A CWI is observing an Eddy Current Testing (ET) inspection being performed on a metallic tube for crack detection. The technician observes a change in both amplitude and phase of the eddy current signal. What does this indicate about the discontinuity?
- AAn increase in tube wall thickness
- BA surface-breaking crack or subsurface flaw
- CA change in material permeability only
- DA change in material conductivity only
Show answer & explanationAnswer & explanation
Correct answer: B. A surface-breaking crack or subsurface flaw
Eddy current signals are sensitive to changes in both amplitude and phase, which are characteristic responses to discontinuities such as cracks (surface-breaking or subsurface), pitting, and corrosion. Changes in conductivity or permeability alone might affect one aspect more significantly, but a change in both indicates a disruption in the material's integrity or geometry. An increase in wall thickness would primarily affect the phase and amplitude in a predictable manner, different from a flaw.
Why the other options are wrong
- A. An increase in wall thickness would affect the signal in a different, more uniform way, not typically described as a 'discontinuity' indication.
- C. A change in permeability would primarily affect phase, but a crack affects both amplitude and phase.
- D. A change in conductivity would primarily affect amplitude, but a crack affects both amplitude and phase.
ET Signal Response to Flaws
Discontinuities in a material, such as cracks, alter the flow of eddy currents, causing changes in both the amplitude and phase of the measured electrical signal.
- Amplitude change relates to the depth/size of the flaw.
- Phase change relates to the type, depth, and orientation of the flaw.
- Experienced operators interpret these changes to characterize discontinuities.
Memory trick: Amplitude is size, Phase is depth, both mean a flaw's adept.