What is one of the primary uses of AC methods in testing?

The primary use of AC methods in testing is indeed associated with eddy current testing. Eddy current testing operates on the principles of electromagnetic induction, where an alternating current generates a magnetic field, which induces eddy currents in conductive materials. These induced currents create their own magnetic fields, which can be measured to identify material properties and detect flaws such as cracks, corrosion, or changes in material thickness.

In contrast to the other options, which pertain to different testing methods or fundamentally different principles, eddy current testing directly utilizes AC to achieve its objectives. This method is particularly effective for non-destructive testing of conductive materials and can provide detailed information about the integrity of the material being inspected, making it a robust choice for applications in industries such as aerospace and automotive.

The incorrect options either suggest applications of AC methods that do not align with established non-destructive testing techniques or focus on methods that primarily rely on DC or other forms of testing.