuities, such as ultrasonic or eddy 

current techniques. 


9.4.3.3 Alternating Current versus Direct Current Magnetization 

Alternating current magnetization is more suitable for detection of outer 

surface discontinuities because it concentrates the magnetic flux at the 

surface. For equal magnetizing forces, an alternating current field is better for 

detecting outside surface imperfections but a direct current field is better for 

detecting imperfections below the surface. In practice, the ends of tubes are 

tested for transverse discontinuities by the following magnetic flux leakage 

techniques. 


1. Where there is a direct current active field from an encircling coil, 

magnetic particles are thrown at the tested material while it is maintained 

at a high level of magnetic induction by a direct current field in the coil. 

This technique is particularly effective for internal cracks. Fatigue cracks in 

drill pipe are often found by this technique. 

2. Where there is an alternating current active field from an encircling coil, 

magnetic particles are thrown at the tested material while it lies inside a 

coil carrying alternating current. Using 50 or 60 Hz alternating current, the 

penetration of the magnetic field into the material is small and the 

technique is good only for the detection of outside surface discontinuities. 

When tests for both outer surface and inner surface discontinuities are 

necessary, it may be best to test first for outer surface discontinuities with 

an alternating current field, then for inner surface discontinuities with a 

direct current field. 


9.4.3.4 Liftoff Control of Scanning Head 

To obtain a stable detection of discontinuities, liftoff between the probe and 

the surface of the material must be kept constant. Usually liftoff is kept 

constant by contact of the probe with the surface but the probe tends to wear 

with this technique. A magnetic floating technique has been used for 

noncontact scanning. In this technique, liftoff is measured by a gap probe and 

the probe holder is moved by a voice coil motor, controlled by the gap signal. 

This system and related technology are described in this volume’s chapter on 

primary metals applications. 


9.4.4 Particular Applications 

9.4.4.1 Wire Ropes 

An interesting example of an elongated steel product inspected by magnetic 

flux leakage testing is wire rope. Such ropes are used in the construction, 

marine and oil production industries, in mining applications and elevators for 

personnel and raw material transportation. Testing is performed to determine 

cross sectional loss caused by corrosion and wear and to detect internal and 

external broken wires. The type of flux loop used (electromagnet or 

permanent magnet) can depend on the accessibility of the rope. Permanent 

magnets might be used where taking power to an electromagnet might cause 

logistic or safety problems. By making suitable estimates of the parameters 

involved, a reasonably good estimate of the flux in the rope can be made. 

Because discontinuities can occur deep inside the rope material, it is 

essential to maintain the rope at a high value of magnetic flux density, 1.6 to 

1.8 T (16 to 18 kG). Under these conditions, breaks in the inner regions of the 

rope will produce magnetic flux leakage at the surface of the rope. 


The problem of detecting magnetic flux leakage from inner discontinuities is 

compounded by the need to maintain the magnetic probes far enough from 

the rope for splices in the rope to pass through the test head. Common 

probes include hall effect detectors and encircling coils. The cross sectional 

area of the rope can be measured by sensing changes in the magnetic flux 

loop that occur when the rope gets thinner. The air gap becomes larger and 

so the value of the field intensity falls. This change can easily be sensed by 

placing hall effect probes anywhere within the magnetic circuit. 


9.4.4.2 Internal Casing or Pipelines 

The testing of in-service well casing or buried pipelines is often performed by 

magnetic flux leakage techniques. Various types of wall loss mechanisms 

occur, including internal and external pitting, erosion and corrosion caused by 

the proximity of dissimilar metals. From the point of view of magnetizing the 

pipe metal in the longitudinal direction, the two applications are identical. The 

internal diameters and metal masses involved in the magnetic flux loop 

indicate that some form of active field excitation must be used. Internal 

diameters of typical production or transportation tubes range from about 100 

mm (4 in.) to about 1.2 m (4 ft). If the material is generally horizontal, some 

form of drive mechanism is required. Because the test device (a robotic 

crawler) may move at differing speeds, the magnetic flux leakage probe 

should have a signal response independent of velocity. 


For devices that operate vertically, such as petroleum well casing test 

systems, coil probes can be used if the tool is pulled from the bottom of the 

well at a constant speed. In both types of instrument, the probes are mounted 

in pads pressed against the inner wall of the pipe. Because both line pipe and 

casing are manufactured to outside diameter size, there is a range of inside 

diameters for each pipe size. Such ranges may be found in specifications. To 

make the air gap as small as possible, soft iron attachments can be screwed 

to the pole pieces. For the pipeline crawler, a recorder package is added and 

the signals from discontinuities are tape recorded. When the tapes are 

retrieved and played back, the areas of damage are located. Pipe welds 

provide convenient magnetic markers. With the downhole tool, the magnetic 

flux leakage signals are sent up the wire line and processed in the logging 

truck at the wellhead. A common problem with this and other magnetic flux 

leakage equipment is the need to determine whether the signals originate 

from discontinuities on the inside or the outside surface of the pipe. 


Production and transmission companies require this information because it 

lets them determine which form of corrosion control to use. The test shoes 

sometimes contain a high frequency eddy current probe system that responds 

only to inside surface discontinuities. Thus, the occurrence of both magnetic 

flux leakage and eddy current signals indicates an inside surface discontinuity 

whereas the occurrence of a magnetic flux leakage signal indicates only an 

outside surface discontinuity. Problems with this form of testing include the 

following: 

1. The magnetic flux leakage system cannot measure elongated changes in 

wall thickness, such as might occur with general erosion. 

2. If there is a second string around the tested string, the additional metal 

contributes to the flux loop, especially in areas where the two strings touch. 

3. A relatively large current must be sent down the wire line to raise the pipe 

wall to saturation. Temperatures in deep wells can exceed 200°C (325 °F). 

4. The tool may stick downhole or underground if external pressures cause 

the pipe to buckle. 


9.4.4.3 Cannon Tubes 

In elongated tubing, the presence of rifling affects the ability to perform a 

good test, especially for discontinuities that occur in the roots of the rifling. 

Despite the presence of extraneous signals from internal rifling, however, 

rifling causes a regular magnetic flux leakage signal that can be distinguished 

from discontinuity signals. As a simulated discontinuity is made narrower and 

shallower, the signal will eventually be indistinguishable from the rifle bore 

noise. In magnetic flux leakage testing, cannon tubes can be magnetized to 

saturation and scanned with hall elements to measure residual induction. 


Cannon Tubes 


9.4.4.5 Round Bars and Tubes 

In some test systems, round bars and tubes have been magnetized by an 

alternating current magnet and rotated under the magnet poles. Because the 

leakage flux from surface discontinuities is very weak and confined to a small 

area, the probes must be very sensitive and extremely small. The system 

uses a differential pair of magnetodiodes to sense leakage flux from the 

discontinuity. The differential output of these twin probes is amplified to 

separate the leakage flux from the background flux. In this system, pipes are 

fed spirally under the scanning station, which has an alternating current 

magnet and an array of probe pairs. The system usually has three scanning 

stations to increase the test rate. In one similar system, round billets are 

rotated by a set of rollers while the billet surface is scanned by a transducer 

array moving straight along the billet axis. Seamless pipes and tubes are 

made from the round billets. In another tube test system, the transducers 

rotate around the pipe as the pipe is conveyed longitudinally. Overlapping 

elliptical printed circuit coils are used instead of magnetodiodes and are 

coupled to electronic circuits by slip rings. The system can separate seams 

into categories according to crack depth. 


9.4.4.6 Billets 

A relatively common problem with square billets is elongated surface 

breaking cracks. By magnetizing the billet circumferentially, magnetic flux 

leakage can be induced in the resulting residual magnetic field. Magnetic flux 

leakage systems for testing tubes exhibit the same general ability to classify 

seam depth. It is generally accepted that even with the lack of correlation 

between some of the instrument readings and the actual discontinuity depths, 

the automatic readout of these two systems still represents an improvement 

over visual or magnetic particle testing. One technique, often called 

magnetography, for the detection of discontinuities uses a belt of flux 

sensitive material, magnetic tape, to record indications. Discontinuity fields 

magnetize the tape, which is then scanned with an array of microprobes or 

hall effect detectors. Finally, the tape passes through an erase head before 

contacting the billet again. Because the field intensity at the corners is less 

than at the center of the flat billet face, a compensation circuit is required for 

equal sensitivity across the entire surface. 


9.4.5 Damage Assessment 

In most forms of magnetic flux leakage testing, discontinuity dimensions 

cannot be accurately measured by using the signals they produce. The final 

signal results from more than one dimension and perhaps from changes in 

the magnetic properties of the metal surrounding the discontinuity. Signal 

shapes differ widely, depending on location, dimensions and magnetization 

level. It is therefore impossible to accurately assess the damage in the test 

object with existing equipment. Under special circumstances (for example, 

when surface breaking cracks can be assumed to share the same width and 

run normal to the material surface), it may be possible to correlate magnetic 

flux leakage signals and discontinuity depths. This correlation is normally 

impossible. Commercially available equipment does not reconstruct all the 

desired discontinuity parameters from magnetic flux leakage signals. For 

example, the signal shape caused by a surface breaking forging lap is 

different from that caused by a perpendicular crack but no automated 

equipment uses this difference to distinguish between these discontinuities. 


As with many forms of nondestructive testing, the detection of a discontinuity 

and subsequent follow up by either nondestructive or destructive methods 

pose no serious problems for the inspector. 

Ultrasonic techniques, especially a combination of shear wave and 

compression wave techniques, work well for discontinuity assessment after 

magnetic flux leakage has detected them. In some cases, however, the 

discontinuity is forever hidden. Such is very often the case for corrosion in 

downhole and subterranean pipes. 


9.5 PART 5. Residual Magnetic Flux Leakage: A Possible 

Tool for Studying Pipeline Defects 

Vijay Babbar and Lynann Clapham 

9.5.0 Preface 

Simulated defects of different shapes and sizes were created in a section of 

API X70 steel line pipe and were investigated using a residual magnetic flux 

leakage (MFL) technique. The MFL patterns reflected the actual shape and 

size of the defects, although there was a slight shift in their position. The 

defect features were apparent even at high stresses of 220 MPa when the 

samples were magnetized at those particular stresses. However, unlike the 

active flux technique, the residual MFL needs a sensitive flux detector to 

detect the comparatively weaker flux signals. 


Journal of Nondestructive Evaluation, Vol. 22, No. 4, December 2003 (© 2004)


The magnetic flux leakage (MFL) technique is frequently used for in-service 

monitoring of oil and gas steel pipelines, which may develop defects such as 

corrosion pits as they age in service. Under the effect of typical operating 

pressures, these defects act as “stress raisers” where the stress 

concentrations may exceed the yield strength of the pipe wall. The main 

objective of MFL inspection is thus to determine the exact location, size, and 

shape of the defects and to use this information to determine the optimum 

operating pressure and estimate the life of a pipeline. Most MFL tools rely on 

active magnetization in which the pipe wall is magnetized to near saturation 

by using a strong permanent magnet, and the flux leaking out around a defect 

is measured at the surface of the pipeline. 

Keywords: 

■ Near saturation 

■ Active magnetization 

■ Flux leaking out 


The magnitude of the leakage flux density depends on the strength of the 

magnet, the width and depth of the defect, the magnetic properties of the 

pipeline material. and running conditions such as velocity and stress. A 

typical peak-to-peak value of leakage flux density from a surface defect may 

be around 30 G. Another way of employing the MFL technique for studying 

the pipeline defects is through residual magnetization. After a magnet is 

passed over a portion of the steel pipe, some residual magnetization remains. 

A study of the residual magnetization MFL signal can provide useful 

information about the size and shape of the defect. However, little published 

work exists about residual MFL, probably because of the comparatively weak 

leakage flux signals, which require sensitive detectors. 

Keywords: 

■ 30 G (Gauss) 


An earlier study of samples magnetized by strong electric currents revealed 

that the residual flux patterns are basically similar to the active flux patterns, 

with exceptions that they are very weak and may have opposite magnetic 

polarity in comparison to the latter. The opposite polarity occurs only when 

the excitation current is low, whereas for high excitation current level, there is 

no reversal of polarity. A finite element modeling technique has been 

proposed by Satish to predict the reversal of the residual leakage field. 

Keywords: 

■ Residual flux patterns are basically similar to the active flux patterns. 

■ Very weak and may have opposite magnetic polarity 

■ For high excitation current level, there is no reversal of polarity 


The present work investigates the residual flux patterns of defects after the 

passing of a permanent magnet (similar to the situation in pipeline inspection). 

The residual flux patterns of three different blind defects, that is, circular, 

elongated pit (henceforth named racetrack), and irregular gouge, are 

investigated. The effect of pipe wall stresses on the active and residual 

leakage flux signals from some of the defects is also reported. 

Note: “Blind” indicates a hole that is not completely through-wall. 


9.5.2 EXPERIMENTAL 

Three simulated defects were used in the present study: a circular blind hole, 

a blind racetrack-shaped defect, and a gouge. The first two defects were 

produced on the surface of a hydraulic pressure vessel (HPV) constructed for 

a previous study and were nearly 50% of the wall thickness. These are 

illustrated in Figure 1. The circular defect has a 15-mm diameter and 5-mm 

depth; the racetrack has about a 53-mm length, 15-mm width and 4.4-mm 

depth. An electrochemical-milling process, which prevents the introduction of 

additional stresses around the defects, was used for creating the first two 

defects in the HPV. The gouge of about 125-mm length, 26-mm width, and a 

graded maximum depression of about 14 mm was created on another section 

of similar steel pipe by using a single backhoe tooth. It is shown in Figure 2. 


Fig. 1. Geometric details of blind hole (a) and blind racetrack (b) defects. 


Fig. 2. Camera picture of a 

gouge on a steel line pipe 

section. The main groove is 

nearly rectangular, having 

dimensions of 53 mm 15 mm 

and depth varying from zero to 

4.4 mm maximum. An 

extended depression as 

indicated by a closed contour is 

present around the gouge. 


The HPV used in the present study is shown in Figure 3 and is briefly 

described here; the details can be found elsewhere. It consists of an outer 

section of API X70 steel pipeline of 635-mm length, 610-mm diameter, and 9- 

m wall thickness separated from an inner steel spool by a hydraulic chamber 

that contains hydraulic oil. On pressurizing the chamber, circumferential 

(hoop) stresses can be created in the outer wall of the pipeline and hence the 

in-service pressure stresses can be simulated. Axial stresses are minimized 

because they are carried by free end caps sealed with O-rings to prevent 

leakage. 


Fig. 3. Outline of pipeline sample (high-pressure vessel), magnet, the Hall 

probe, and scanning system assembly. 


The pipe wall was magnetized by using an assembly of strong permanent 

magnets. High-strength NdFeB permanent magnet blocks, approximately 

55 x 55 x 6 mm 3 , were connected in parallel and held in place by aluminum 

cover plates at each pole piece. Steel brushes, having the same curvature as 

the pipe, were used to couple the flux into the pipe wall. A back-iron mounting 

plate was connected to the pole pieces, thus completing the magnetic circuit 

from the NdFeB magnets through to the pipe wall and back again. To 

magnetize the defect, the magnet was pulled along the axis and across the 

surface of the HPV over the defect from left to right with south pole ahead. 

This is consistent with typical inspection procedures, although in this case the 

detector is on the outer wall of the pipe while inspection is internal. The 

magnet was pushed from the pipe end to a cylindrical aluminum platform, 

where it was lifted off, turned in a direction perpendicular to the axis, and 

returned to the left of the pressure vessel. This procedure was repeated three 

times for each magnetization process. 


After the three magnetization cycles the magnet remained on the HPV 

producing a flux density of 1.4 T (tesla). The gouge was similarly magnetized. 

All the measurements were repeated three times with time intervals of several 

days to verify the reproducibility of results, keeping the direction of 

magnetization always the same. The scanning system used in the present 

investigation can be seen in Figure 3. More details are available in a previous 

paper. It consisted of an SS94A1 Micro-Switch Hall probe that was controlled 

by a computer software and moved smoothly over the surface of defects in a 

two- imensional grid with increments of 1 x 1 mm 2 . It was connected to a 

Roland DXY-1100 XY digital plotter, which was controlled by a Tecmar A/D 

board operated by a compiled Microsoft Visual BASIC 4.0 program called 

Aquis. Finally, a three-dimensional plotting package called Surfer 7.0 from 

Golden Software was used for obtaining surface and contour maps. 


9.5.3 RESULTS AND DISCUSSION 

9.5.3.1 Active and Residual MFL Results in an Unstressed Pipe Wall 

(A) Active MFL 

The contour map of the active radial MFL scan from the circular blind-hole 

defect is shown in Figure 4. The magnetic field lies along the axial direction, 

whereas the stress is circumferential. A corresponding axial line scan through 

the center of the blind hole is shown in Figure 5, where the solid line is only a 

guide to the eye. The scan is approximately symmetric along the axis of the 

pipe; a region of high positive flux is present on one side of the defect and a 

high negative flux on the other. The peak-to-peak value of the radial leakage 

flux (MFL pp ) is about 27.0 Gauss. The shape of the flux pattern is well 

understood and has been reported by many workers. Although the size and 

shape of the circular defect are not obvious from this contour map, some 

useful information can be obtained. For example, this type of circular defect is 

typically located between high positive and high negative flux regions, with its 

center almost on the zero flux line. Also, the MFL pp is used to determine the 

defect depth. However, for irregular defect shapes, such contour maps may 

not reveal very useful information about the defect geometry. 


Fig. 4. Contour map of radial active magnetic leakage flux density (B) from circular blind-hole 

defect. Solid circle represents the actual location of the defect. The applied magnetic field and 

stress are along the axial and circumferential directions, respectively. 


Fig. 5. Radial active MFL axial line scan through the center of the circular 

defect showing the variation of the radial active magnetic leakage flux density 

(B) along the axial direction. 

MFL pp 


Keywords: 

■ Contour map scan 

■ MFL axial line scan 


(B1) Vertical Lift-Off- Residual MFL 

The residual radial MFL scan and the corresponding axial line scan through 

the center of the defect are shown in Figures 6 and 7, respectively. These 

were obtained after lifting the magnet perpendicularly upward from the defect. 

The residual flux pattern shows magnetic polarity exactly opposite to that of 

active flux pattern of Figure 4. This is consistent with reports by Heath for 

comparatively low excitation levels. The residual peak-to-peak flux density in 

the present case is about 4.3 Gauss. It may also be noted from Figures 6 and 

7 that, as for active flux patterns, the regions of positive and negative flux in 

the residual pattern appear to exhibit axial symmetry around the center of the 

defect. A small change in orientation of the flux pattern with respect to the 

axial direction is believed to be due to the rotation of the magnet after lifting it 

off the pipe. To summarize, as for active MFL patterns, the residual patterns 

with perpendicular liftoff can reveal information about the size and shape of 

the defect only on the basis of positions of high positive and negative flux 

regions. However, as with active MFL patterns, the shape of the defect is not 

directly obvious from the signal. 


Fig. 6. Contour map of radial residual magnetic leakage flux density (B) after 

perpendicular lift-off of the magnet from the circular defect. Solid circle 

represents the actual location of the defect. 


Fig. 7. Radial residual MFL axial line scan through the center of the circular 

defect after perpendicular lift-off of the magnet. B represents the radial 

residual magnetic leakage flux density. 


Compare the magnetic flux density of active & residual MFLT 


(B2) Sliding the magnet along the axial direction- Residual MFL 

During actual service conditions the magnets always slide along the pipe axis; 

therefore subsequent residual scans were made after sliding the magnet 

along the axial direction on the outer surface of the pipe wall with south pole 

leading. The contour map and the line scan obtained with this end lift-off 

method are shown in Figures 8 and 9 and are markedly different from those 

shown for perpendicular lift-off. There is now a marked asymmetry between 

the regions of positive and negative flux; the center of positive and negative 

regions no longer coincide with the edges of the defect, and the region of 

positive flux is more spread out over the defect. 


Fig. 8. Contour map of radial residual magnetic leakage flux density (B) after 

end lift-off of the magnet. Solid and dotted circles represent the actual and 

apparent locations of the defect, respectively. 


Fig. 9. Radial residual MFL axial line scan through the center of the circular 

defect after end lift-off of the magnet. B represents the radial residual 

magnetic leakage flux density. 


Active and residual flux distributions 

The possible active and residual flux distributions for the above cases are 

depicted in Figure 10. In the active case when magnet is on the defect, the 

flux and hence the domains are parallel to the top horizontal surface of the 

pipe, while those near the sides are oriented almost vertically. The path of flux 

lines near the edges of the defect is shown in Figure 10(a). When the magnet 

is lifted perpendicularly, the domains on either side of the defect tend to 

remain in the vertical orientation. A localized symmetric flux distribution is 

thus established around the defect, with flux being directed downward on the 

left, upward on the right, and from right to left over the defect. The flux path is 

shown in Figure 10(b) and is similar to that reported by Heath. There appear 

to be induced south and north polarities near the edges of the defect along 

the axial direction. 


In the third case of end lift-off with north pole leaving the pipe at the end, the 

asymmetric flux distribution shown in Figure 10(c) appears to account for the 

asymmetric MFL pattern of Figure 9. This is due apparently to the slight 

displacement of the S-N dipole developed on the axial diameter of the defect 

toward the left, owing to the repulsion from the north pole of the magnet 

before end lift-off. However, there is a need to verify these results by other 

methods. Unfortunately, finite element model simulations cannot be used for 

this purpose unless the domain level phenomena are incorporated into the 

model. 


Fig. 10. Probable flux distributions around the circular defect: (a) active, (b) 

residual with perpendicular lift-off, and (c) residual with end lift-off. 

Active 

magnetization 


Fig. 10. Probable flux distributions around the circular defect: (a) active, (b) 

residual with perpendicular lift-off, and (c) residual with end lift-off. 

Vertical lift-off 

S 

N 

End lift-off 


One of the interesting features of the asymmetric contour map of residual 

MFL scan with end lift-off of the magnet is that the defect shape is reflected in 

the radial MFL signal. It is also easy to estimate the size and position of the 

defect. A close look at Figure 8 indicates an almost circular defect centered 

on a point of high positive flux marked by the dotted circle. The true location is 

marked by the solid circle and is slightly toward the negative flux region. The 

magnitude of the shift in the position of the defect apparently depends on the 

strength of the magnet and the magnetic properties of the pipeline and can be 

determined experimentally. It is about 3 mm for the present system. It is also 

possible to estimate the size of the defect from the axial line scan shown in 

Figure 9. The diameter of the apparent defect is approximately the length of 

the horizontal projection of the positive peak. 


Racetrack defect 

The active and residual radial MFL contour maps of the racetrack defect are 

shown in Figure 11. The solid racetrack boundary in Figure 11(a) indicates 

the true location of the defect, and the broken boundary in Figure 11(b) 

indicates its apparent location according to the residual signal. In the active 

scan, the ends of the defect are located slightly outside the positive and 

negative peak positions of the flux density but the shape and size of the 

defect cannot be seen clearly. Conversely, the residual scan gives a clear 

view of the size and shape of the defect, except with an axial shift of about 3 

mm as observed in case of circular defect. The nature of flux pattern of this 

residual scan, however, differs from that of circular defect. In the residual 

racetrack pattern, the region of high negative flux is not concentrated at the 

end of the defect, but on the axial side of it, while the region of high positive 

flux is present almost everywhere over the defect as observed for circular 

defect. 


Fig. 11. Active (a) and residual (b) MFL radial contour maps of racetrack 

defect in the absence of stress. The actual and apparent locations of defect 

are indicated by the solid and broken racetrack boundaries, respectively. 


This 90-degree rotation of the magnetic flux pattern from the expected axial 

direction is probably due to the large length of the defect, which does not 

permit the flux to make long axial loops. Instead, short circumferential flux 

loops around the defect are energetically more favorable wherein most of the 

flux lines emerge out of the defect, make loops around one of the long axial 

sides, and reenter the pipe slightly outside the region of defect. The domains 

are apparently aligned horizontally along the circumferential direction beneath 

the defect, but vertically along the axial wall of the defect. This is in spite of 

the fact that, even in the absence of applied stress, there exists a 

macroscopic easy axis that is parallel to the axis of the steel pipe section. 


Irregular gouge 

The active and residual MFL scans of the third defect, an irregular gouge, are 

shown in Figure 12. Although the actual length, width, and maximum 

depression of the gouge are about 125 mm, 26 mm, and 14 mm, respectively, 

the overall depression is not limited to an area of just 125 mm 26 mm 

because of depression of the surrounding region during the gouge formation. 

The defect is spread over a non-uniform area of about 155 mm 65 mm as 

indicated in Figure 12 by the elongated closed contour. The active flux pattern 

of the gouge, as shown in Figure 12(a), does not exhibit longitudinal 

symmetry, which is expected owing to the non-uniformity in depression as 

well as width. The only resemblance this pattern has to the racetrack flux 

pattern of Figure 11(a) is that the upper half pattern shows a region of positive 

active flux and the lower half shows a region of relatively weak negative flux. 

The shape of the gouge is not apparent from this pattern. 


The extreme axial regions of high positive and negative flux are not due to the 

defect itself, but to the closer approach of the Hall probe detector to the 

magnetic brushes, where the induced magnetic poles produce spurious flux 

leakage signals. The residual flux pattern of Figure 12(b), on the other hand, 

shows a region of positive flux spread over the defect, which helps to 

estimate the size of the defect more conveniently. Thus, instead of active 

scans, the residual scans look more promising to reveal the size and shape of 

this type of irregular defect. 


Fig. 12. Active (a) and residual (b) radial contour maps of the gouge in the 

absence of stress. The approximate location of the defect is shown in both. 


9.5.3.2 Active and Residual MFL Results as a Function of Pipe 

Wall Stress 

In-service oil and gas pipelines are subjected to high stresses (up to 70% of 

the yield strength); thus the variations in the active MFL patterns brought 

about by the increased level of stress have been the subject of study. When 

the pipe is axially magnetized, the higher circumferential stresses are known 

to affect the active MFL signals and patterns from circular blind-hole defects 

in two ways: 

(1) they rotate the macroscopic magnetic easy axis of the pipe from the axial 

direction toward the circumferential direction, which causes the change in 

MFL pp , and 

(2) they modify the MFL pattern by producing localized flux variations as a 

result of stress concentrations around defects. 

To study such changes in the residual MFL patterns, measurements were 

made on circular and racetrack defects at different stress levels. 


The main interest was to determine if, as at zero stress, the residual patterns 

could reveal the shape and size of the defects at high stress levels. Figure 13 

depicts the residual MFL patterns of both circular and racetrack defects, 

which were magnetized at a stress level of 0 MPa but then studied at 220 

MPa. The corresponding 0 MPa patterns are shown in Figure 8 and 11(b). A 

comparison of these patterns indicates that a flux rotation of 180 degrees 

occurs at stress values of 220 MPa, with positive and negative flux regions 

interchanging their locations. In the case of a circular defect, the negative flux 

region has two localized regions of comparatively higher flux along the 

circumferential or stress direction where the stress concentration is higher. 

Two similar localized positive flux regions, though not clearly seen in Figure 

13(a), are developed on the positive side of the flux at higher stresses. The 

positions of such localized flux regions may be linked to the localized stress 

concentrations around the defect. The residual pattern of the racetrack defect 

in Figure 13(b) also shows two pockets of positive and negative flux regions 

near the four corners of the racetrack. 


The residual patterns of Figure 13 do not depict the shape of the defects as 

clearly as seen from patterns of Figures 8 and 11(b), which indicates that the 

application of stress reorients the magnetic domains along the stress direction, 

thus disturbing the original pattern. However, if the stress is applied before 

magnetization, as is done during in-service operation, the residual patterns 

can still be employed to get useful information about the shape and size of 

the defect. This is obvious from the residual patterns shown in Figure 14, 

where the defects were magnetized and also scanned at 220 MPa. 


Fig. 13. Residual MFL scans of circular (a) and racetrack (b) defects taken at 

a stress of 220 MPa after magnetizing at 0 MPa. The actual defect locations 

are shown. 


Fig. 14. Residual MFL scans of circular (a) and racetrack (b) defects taken at 

a stress of 220 MPa after magnetizing at the same stress. The actual defect 

locations are shown. 


9.5.4 CONCLUSIONS 

The residual MFL technique with end lift-off of the magnet appears to be very 

promising to provide useful information about defect geometry. Although the 

flux leakage signals weaken at high pressures, the technique still can be used 

to obtain reasonably good information provided the samples are magnetized 

at the same high pressure. However, the technique involves the use of 

sensitive probes to detect the flux leakage signals, which have about one 

tenth of the strength of the active flux leakage commonly used. 




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Chong/ Fion Zhang

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