Effects of Discontinuities on Materials
SURFACE DISCONTINUITIES
The magnetic field surrounding subsurface discontinuities drops off very rapidly with distance below the surface. Magnetic particle movement is critical to the collection of contrastingly colored or fluorescent particles at a discontinuity site. Thus, it is desirable to have fluctuating magnetic fields on the surface of the test object.
Certain metallurgical discontinuities produce much higher collections of magnetic particles than other kinds of discontinuities of the same size.
The important condition of surface cracks includes metal grainto-grain tearing induced by surface tensile forces that produce the highest amount of magnetic field disruption for a given size of discontinuity. A leakage field at an air gap in a longitudinally magnetized test object is shown in Figure. This disruption is
produced by the alignment of magnetic domains in the stretched metal crack site. For this reason, cracks in the formation state are highly detectable and normally produce sharp, well defined indications. Open cracks that have been subject to large thermal, chemical or mechanical forces may have had their magnetic field
disruption characteristics greatly or entirely reduced and may not be detectable using magnetic particle testing.
For high detectability of forming surface cracks, magnetic particle testing uses induced alternating field magnetization techniques as the primary method to test ferromagnetic objects inservice. Using wet alternating current fluorescent techniques will generally produce the highest sensitivity obtainable with this
technology.
Leakage field at an air gap in a longitudinally magnetized test object.
SCRATCHES
Scratches, unlike cracks, exhibit metal grain plastic tearing that lowers the ability of this discontinuity to repeatably attract magnetic particles in a uniform manner for a given scratch size. Very deep scratches should be evaluated by their visual characteristics rather than their relative magnetic particle indication sizing.
SUBSURFACE DISCONTINUITIES
A typical subsurface discontinuity is shown in Figure. The size, orientation and composition of subsurface discontinuities affect evaluation capabilities other than general locating of the discontinuity. In general, the magnetic image is very broad and fuzzy. Additionally, changes in test object thickness and holes from
the opposite side may be misleading. The use of alternate testing technologies after demagnetization and retesting may be required to confirm the magnetic particle indication.
Magnetic particles attracted by a leakage field from a subsurface discontinuity.
