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Electromagnetic Testing Eddy Current in Brief

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<strong>Electromagnetic</strong> <strong>Test<strong>in</strong>g</strong><br />

<strong>Eddy</strong> <strong>Current</strong> <strong>in</strong> <strong>Brief</strong><br />

2014-December<br />

My ASNT Level III Pre-Exam Preparatory Self Study Notes<br />

外 围 学 习 中<br />

Charlie Chong/ Fion Zhang


<strong>Eddy</strong> current test<strong>in</strong>g is used to f<strong>in</strong>d surface and near surface<br />

defects <strong>in</strong> conductive materials. It is used by the aviation <strong>in</strong>dustry for<br />

detection of defects such as cracks, corrosion damage, thickness verification,<br />

and for materials characterization such as metal sort<strong>in</strong>g and heat treatment<br />

verification. Applications range from fuselage and structural <strong>in</strong>spection,<br />

eng<strong>in</strong>es, land<strong>in</strong>g gear, and wheels. <strong>Eddy</strong> current <strong>in</strong>spection <strong>in</strong>volves <strong>in</strong>itial<br />

setup and calibration procedures with known reference standards of the same<br />

material as the part. Probes of appropriate design and frequency must be<br />

used.<br />

<strong>Eddy</strong> current <strong>in</strong>spection is based on the pr<strong>in</strong>ciple of electromagnetic <strong>in</strong>duction.<br />

An electric coil <strong>in</strong> which an alternat<strong>in</strong>g current is flow<strong>in</strong>g is placed adjacent to<br />

the part. S<strong>in</strong>ce the method is based on <strong>in</strong>duction of electromagnetic fields,<br />

electrical contact is not required.<br />

Charlie Chong/ Fion Zhang


Figure 1. Schematic of <strong>Eddy</strong> <strong>Current</strong> absolute probe<br />

Charlie Chong/ Fion Zhang


An alternat<strong>in</strong>g current flow<strong>in</strong>g through the coil produces a primary magnetic<br />

field that <strong>in</strong>duces eddy currents <strong>in</strong> the part. Energy is needed to generate the<br />

eddy currents, and this energy shows up as resistance losses <strong>in</strong> the coil.<br />

Typical NDE application are designed to measure these resistance losses.<br />

<strong>Eddy</strong> currents flow with<strong>in</strong> closed loops <strong>in</strong> the part.<br />

Figure 2. Diagram illustrat<strong>in</strong>g <strong>Eddy</strong> <strong>Current</strong>s created <strong>in</strong> a port<br />

Charlie Chong/ Fion Zhang


As a result of eddy currents, a second magnetic field is generated <strong>in</strong> the<br />

material. The magnetic fields of the core <strong>in</strong>teract with those <strong>in</strong> the part and<br />

changes <strong>in</strong> the material be<strong>in</strong>g <strong>in</strong>spected affect the <strong>in</strong>teraction of the magnetic<br />

fields. The <strong>in</strong>teraction, <strong>in</strong> turn, affects the electrical characteristics of the coil.<br />

Resistance and <strong>in</strong>ductive reactance add up to the total impedance of the coil.<br />

Changes <strong>in</strong> the electrical impedance of the coil are measured by commercial<br />

eddy current <strong>in</strong>struments. So, what does all of this have to do with<br />

nondestructive test<strong>in</strong>g?<br />

The ma<strong>in</strong> method used <strong>in</strong> eddy current <strong>in</strong>spection is one <strong>in</strong> which the<br />

response of the sensor depends on conductivity and permeability of the test<br />

material and the frequency selected.<br />

Charlie Chong/ Fion Zhang


How eddy currents are created and sensed:<br />

An alternat<strong>in</strong>g current creates a magnetic field (Oersted's Law).<br />

The magnetic field causes a result<strong>in</strong>g eddy current <strong>in</strong> a part, which creates an<br />

<strong>in</strong>duced magnetic field (Faraday's Law).<br />

The magnetic field from the coil is opposed to the <strong>in</strong>duced magnetic field from<br />

the eddy current.<br />

A defect (surface or near surface) modifies the eddy current and therefore the<br />

magnetic field as well.<br />

This change <strong>in</strong> the magnetic field is detected by a sensor and is <strong>in</strong>dicative of<br />

a flaw.<br />

Charlie Chong/ Fion Zhang


How eddy currents are created and sensed:<br />

• An alternat<strong>in</strong>g current creates a magnetic field (Oersted's Law).<br />

• The magnetic field causes a result<strong>in</strong>g eddy current <strong>in</strong> a part, which creates<br />

an <strong>in</strong>duced magnetic field (Faraday's Law).<br />

• The magnetic field from the coil is opposed to the <strong>in</strong>duced magnetic field<br />

from the eddy current.<br />

• A defect (surface or near surface) modifies the eddy current and therefore<br />

the magnetic field as well.<br />

• This change <strong>in</strong> the magnetic field is detected by a sensor and is <strong>in</strong>dicative<br />

of a flaw.<br />

Charlie Chong/ Fion Zhang


How far do the eddy currents penetrate <strong>in</strong>to a test piece?<br />

The strength of the response from a flaw is greatest at the surface of the<br />

material be<strong>in</strong>g tested, and decreases with depth <strong>in</strong>to the material. The<br />

"Standard depth of penetration" is mathematically def<strong>in</strong>ed as the po<strong>in</strong>t when<br />

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

penetration" is def<strong>in</strong>ed as three times the standard depth of penetration,<br />

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

depth there is no effective impact on the eddy current and a valid <strong>in</strong>spection is<br />

not feasible.<br />

Penetration depth will:<br />

- Decrease with an <strong>in</strong>crease <strong>in</strong> conductivity<br />

- Decrease with an <strong>in</strong>crease <strong>in</strong> permeability<br />

- Decrease with an <strong>in</strong>crease <strong>in</strong> frequency<br />

Charlie Chong/ Fion Zhang


Conductivity is sensitive to cracks and material <strong>in</strong>-homogeneities;<br />

-Cracks<br />

- Defects<br />

-Voids<br />

- Scatter<strong>in</strong>g of electrons<br />

Magnetic permeability is much more sensitive to structural changes <strong>in</strong><br />

magnetic materials;<br />

- Dislocations<br />

- Residual stress<br />

- Second phases<br />

- Precipitates<br />

Frequency selection will greatly affect eddy current response. Selection of<br />

the proper frequency is the essential test factor under the control of the test<br />

operator. The frequency selected affects not only the strength of the response<br />

from flaws and the effective depth of penetration, but also the phase<br />

relationship.<br />

Charlie Chong/ Fion Zhang


The frequency selected affects not only:<br />

(1) the strength of the response from flaws and<br />

(2) the effective depth of penetration, but also<br />

(3) the phase relationship.<br />

Charlie Chong/ Fion Zhang


How do we measure eddy current response?<br />

<strong>Eddy</strong> current response is viewed on an oscilloscope display, show<strong>in</strong>g the<br />

impedance response (Z) from the test material, which is affected by factors<br />

depend<strong>in</strong>g on the specimen and test<strong>in</strong>g conditions.<br />

Specimen conditions affect<strong>in</strong>g response:<br />

• Electrical conductivity,<br />

• Magnetic permeability (unmagnetized ferromagnetic materials can become<br />

magnetized, result<strong>in</strong>g <strong>in</strong> large changes <strong>in</strong> impedance),<br />

• Specimen thickness - thickness should be limited to less then three times<br />

the standard depth of penetration.<br />

<strong>Test<strong>in</strong>g</strong> conditions affect<strong>in</strong>g response:<br />

• AC frequency,<br />

• <strong>Electromagnetic</strong> coupl<strong>in</strong>g between the coil and the specimen - a small liftoff<br />

has a pronounced effect,<br />

• Inspection coil size,<br />

• Number of turns with<strong>in</strong> the coil itself,<br />

• Coil type.<br />

Charlie Chong/ Fion Zhang


On an impedance plane diagram the signal of the resistance (R) component<br />

is displayed on the X axis and the <strong>in</strong>ductive reactance (X L ) component is<br />

displayed on the Y axis.<br />

Figure 3. Electrical Conductivity changes for typical materials.<br />

Charlie Chong/ Fion Zhang


Thickness changes <strong>in</strong> a sample can change the impedance response on an<br />

oscilloscope. Defects such as corrosion are found <strong>in</strong> this fashion.<br />

The th<strong>in</strong>ner the part the greater is<br />

the impedance (>R & >X L )<br />

Figure 4. Changes <strong>in</strong> conductivity curve due to th<strong>in</strong>n<strong>in</strong>g of a part<br />

Charlie Chong/ Fion Zhang


Figure 5. Changes <strong>in</strong> conductivity curve due to corrosion damage<br />

Charlie Chong/ Fion Zhang


There are two basic types of coil probes used <strong>in</strong> eddy current <strong>in</strong>spection; the<br />

absolute probe and the differential probe. An absolute probe consists of a<br />

s<strong>in</strong>gle pickup coil which can be fashioned <strong>in</strong> a variety of shapes. Absolute<br />

probes are very good for sort<strong>in</strong>g metals and detection of cracks <strong>in</strong> many<br />

situations. Absolute coils can detect both sharp changes <strong>in</strong> impedance and<br />

gradual changes. They are however, sensitive to material variations,<br />

temperature changes, etc.<br />

Charlie Chong/ Fion Zhang


Figure 6. Typical response for samples of different conductivity<br />

Charlie Chong/ Fion Zhang


A differential probe consists of two coils sens<strong>in</strong>g different areas of the<br />

material be<strong>in</strong>g tested, which are l<strong>in</strong>ked electrically <strong>in</strong> opposition. The circuit<br />

will become unbalanced when one of the coils encounters a change <strong>in</strong><br />

impedance. The response to this change <strong>in</strong> impedance creates what is known<br />

as a Lissajous figure. In general, the closer the element spac<strong>in</strong>g the wider the<br />

"loop" <strong>in</strong> the signal. Differential probes are relatively unaffected by lift-off as<br />

long as the elements are balanced, and are suited for detection of small<br />

defects.<br />

Keywords:<br />

Differential probes are relatively unaffected by lift-off as long as the elements<br />

are balanced<br />

Charlie Chong/ Fion Zhang


Figure 7. Diagram of response of a differential probe over a defect<br />

Charlie Chong/ Fion Zhang


Lift Off<br />

Lift-off from pa<strong>in</strong>t, coat<strong>in</strong>gs, etc. can cause variations that may mask the<br />

defects of <strong>in</strong>terest. Lift-off may also be useful <strong>in</strong> determ<strong>in</strong><strong>in</strong>g the thickness of<br />

nonconductive coat<strong>in</strong>gs on a conductive component.<br />

Figure 8. Response of a probe due to lift off.<br />

http://www.cnde.iastate.edu/faa-casr/eng<strong>in</strong>eers/Support<strong>in</strong>g%20Info/Support<strong>in</strong>g%20Info%20Pages/<strong>Eddy</strong>%20Pages/<strong>Eddy</strong>-pr<strong>in</strong>ciples.html<br />

Charlie Chong/ Fion Zhang


Birr<strong>in</strong>g NDE Center, <strong>Eddy</strong> <strong>Current</strong> <strong>Test<strong>in</strong>g</strong> # 1 Basic Concept<br />

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

The expert<br />

Mr. Birr<strong>in</strong>g<br />

<strong>Eddy</strong> <strong>Current</strong> test<strong>in</strong>g and impedance plane display. Birr<strong>in</strong>g NDE Center is a NDT school <strong>in</strong> Houston<br />

that provides NDT tra<strong>in</strong><strong>in</strong>g as per SNT-TC-1A. For tra<strong>in</strong><strong>in</strong>g <strong>in</strong>fo see http://www.nde.com/tra<strong>in</strong><strong>in</strong>g/<br />

Charlie Chong/ Fion Zhang


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

Chong/ Fion Zhang


Good Luck!<br />

Charlie Chong/ Fion Zhang


Good Luck!<br />

Charlie Chong/ Fion Zhang

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