While LPT is effective only for fine surface discontinuities, the need remains to detect bigger surface flaws or those present just below the surface. The need is meet by magnetic particle testing (MPT). This technique is applicable only for ferromagnetic materials. MPT is a considered more delicate than LPT. MPT requires a higher degree or operator expertise to ensure that the magnetic fields are aligned in the correct direction in order to find the defect. Flaws oriented perpendicular to the induced magnetic field are only reliably detectable. Hence the test is to induce magnetic field lines in a given work piece so that they are most likely to be perpendicular to the flaw orientation. Therefore, preceding knowledge on flaw orientation and or introduction of magnetic fields in several directions are is essential.
It is commonly agreed that defects breaking the surface are most severe amongst the various discontinuities occurring in component materials. For dynamically loaded structures, their removal is crucial. Their detection in ferritic materials is most easily accomplished by MPT. Since the depth calculation of the surface breaking cracks is almost absurd, crack depth measurements may be additionally employed to asses them. Examining the subsurface defects by MPT is impossible if the components are thin, but usually it requires ideal testing conditions.