Examination method
Magnetic particle testing (MT)
A magnetic field is induced in ferromagnetic material, and fine iron particles gather at the flux leakage caused by a crack, marking surface and slightly subsurface flaws.
How it works
A magnetic field is set up in the part with a yoke, prods, a coil, or direct current passage. In sound material the flux flows through the metal undisturbed. Where a discontinuity interrupts the flux path, the field is forced out of the surface and back in again, creating a small local leakage field with north and south poles at the flaw.
Fine ferromagnetic particles, applied dry or suspended in a liquid carrier, are attracted to that leakage field and pile up over the flaw. The accumulation is wider than the crack itself, so a tight discontinuity produces a visible indication. Fluorescent particles viewed under ultraviolet light give far greater contrast than plain visible particles.
Flaw orientation relative to the field decides everything. Maximum leakage occurs when the discontinuity lies across the flux lines, and a flaw parallel to the field barely disturbs it at all. That is why examinations are performed twice with the field rotated roughly ninety degrees between passes, so no orientation escapes.
Strengths
- Fast and inexpensive, with immediate results and portable yoke equipment
- Detects flaws slightly below the surface as well as surface-breaking ones, unlike penetrant testing
- Tolerates thin coatings and less than perfect surface cleanliness better than penetrant methods
- Very sensitive to the tight, planar cracks that matter most in welded joints
Limitations
- Works only on ferromagnetic materials, ruling out austenitic stainless, aluminium, and copper alloys
- Requires two examinations at different field directions to cover all flaw orientations
- Parts may need demagnetising afterwards, since residual magnetism interferes with machining and instruments
- Prod techniques can leave arc strikes on the part, creating the very kind of defect being looked for
Key variables
- Field direction
- Maximum sensitivity occurs when the flaw lies across the flux lines, so orientation drives the whole technique and demands two passes.
- Field strength
- Must be enough to leak at a tight flaw but not so high that the surface saturates and produces heavy irrelevant background.
- Magnetising method
- Yokes, prods, coils, and direct passage each set up different field geometry and suit different part shapes.
- Particle type and application
- Dry powder suits rough hot surfaces; wet suspensions and fluorescent particles give higher sensitivity on smooth ones.
- Demagnetisation
- Residual field after examination can attract swarf and upset instruments, so it is removed where the part's service requires it.
What an inspector watches for
- Confirm the field was applied in two directions, since a single-direction examination misses flaws lying along the flux lines
- Check field adequacy with a suitable indicator rather than assuming the yoke is producing enough flux
- Distinguish non-relevant indications at changes of section, permeability, or magnetic writing from genuine cracks
- Watch for prod burns on the part, which are arc strikes and must be dressed out and re-examined