Eddy Current Testing (ET) Level I Practice Test

In ET, what indicates a flaw in the material?

A change in the impedance due to altered eddy currents. A drop in thermal emissivity. An increase in acoustic noise. A change in the color of the surface.

Correct Answer:

A change in the impedance due to altered eddy currents.

Explanation:

In eddy current testing, a flaw is indicated by a change in impedance caused by the disruption of eddy currents. When AC current flows in the probe, it induces circular currents in the material; if there’s a flaw like a crack or void, these currents are disturbed, which changes the distribution of the currents and the magnetic coupling to the probe. That disturbance shows up as a change in the impedance (and often the phase) read by the instrument, signaling a flaw. The other options don’t fit ET: a drop in thermal emissivity relates to infrared thermography, increased acoustic noise points to ultrasonic or mechanical issues, and a surface color change isn’t a reliable indicator of subsurface flaws in ET.

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