NDT Eddy Current Testing Theory 2 — Questions and Answers
Question 1: What happens to eddy current penetration depth when test frequency is increased?
- Penetration depth increases
- Penetration depth decreases (Correct answer)
- Penetration depth remains unchanged
- Penetration depth first increases then decreases
Correct 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.
Question 2: Which parameter is NOT directly part of the standard depth of penetration formula for eddy currents?
- Electrical conductivity
- Magnetic permeability
- Test frequency
- Surface roughness (Correct answer)
Correct answer: Surface roughness
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?
- The probe leaving the test surface accidentally
- The change in signal caused by varying probe-to-surface distance (Correct answer)
- Removing the coil from the magnetizing circuit
- The effect of high-frequency signals bouncing off defects
Correct 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.
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?
- Use a lower frequency to overcome increased skin effect
- Use a higher frequency to compensate for reduced conductivity
- Apply a DC saturation field to reduce permeability effects (Correct answer)
- Increase probe diameter to improve signal strength
Correct 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.
Question 5: What type of eddy current probe is best suited for detecting surface cracks oriented in any direction?
- Pencil probe
- Reflection probe
- Rotating scanner probe (Correct answer)
- Pancake coil probe
Correct 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.
Question 6: On an impedance plane display, where does a large subsurface void typically appear relative to a surface crack signal?
- At the same location, indistinguishable from surface cracks
- Closer to the inductive reactance axis with less resistive component
- Further along the resistive axis with greater phase lag (Correct answer)
- Always below the operating point regardless of frequency
Correct 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.
Question 7: What is the primary purpose of using a differential eddy current probe rather than an absolute probe?
- To measure absolute conductivity values
- To cancel out gradual changes and highlight abrupt discontinuities (Correct answer)
- To increase the penetration depth by 50%
- To reduce the frequency required for testing
Correct 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.
NDT Eddy Current Testing Theory 2
What happens to eddy current penetration depth when test frequency is increased?