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