NDT - Non-Destructive Testing Eddy Current Testing Theory Questions and Answers

Question 1

An inspector needs to detect subsurface defects in a thick aluminum plate. According to the principles of eddy current testing, which adjustment to the test frequency would be most effective?

Accepted Answer

Decrease the frequency to increase the depth of penetration.

Answer

In eddy current testing, there is an inverse relationship between the test frequency and the depth of penetration. Lower frequencies generate eddy currents that penetrate deeper into the material, which is necessary for detecting subsurface defects. Higher frequencies concentrate the eddy currents near the surface (the skin effect) and are better for surface flaw detection.

Question 2

The 'standard depth of penetration' in eddy current testing is defined as the depth at which the eddy current density decreases to approximately what percentage of its surface density?

Accepted Answer

37%

Answer

The standard depth of penetration (δ) is a key theoretical concept where the density of the induced eddy currents falls to 1/e (where 'e' is Euler's number, approx. 2.718) of its value at the surface. This is approximately 36.8%, commonly rounded to 37%.

Question 3

Which of the following material properties has the MOST significant influence on the magnitude and distribution of induced eddy currents?

Accepted Answer

Electrical Conductivity

Answer

Eddy current testing is fundamentally an electromagnetic method. The ease with which eddy currents can be induced and flow within a material is primarily determined by the material's ability to conduct electricity, i.e., its electrical conductivity. Variations in conductivity directly alter the impedance of the test coil.

Question 4

An inspector is using an eddy current surface probe on a flat plate and observes a signal change as the probe is moved slightly away from the surface. This phenomenon is known as:

Accepted Answer

Lift-Off

Answer

Lift-off refers to the effect of the distance, or gap, between the test probe and the surface of the conductive material. Even small changes in this distance can cause significant changes in the coil's impedance, which must be accounted for during inspection. 'Fill Factor' is the analogous term used for encircling coils.

Question 5

On an impedance plane diagram, the phase angle (or phase lag) of a defect signal provides crucial information about the defect's:

Accepted Answer

Depth

Answer

The phase of the eddy current signal lags as it penetrates deeper into the material. Because of this, the phase angle of a signal from a disruption (defect) correlates to its depth below the surface. Surface defects will have a different phase angle compared to subsurface defects.

NDT - Non-Destructive Testing Practice Test

NDT - Non-Destructive Testing Practice Test

NDT - Non-Destructive Testing Eddy Current Testing Theory Questions and Answers