NDT Eddy Current Testing Theory 2

Question 1

What happens to eddy current penetration depth when test frequency is increased?

Accepted Answer

Penetration depth decreases

Answer

Higher frequency reduces the standard depth of penetration (skin depth) because eddy currents concentrate closer to the surface at higher frequencies.

Question 2

Which parameter is NOT directly part of the standard depth of penetration formula for eddy currents?

Accepted Answer

Surface roughness

Answer

The standard depth of penetration depends on conductivity, permeability, and frequency — surface roughness is not a factor in the formula.

Question 3

In eddy current testing, what does a 'lift-off' effect refer to?

Accepted Answer

The change in signal caused by varying probe-to-surface distance

Answer

Lift-off is the change in eddy current signal that occurs when the distance between the probe coil and the test surface varies, causing signal noise.

Question 4

A ferromagnetic material is being tested with eddy currents. Compared to a non-magnetic material of the same conductivity, what adjustment is typically needed?

Accepted Answer

Apply a DC saturation field to reduce permeability effects

Answer

Ferromagnetic materials have high permeability that dramatically reduces skin depth, so a DC bias field is applied to magnetically saturate the material and minimize permeability variations.

Question 5

What type of eddy current probe is best suited for detecting surface cracks oriented in any direction?

Accepted Answer

Rotating scanner probe

Answer

A rotating scanner probe rotates the coil or uses an array that covers all orientations, making it effective at detecting cracks regardless of their direction.

NDT - Non-Destructive Testing Practice Test

NDT - Non-Destructive Testing Practice Test

NDT Eddy Current Testing Theory 2