NDT Eddy Current Testing Theory 5 — Questions and Answers
Question 1: What is the primary purpose of a reference standard (calibration standard) in eddy current testing?
- To magnetize the test piece before inspection
- To establish known signal responses for instrument setup and defect sizing (Correct answer)
- To set the probe frequency to the correct value automatically
- To measure the exact conductivity of the test material
Correct answer: To establish known signal responses for instrument setup and defect sizing
Reference standards contain artificial or natural discontinuities of known size that allow the technician to calibrate instrument settings and correlate signal amplitudes/phases to defect sizes.
Question 2: In eddy current testing, 'noise' most commonly refers to:
- Audible sounds produced by the test instrument
- Unwanted signals from material variations, geometry, or probe wobble that mask defect indications (Correct answer)
- The electrical hum from the AC power supply
- Reflections from the far wall of the test piece
Correct answer: Unwanted signals from material variations, geometry, or probe wobble that mask defect indications
Noise in ECT is any unwanted signal — from surface roughness, microstructural variation, or probe instability — that reduces the signal-to-noise ratio and makes defect detection harder.
Question 3: An eddy current signal from a through-wall hole in a tube will appear on the impedance plane with:
- Very small amplitude because there is no material to interact with
- Maximum amplitude and a phase angle near 0°
- Large amplitude and a phase angle that places it well below the lift-off line (Correct answer)
- The same phase angle as a shallow surface pit
Correct answer: Large amplitude and a phase angle that places it well below the lift-off line
A through-wall hole represents 100% wall loss and produces the largest signal with a phase angle characteristic of very deep or complete penetration, distinct from surface indications.
Question 4: When testing for cracks parallel to the coil winding direction in an encircling coil, what happens to sensitivity?
- Sensitivity is maximized because the crack interrupts the maximum eddy current path
- Sensitivity is minimized because cracks parallel to eddy current flow cause little disruption (Correct answer)
- Sensitivity doubles compared to perpendicular cracks
- The signal phase inverts but amplitude stays the same
Correct answer: Sensitivity is minimized because cracks parallel to eddy current flow cause little disruption
Eddy currents flow circumferentially in a cylindrical test piece under an encircling coil, so longitudinal cracks running parallel to that flow cause minimal disruption and very weak signals.
Question 5: The 'operating point' on an impedance plane diagram is determined by:
- The defect size in the reference standard
- The coil impedance when placed on a defect-free section of the test material at the selected frequency (Correct answer)
- The intersection of the lift-off curve and the defect curve
- The maximum voltage output of the eddy current instrument
Correct answer: The coil impedance when placed on a defect-free section of the test material at the selected frequency
The operating point is where the coil's normalized impedance falls on the impedance plane when coupled to a sound (defect-free) test specimen at the chosen test frequency.
Question 6: Pulsed eddy current (PEC) testing differs from conventional eddy current testing primarily in that it:
- Uses multiple simultaneous sinusoidal frequencies instead of a single frequency
- Applies a step-change pulse excitation and analyzes the time-domain decay response (Correct answer)
- Requires direct contact with the conductive surface for signal coupling
- Operates only on ferromagnetic materials at saturation
Correct answer: Applies a step-change pulse excitation and analyzes the time-domain decay response
PEC excites the coil with a sharp current pulse and measures the time-domain decay of the induced eddy currents, providing simultaneous multi-depth information from a single measurement.
Question 7: When comparing eddy current indications from a 20% deep notch versus a 40% deep notch (percent of wall thickness), which statement is correct?
- Both notches produce identical signals if they have the same length
- The 40% notch typically produces a larger amplitude signal with greater phase lag (Correct answer)
- The 20% notch produces a larger signal because it is closer to the probe
- Phase angles are the same; only amplitude differs between the two depths
Correct answer: The 40% notch typically produces a larger amplitude signal with greater phase lag
Deeper notches cause stronger disruption to eddy current flow (greater amplitude) and accumulate more phase lag because the interaction occurs at greater depth.
NDT Eddy Current Testing Theory 5
What is the primary purpose of a reference standard (calibration standard) in eddy current testing?