MAGNETIC PARTICLE INSPECTION

Magnetic Particle Inspections (MPI or MT) identify surface and subsurface discontinuities in ferromagnetic materials. A magnetic field is induced into the part and then iron (magnetic) particles are applied. Flaws become visible when the particles collect and form an indication on the magnetic "flux leakage" surrounding discontinuities.

Magnetic particle inspections are commonly performed on ferromagnetic materials, such as welds, castings and forgings, valves and their components, machined parts, pressure vessels, cranes and structural steel.

When performing magnetic particle inspections, the surface must be accessible and must be clean and free from deleterious material such as oil, grease, scale, etc. Thin well-adhered paint may be allowed where a procedure test can show that sensitivity is not compromised. Testing is carried out utilizing either alternating current (AC) or direct current (DC). AC is limited to identifying surface discontinuities while (DC) can be used to identify subsurface discontinuities (subject to the type of material and size of discontinuities). The DC technique is not as effective as AC for the detection of surface discontinuities. The magnetism is most commonly applied in the field by use of a hand held "yoke". Bench units are more common for high production requirements in manufacturing situations on items such as small shafts and studs.

The magnetic particles are applied either as a dry powder or in a wet medium such as water, solvent (Varsol) or light oil (Bayol). Dry powders come in various colours such as grey, red and yellow and the colour chosen should contrast with the colour of the material being tested to promote the highest visibility of indications.

The wet particles are either visible (black) or fluorescent particles. Prior to the application of the visible particles, a removable white contrast aid paint is applied to the component and allowed to dry. The visible "medium" is applied to the magnetized area and any discontinuities will show as black indications on a white background.

The fluorescent "medium" is applied to clean and magnetized component areas. This is then viewed under ultraviolet light, sometimes called a "black light." Any discontinuities result in bright green fluorescent indications.

Instrumentation and aircraft components require demagnetization after being inspected; residual magnetization may cause them to malfunction.

Test results are interpreted in accordance with the appropriate specifications, or customer requirements. A detailed report is issued clearly identifying the locations of any discontinuities.