NDT Eddy Current Testing Theory 4

Question 1

The 'characteristic frequency' (fg) in eddy current tube testing is used to:

Accepted Answer

Normalize test frequency selection relative to material properties and tube dimensions

Answer

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:

Accepted Answer

Suppress a specific interfering signal while retaining defect sensitivity

Answer

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:

Accepted Answer

Double

Answer

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?

Accepted Answer

It scans a wider area in fewer passes with real-time C-scan imaging

Answer

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?

Accepted Answer

The additional phase lag accumulated as eddy currents travel through material depth

Answer

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.

NDT - Non-Destructive Testing Practice Test

NDT - Non-Destructive Testing Practice Test

NDT Eddy Current Testing Theory 4