What is a potential issue when using alternating current in the detection of subsurface discontinuities?

Using alternating current for magnetic particle testing can indeed affect the effectiveness of the magnetic particles used in the detection of discontinuities, particularly subsurface ones. When an alternating current is applied, it induces a magnetic field that reverses itself constantly. This rapid reversal can cause the magnetic particles to lose their adherence to the test surface and fail to accumulate at areas of discontinuity effectively.

The main function of magnetic particles is to concentrate around defects to create visible indications under proper lighting conditions. If the magnetic field is alternating, the particles may not align as effectively with the discontinuities, leading to a reduction in their ability to show these defects. Consequently, this can make it challenging for inspectors to identify subsurface defects, which is critical in various applications, particularly in structural integrity assessments.

Alternating current does have other implications, such as causing heating or potentially generating false indications, but the direct effect on the effectiveness of the magnetic particles in detecting subsurface discontinuities is a key point to focus on in this context.