When eddy current testing a rod for discontinuities, which of the following conditions could produce extraneous indications?

When conducting eddy current testing on a rod for discontinuities, multiple factors can lead to extraneous indications, making the selection of "all of the above" the most comprehensive response.

Improper adjustment of the hold-down rollers can cause the rod to be misaligned within the coil. This misalignment can alter the electromagnetic field distribution and, consequently, affect the eddy currents generated and their measurement. If the rod is not centered, it may pass through regions of the coil where the magnetic field strength varies, leading to false indications of discontinuities.

Feeding the rod through the coil at varying speeds can also introduce errors in the test results. Eddy current measurements rely on stable and consistent conditions to produce accurate readings. Varying speeds can cause fluctuations in the induced eddy currents, leading to inconsistent readings that can mimic genuine discontinuities in the material.

Changes in dimension, such as variations in diameter or surface irregularities, can naturally alter the eddy current response. These variations can lead to false indications as they affect the impedance of the coil differently than expected. For instance, if there’s an unintentional change in the rod’s dimensions, it can either enhance or reduce the amplitude of the eddy currents, leading to misinterpretations.