Fundamentals of Magnetism and Magnetic Particle Testing Theory
Why MT works
Magnetic particle testing works because a ferromagnetic part conducts magnetic flux far better than the air around it. Inside the metal, small magnetic domains align under an applied magnetizing force H to produce a flux density B, related by B = µH. Reluctance is the magnetic analogue of resistance and is inversely proportional to permeability. A discontinuity - a crack, void, inclusion or lap - is a region of low permeability, so flux is forced out into the air, forming a leakage field that attracts and holds particles.
Concepts a candidate must control
Domains, saturation, hysteresis, retentivity and coercivity; the Curie temperature at which ferromagnetism is lost (about 770 °C for iron); diamagnetic, paramagnetic and ferromagnetic classification; and the difference between surface and subsurface detection. Flux enters and leaves the part at its poles, so leakage is strongest where the flux lines are perpendicular to the discontinuity. A crack lying parallel to the flux produces little or nothing. That is why practice requires two magnetizations roughly 90° apart unless the geometry fixes a known flux direction.
Traps
Questions test whether you know H (magnetizing force, oersted or A/m) and B (flux density, gauss or tesla, 1 T = 10⁴ G) are not interchangeable; that permeability falls as a part approaches saturation; that AC is concentrated at the surface by the skin effect while DC and rectified current penetrate deeper; and that a soft material with high permeability and low retentivity is a poor choice for residual techniques.
Rule of thumb
Aim the field so flux crosses the discontinuity's long axis at right angles: axial seams get circular magnetization (head shot or central conductor); circumferential seams get longitudinal magnetization (coil or yoke). Tight, shallow surface cracks leak strongly; deep, rounded porosity leaks weakly.