Electromagnetic Testing Eddy Current in Brief

Electromagnetic Testing 

Eddy Current in Brief 

2014-December 

My ASNT Level III Pre-Exam Preparatory Self Study Notes 

外围学习中 

Charlie Chong/ Fion Zhang

Eddy current testing is used to find surface and near surface 

defects in conductive materials. It is used by the aviation industry for 

detection of defects such as cracks, corrosion damage, thickness verification, 

and for materials characterization such as metal sorting and heat treatment 

verification. Applications range from fuselage and structural inspection, 

engines, landing gear, and wheels. Eddy current inspection involves initial 

setup and calibration procedures with known reference standards of the same 

material as the part. Probes of appropriate design and frequency must be 

used. 

Eddy current inspection is based on the principle of electromagnetic induction. 

An electric coil in which an alternating current is flowing is placed adjacent to 

the part. Since the method is based on induction of electromagnetic fields, 

electrical contact is not required. 


Figure 1. Schematic of Eddy Current absolute probe 


An alternating current flowing through the coil produces a primary magnetic 

field that induces eddy currents in the part. Energy is needed to generate the 

eddy currents, and this energy shows up as resistance losses in the coil. 

Typical NDE application are designed to measure these resistance losses. 

Eddy currents flow within closed loops in the part. 

Figure 2. Diagram illustrating Eddy Currents created in a port 


As a result of eddy currents, a second magnetic field is generated in the 

material. The magnetic fields of the core interact with those in the part and 

changes in the material being inspected affect the interaction of the magnetic 

fields. The interaction, in turn, affects the electrical characteristics of the coil. 

Resistance and inductive reactance add up to the total impedance of the coil. 

Changes in the electrical impedance of the coil are measured by commercial 

eddy current instruments. So, what does all of this have to do with 

nondestructive testing? 

The main method used in eddy current inspection is one in which the 

response of the sensor depends on conductivity and permeability of the test 

material and the frequency selected. 


How eddy currents are created and sensed: 

An alternating current creates a magnetic field (Oersted's Law). 

The magnetic field causes a resulting eddy current in a part, which creates an 

induced magnetic field (Faraday's Law). 

The magnetic field from the coil is opposed to the induced magnetic field from 

the eddy current. 

A defect (surface or near surface) modifies the eddy current and therefore the 

magnetic field as well. 

This change in the magnetic field is detected by a sensor and is indicative of 

a flaw. 


How eddy currents are created and sensed: 

• An alternating current creates a magnetic field (Oersted's Law). 

• The magnetic field causes a resulting eddy current in a part, which creates 

an induced magnetic field (Faraday's Law). 

• The magnetic field from the coil is opposed to the induced magnetic field 

from the eddy current. 

• A defect (surface or near surface) modifies the eddy current and therefore 

the magnetic field as well. 

• This change in the magnetic field is detected by a sensor and is indicative 

of a flaw. 


How far do the eddy currents penetrate into a test piece? 

The strength of the response from a flaw is greatest at the surface of the 

material being tested, and decreases with depth into the material. The 

"Standard depth of penetration" is mathematically defined as the point when 

the eddy current is 1/e or 37% of its surface value. The "effective depth of 

penetration" is defined as three times the standard depth of penetration, 

where the eddy current has fallen to about 3% of its surface value. At this 

depth there is no effective impact on the eddy current and a valid inspection is 

not feasible. 

Penetration depth will: 

- Decrease with an increase in conductivity 

- Decrease with an increase in permeability 

- Decrease with an increase in frequency 


Conductivity is sensitive to cracks and material in-homogeneities; 

-Cracks 

- Defects 

-Voids 

- Scattering of electrons 

Magnetic permeability is much more sensitive to structural changes in 

magnetic materials; 

- Dislocations 

- Residual stress 

- Second phases 

- Precipitates 

Frequency selection will greatly affect eddy current response. Selection of 

the proper frequency is the essential test factor under the control of the test 

operator. The frequency selected affects not only the strength of the response 

from flaws and the effective depth of penetration, but also the phase 

relationship. 


The frequency selected affects not only: 

(1) the strength of the response from flaws and 

(2) the effective depth of penetration, but also 

(3) the phase relationship. 


How do we measure eddy current response? 

Eddy current response is viewed on an oscilloscope display, showing the 

impedance response (Z) from the test material, which is affected by factors 

depending on the specimen and testing conditions. 

Specimen conditions affecting response: 

• Electrical conductivity, 

• Magnetic permeability (unmagnetized ferromagnetic materials can become 

magnetized, resulting in large changes in impedance), 

• Specimen thickness - thickness should be limited to less then three times 

the standard depth of penetration. 

Testing conditions affecting response: 

• AC frequency, 

• Electromagnetic coupling between the coil and the specimen - a small liftoff 

has a pronounced effect, 

• Inspection coil size, 

• Number of turns within the coil itself, 

• Coil type. 


On an impedance plane diagram the signal of the resistance (R) component 

is displayed on the X axis and the inductive reactance (X L ) component is 

displayed on the Y axis. 

Figure 3. Electrical Conductivity changes for typical materials. 


Thickness changes in a sample can change the impedance response on an 

oscilloscope. Defects such as corrosion are found in this fashion. 

The thinner the part the greater is 

the impedance (>R & >X L ) 

Figure 4. Changes in conductivity curve due to thinning of a part 


Figure 5. Changes in conductivity curve due to corrosion damage 


There are two basic types of coil probes used in eddy current inspection; the 

absolute probe and the differential probe. An absolute probe consists of a 

single pickup coil which can be fashioned in a variety of shapes. Absolute 

probes are very good for sorting metals and detection of cracks in many 

situations. Absolute coils can detect both sharp changes in impedance and 

gradual changes. They are however, sensitive to material variations, 

temperature changes, etc. 


Figure 6. Typical response for samples of different conductivity 


A differential probe consists of two coils sensing different areas of the 

material being tested, which are linked electrically in opposition. The circuit 

will become unbalanced when one of the coils encounters a change in 

impedance. The response to this change in impedance creates what is known 

as a Lissajous figure. In general, the closer the element spacing the wider the 

"loop" in the signal. Differential probes are relatively unaffected by lift-off as 

long as the elements are balanced, and are suited for detection of small 

defects. 

Keywords: 

Differential probes are relatively unaffected by lift-off as long as the elements 

are balanced 


Figure 7. Diagram of response of a differential probe over a defect 


Lift Off 

Lift-off from paint, coatings, etc. can cause variations that may mask the 

defects of interest. Lift-off may also be useful in determining the thickness of 

nonconductive coatings on a conductive component. 

Figure 8. Response of a probe due to lift off. 

http://www.cnde.iastate.edu/faa-casr/engineers/Supporting%20Info/Supporting%20Info%20Pages/Eddy%20Pages/Eddy-principles.html 


Birring NDE Center, Eddy Current Testing # 1 Basic Concept 

https://www.youtube.com/watch?v=dxzsPzCnpVc 

The expert 

Mr. Birring 

Eddy Current testing and impedance plane display. Birring NDE Center is a NDT school in Houston 

that provides NDT training as per SNT-TC-1A. For training info see http://www.nde.com/training/ 


Charlie https://www.yumpu.com/en/browse/user/charliechong 

Chong/ Fion Zhang

Good Luck! 


Good Luck!

Electromagnetic Testing Eddy Current in Brief

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