Methods, Techniques and Equipment (Basic)
What you must know
This domain covers the physics and practical limits of the basic methods.
- PT – capillary action and surface tension; only surface-breaking discontinuities; penetrant type (dye vs fluorescent), removal method (water-wash, post-emulsification, solvent), developer form; cleanliness before and after.
- MT – magnetisation is directional. Maximum flux leakage occurs when the discontinuity is perpendicular to the flux. Compare circular (head shot, prod, central conductor) with longitudinal (yoke, coil), and residual with continuous technique; demagnetisation; field direction checked with a field indicator or pie gauge.
- ET – impedance-plane analysis; skin depth falls as frequency rises; lift-off, fill factor and phase angle; conductive materials only.
- UT – piezoelectric effect, near-field length, beam spread, Snell's law and mode conversion, 6 dB drop sizing, DAC/TCG, couplant, dead zone.
- RT – geometric unsharpness Ug = F t / d (F = source size, t = source-side-to-film distance, d = source-to-object distance); image quality indicators; density and contrast; radiation safety by time, distance, shielding.
Traps
- PT cannot find subsurface flaws or flaws sealed under a coating or smeared metal.
- MT misses discontinuities lying parallel to the field lines - a lamination parallel to the surface survives surface magnetisation.
- ET cannot inspect plastics or ceramics, and high frequency means shallow penetration.
- The UT dead zone under the probe limits near-surface resolution; a fine surface-breaking crack is often better served by PT or MT.
- RT favours a discontinuity whose plane is perpendicular to the beam; a tight crack parallel to the beam may be invisible.
Worked example
Focal spot F = 3 mm, d = 700 mm, t = 100 mm. Ug = 3 x 100 / 700 = 0.43 mm.
Rule of thumb: reduce Ug with a smaller source, larger source-to-object distance, or thinner object (film tight to the part).