The choice of test frequency when eddy current testing a nonferrous material is determined by what?

The selection of test frequency in eddy current testing (ET) of nonferrous materials is influenced by multiple factors, making the option "all of the above" the most comprehensive answer.

When considering the degree of phase discrimination required, a higher frequency can improve the resolution of defects by providing clearer phase information, which is critical for differentiating between various types of discontinuities. This is especially important in complex geometries or in parts with multiple features.

Moreover, the depth of eddy current penetration is crucial because it determines how deep the induced currents will go into the material. Lower frequencies allow for deeper penetration, which can be necessary for detecting subsurface flaws. Conversely, higher frequencies result in shallower penetration and are used for detecting surface-breaking defects.

Finally, the rate of response required during testing is also a factor. Higher frequencies enable faster response times, which can be essential in applications requiring rapid assessments or in automated systems where speed is crucial.

Considering all these interactions, it becomes clear that choosing the appropriate frequency incorporates all the listed factors, justifying why the correct answer is "all of the above."