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
The 'characteristic frequency' (fg) in eddy current tube testing is used to:
Answer: Normalize test frequency selection relative to material properties and tube dimensions
The characteristic frequency fg relates tube dimensions and material properties so that the ratio f/fg can be used to select optimal test frequencies across different tube types.
When two eddy current frequencies are used simultaneously (multifrequency technique), the primary reason for mixing channels is to:
Answer: Suppress a specific interfering signal while retaining defect sensitivity
Multifrequency mixing subtracts one channel's signal from another to cancel a known interference (such as support plate signals) while preserving sensitivity to defects.
A probe operating at 500 kHz detects a surface crack in titanium. If the frequency is reduced to 125 kHz, the standard depth of penetration will:
Answer: Double
Standard depth of penetration is proportional to 1/√f, so reducing frequency by a factor of 4 (500 to 125 kHz) doubles the penetration depth.
In eddy current array (ECA) testing, what advantage does it offer over single-element scanning?
Answer: It scans a wider area in fewer passes with real-time C-scan imaging
ECA probes contain multiple coil elements that are multiplexed electronically, covering a wider swath and producing C-scan images without the need for raster scanning.
What phenomenon causes eddy current signals from a defect to appear at DIFFERENT phase angles depending on whether the crack is on the near-surface or far-surface of the test piece?
Answer: The additional phase lag accumulated as eddy currents travel through material depth
Each additional skin depth of material the eddy current travels through introduces approximately 1 radian of phase lag, so far-surface defects produce signals at much larger phase angles.
During heat exchanger tube inspection with eddy currents, a signal at the tube sheet location is noted. The technician should:
Answer: Recognize it as a geometric/material discontinuity and use multifrequency mixing to suppress it
Tube sheets cause consistent geometric signals that are not defects; multifrequency mixing suppresses these known interferences so true defects can be identified.
Which statement best describes the relationship between electrical conductivity and eddy current signal response for non-ferromagnetic metals?
Answer: Higher conductivity increases eddy current intensity but reduces skin depth at the same frequency
In non-ferromagnetic materials, higher conductivity allows stronger eddy currents to flow but also concentrates them nearer the surface (reduced skin depth) at the same test frequency.