Magnetic Particle Testing: Method and Interpretation
What the candidate must understand
Magnetic particle testing (MT) detects surface and near-surface discontinuities in ferromagnetic materials. Candidates must know magnetization methods (direct and indirect, longitudinal and circular), the difference between continuous and residual magnetization, and the use of wet versus dry particles and fluorescent versus visible techniques. Understand field direction relative to defect orientation: a defect is best detected when it is perpendicular to the magnetic flux. Know the need for demagnetization, the role of surface preparation, and the interpretation of indications versus relevant indications. Familiarity with the equipment checks and the sequence of the test is essential.
Traps in questions
A common trap is assuming MT works on all metals; it applies only to ferromagnetic materials, so aluminum and austenitic stainless are excluded. Another is confusing the direction rule. Watch for questions about whether a discontinuity parallel to the flux will be detected; it generally will not. Also expect confusion between indications and defects: not every indication is a relevant defect.
Worked example / rule of thumb
Rule of thumb: flux perpendicular to the defect. Worked example: a stem describes a longitudinal crack in a shaft and asks which magnetization detects it best. A longitudinal crack runs along the axis, so circular magnetization around the axis produces flux crossing it, making that the correct method. If a candidate selects longitudinal magnetization, the flux runs parallel to the crack and detection is poor.
Study approach
Sketch field directions for each method and map defect orientations to the best method. Then review particle types and the continuous-versus-residual distinction.