MT is typically applied using a “dry powder” method for crack detection in steel bridge components. Magnetic particles infused with a colored dye are broadcast onto the surface of the material under test. A blower may be used to remove excess powder from the surface; particles not confined by a magnetic field are removed, particles confined by magnetic forces remain forming the indication.
Surface preparation for applying MT includes removing coatings to ensure effective coupling of the magnetic field into the material under test. Removing the coating also reduces the likelihood of nonrelevant indications forming from debris on the surface or imperfections in the coating itself. Debris or imperfections in the coating can mechanically confine the particles resulting in a nonrelevant indication. Care must be taken to ensure crack openings are not obscured during coating removal using mechanical removal processes (e.g., grinding).
The orientation of the magnetic fields induced in the material must be orthogonal to a crack opening to ensure sufficient leakage field to capture magnetic particles. Direct magnetization using prods develops circular magnetic fields around each prod. Consequently, the orientation of a crack must lie parallel to an imaginary line connecting the prods. In contrast, indirect magnetization results in a magnetic field in the material that is parallel to an imaginary line connecting the yokes. Therefore, the crack must be oriented orthogonal to an imaginary line connecting the yokes. When the orientation of cracks is not known, procedures for MT typically require reorientation of the yokes or prods in orthogonal positions to ensure cracks are detected reliably.(1)