NDT Eddy Current Testing Theory 4 — Questions and Answers
Question 1: The 'characteristic frequency' (fg) in eddy current tube testing is used to:
- Set the exact test frequency for all tube materials
- Normalize test frequency selection relative to material properties and tube dimensions (Correct answer)
- Define the upper limit of usable frequencies for a given instrument
- Calculate the maximum scan speed for a given probe
Correct 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.
Question 2: When two eddy current frequencies are used simultaneously (multifrequency technique), the primary reason for mixing channels is to:
- Double the penetration depth into the test material
- Suppress a specific interfering signal while retaining defect sensitivity (Correct answer)
- Increase the overall test speed by a factor of two
- Eliminate the need for reference standards
Correct 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.
Question 3: 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:
- Double (Correct answer)
- Quadruple
- Remain the same
- Halve
Correct 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.
Question 4: In eddy current array (ECA) testing, what advantage does it offer over single-element scanning?
- It eliminates the need for couplant
- It provides full-volume coverage without mechanical scanning
- It scans a wider area in fewer passes with real-time C-scan imaging (Correct answer)
- It works on ferromagnetic materials without saturation
Correct 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.
Question 5: 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?
- Hysteresis in the coil core material
- The additional phase lag accumulated as eddy currents travel through material depth (Correct answer)
- Frequency modulation caused by the crack geometry
- Reflection of electromagnetic waves at the far surface
Correct 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.
Question 6: During heat exchanger tube inspection with eddy currents, a signal at the tube sheet location is noted. The technician should:
- Flag it immediately as a defect requiring tube plugging
- Recognize it as a geometric/material discontinuity and use multifrequency mixing to suppress it (Correct answer)
- Increase frequency until the signal disappears
- Recalibrate using the tube sheet signal as the 100% reference
Correct 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.
Question 7: Which statement best describes the relationship between electrical conductivity and eddy current signal response for non-ferromagnetic metals?
- Higher conductivity always means weaker eddy current signals regardless of frequency
- Higher conductivity increases eddy current intensity but reduces skin depth at the same frequency (Correct answer)
- Conductivity has no effect on eddy current behavior in non-ferromagnetic metals
- Higher conductivity shifts the impedance operating point down the inductive axis only
Correct 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.
NDT Eddy Current Testing Theory 4
The 'characteristic frequency' (fg) in eddy current tube testing is used to: