This article explains the principle behind magnetic particle test. when this test is done on a ferromagnetic material we can see that magnetic flux is easily distributed into the material without any interruption as because the material is entirely defect free .the magnetic flux is aligned into the material along north to south direction.
When there is a defect inside the test material, and when we apply magnetic field into it, we can see that flux being distributed every where uniformly except the defect area. flux leaks out of the material under test where there is a defect. then the next step is to apply iron powder onto the area being magnetized, then the iron particles will be collected by the magnetic flux, the collected iron powder helps us to determine the size shape and depth of the defect.
However flux leakage happens only when the applied magnetic field is perpendicular to the defect alignment ,if there is a crack in the test material and if magnetic field is applied in such a way that the applied field is parallel to the crack, then no leakage in flux can be seen and no indication is observed. so to resolve such an issue, the material under test should be subjected to magnetic field in two different ways. the second way of examination should be perpendicular to the first one so that the defects at any orientation is identified.