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 is the primary purpose of a reference standard (calibration standard) in eddy current testing?
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.
In eddy current testing, 'noise' most commonly refers to:
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.
An eddy current signal from a through-wall hole in a tube will appear on the impedance plane with:
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.
When testing for cracks parallel to the coil winding direction in an encircling coil, what happens to sensitivity?
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.
The 'operating point' on an impedance plane diagram is determined by:
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.
Pulsed eddy current (PEC) testing differs from conventional eddy current testing primarily in that it:
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.
When comparing eddy current indications from a 20% deep notch versus a 40% deep notch (percent of wall thickness), which statement is 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.