Eddy Current Testing Theory Flashcards
7 cards from real NDT practice questions. Tap to flip, then mark Knew It or Still Learning — missed cards come back until you master them.
Read the first 7 Eddy Current Testing Theory flashcards as text
What happens to eddy current penetration depth when test frequency is increased?
Answer: Penetration depth decreases
Higher frequency reduces the standard depth of penetration (skin depth) because eddy currents concentrate closer to the surface at higher frequencies.
Which parameter is NOT directly part of the standard depth of penetration formula for eddy currents?
Answer: Surface roughness
The standard depth of penetration depends on conductivity, permeability, and frequency — surface roughness is not a factor in the formula.
In eddy current testing, what does a 'lift-off' effect refer to?
Answer: The change in signal caused by varying probe-to-surface distance
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.
A ferromagnetic material is being tested with eddy currents. Compared to a non-magnetic material of the same conductivity, what adjustment is typically needed?
Answer: Apply a DC saturation field to reduce permeability effects
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.
What type of eddy current probe is best suited for detecting surface cracks oriented in any direction?
Answer: Rotating scanner probe
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.
On an impedance plane display, where does a large subsurface void typically appear relative to a surface crack signal?
Answer: Further along the resistive axis with greater phase lag
Subsurface defects show greater phase lag than surface defects because the eddy currents travel deeper before interacting with the discontinuity.
What is the primary purpose of using a differential eddy current probe rather than an absolute probe?
Answer: To cancel out gradual changes and highlight abrupt discontinuities
Differential probes use two coils wound in opposition, canceling gradual property changes (like temperature drift) while responding strongly to abrupt local discontinuities.