What type of test should be used to detect certain kinds of subsurface flaws?

To detect subsurface flaws, a method that can penetrate the surface of the material and reveal flaws below the surface is required. Visual inspection and liquid penetrant tests are typically used for surface defects, while the Rockwell test measures hardness. Magnetic particle testing, however, is

Magnetic particle testing (MT) is a non-destructive testing method used to detect surface and near-surface flaws in ferromagnetic materials. It involves magnetizing the material and applying ferrous particles that gather around discontinuities, making subsurface flaws detectable.