Tutorial - Eddy Current Testing

Eddy Current Testing

By : Ethan Gros

Eddy Current Principle. Digital image. Eddy Current Inspection Solution. Kontroll Technik, n.d. Web. 27 May 2015.

OutlineEddy Current Probe……………………………………………………………………....Slides 3-5

Eddy Current Penetration……………………………………………………………….Slide 6

Eddy Current Factors……………………………………………………………………..Slide 7

Conductivity………………………………………………………………………………….Slide 8

Permeability………………………………………………………………………………….Slide 8

Frequency…..…………………………………………………………………………………Slide 9

Eddy Current Edge Effect……………………………………………………………….Slide 10

Eddy Current Lift Off……………………………………………………………………..Slide 11

Eddy Current Array Testing…………………………………………………………….Slide 12

Eddy current testing is based on the physics of electromagnetic induction. In an eddy current probe, an alternating current flows through a coil and generates an

oscillating magnetic field - Ampere’s Law

If the probe and its magnetic field are brought close to a conductive material like a metal test piece, an electric magnetic field (emf) is induced resulting in induced currents called eddy currents (like swirling water in a stream) - Faraday’s Law

Eddy Current Probe

Eddy Current Detection. Digital image. Eddy Current Inspection Solution. Kontroll Technik, n.d. Web. 27 May 2015.

Eddy Current Probe Cont. That eddy current flowing through the metal will in turn generate its own magnetic

field, which will interact with the coil and its field through mutual inductance.

The eddy current instrument plots changes in the impedance amplitude and phase angle, which can be used to identify changes in the test piece.

Nelligan, Tom, and Cynthia Calderwood. Eddy Current Testing. Digital image. Olympus. Olympus Corporation, n.d. Web. 27 May 2015.

Eddy Current Probe Cont.

Changes in metal thickness or defects like near-surface cracking will interrupt or alter the amplitude and pattern of the eddy current and the resulting magnetic field. This in turn affects the (inductance and resistance) impedance of the coil.

Eddy current density is highest near the surface of the part, so that is the region of highest test resolution.

Eddy current density decreases exponentially with depth. Hence To detect surface breaking defects in a material, and also to measure the thickness of thin

sheets, very high frequencies are used. Similarly, to detect subsurface defects, and to test highly conductive, magnetic, or thick

materials, lower frequencies must be used.


Eddy Current Penetration The standard depth of penetration is defined as the depth at which the eddy current density is

37% of its surface value - can be calculated from the test frequency and the magnetic permeability and conductivity of the test material.

The formula to calculate the Standard depth of penetration (aka. Skin Depth):

Where: = Standard depth of penetration (mm) = 3.14… = Test Frequency (Hz) = Magnetic Permeability (H/mm) = Electrical Conductivity

Depth of Penetration. Digital image. Olympus. Olympus Corporation, n.d. Web.

26 May 2015.

Eddy Current FactorsVariations in the conductivity of the test material, its

magnetic permeability, the frequency of the AC driving the coil, and coil geometry will all have an effect on test sensitivity, resolution, and penetration as seen in the skin depth equation

𝛿≈ 1√𝜋∗𝒻∗𝜇∗𝜎

Conductivity/ Permeability Higher conductivity implies smaller skin depth or less penetration i.e.

more sensitive to surface defects and oxide layer thickness

Lower conductivity implies larger skin depth or more penetration but more diffuse fields, i.e. less sensitivity to surface defects and oxide layer thickness.

Variations in permeability of a material generate noise that can limit flaw resolution because of greater background variations.

𝛿≈ 1√𝜋∗𝒻∗𝜇∗𝜎

For fixed frequency and permeability -

For fixed frequency and conductivity - 𝛿≈ 1√𝜋∗𝒻∗𝜇∗𝜎

Frequency Higher test frequencies increase near surface resolution, but limit the

depth of penetration.

Lower test frequencies decreases near surface resolution, but increase penetration.

Sweeping through multiple frequencies helps to optimize results, or utilizing multiple probes to obtain the best resolution and penetration required to detect all possible flaws.

Wave Frequency Definition. Digital image. Wave Frequency. TutorVisa, n.d. Web. 10 June 2015.

For most eddy current techniques, the eddy current path is circular and parallel to the surface of the part. If the flow of eddy currents intercepts an edge, corner, or radius of the part, the circular pattern is disrupted and the eddy currents are confined to a smaller volume. This action changes the magnitude and distribution of the eddy currents and is known as edge effect.

As illustrated, the current density will be slightly greater at the edge of the part than at the interior. Basically the edge acts as a large defect producing a large signal, which can mask a small defect indication near the edge – making it difficult to detect defects near the edge

Eddy Current Edge Effect

Figure 4-7. Distortion of Eddy Current Flow at the Edge of a Part. Digital image. Figure 4-7. Distortion of Eddy Current Flow at the Edge of a Part. Integrated Publishing, Inc., n.d. Web. 10 June 2015.

Eddy Current Lift Off



The distance between the coil and the conductive material is called liftoff, and this distance affects the mutual inductance.

Liftoff can be used to make measurements of the thickness of nonconductive coatings, such as an oxide layer, that hold the probe a certain distance from the surface of the conductive material.

Increasing the lift off, decreases the eddy current probe signal because the magnetic flux interacts less with the conductive material.

Decreasing the lift off, increases the eddy current probe signal because the magnetic flux interacts more with the conductive material.

Eddy Current Array TestingEddy Current Array testing, or ECA, is a technology that

provides the ability to simultaneously use multiple eddy current coils that are placed side by side in the same probe assembly. This allows for larger areas to be covered in a single pass

while maintaining high resolution.

A New Method of Eddy Current Testing Insensitive to Defect Orientation. Digital image. E-Journal of Advanced Maintenance (EJAM) - Top Page. TOSHIBA Corporation, n.d. Web. 10 June 2015.

Depth of Penetration. Digital image. Olympus. Olympus Corporation, n.d. Web. 26 May 2015.

"Distortion of Eddy Current Flow at the Edge of a Part." Integrated. Integrated Publishing, Inc., n.d. Web. 10 June 2015.



Figure 4-7. Distortion of Eddy Current Flow at the Edge of a Part. Digital image. Figure 4-7. Distortion of Eddy Current Flow at the Edge of a Part. Integrated Publishing, Inc., n.d. Web. 10 June 2015.

"Mutual Inductance." Mutual Inductance. National Science Foundation, n.d. Web. 28 May 2015.

Nelligan, Tom, and Cynthia Calderwood. Eddy Current Testing. Digital image. Olympus. Olympus Corporation, n.d. Web. 27 May 2015.

Nelligan, Tom, and Cynthia Calderwood. "Knowledge." Introduction to Eddy Current Testing. Olympus Corporation, n.d. Web. 26 May 2015.

A New Method of Eddy Current Testing Insensitive to Defect Orientation. Digital image. E-Journal of Advanced Maintenance (EJAM) - Top Page. TOSHIBA Corporation, n.d. Web. 10 June 2015.

Wave Frequency Definition. Digital image. Wave Frequency. TutorVisa, n.d. Web. 10 June 2015.

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