Surface Methods – Liquid Penetrant Inspection (LPI) Exam 1 Practice

Magnetic Particle Inspection is the preferred method for ferromagnetic metals because it directly visualizes the leakage field around surface cracks. When you magnetize iron or steel, any small surface-breaking flaw creates a local disturbance in the magnetic field. Fine particles—either dry or suspended in a liquid—gather at that flux leakage, forming a bright, visible indication of the crack. This makes defects highly detectable, fast to apply over large areas, and effective even on painted or rough surfaces.

Liquid penetrant inspection relies on penetrant seeping into defects and then being developed to reveal them. Some tight or closed cracks in ferromagnetic materials may not allow penetrant to enter or drain well, reducing sensitivity, and surface preparation can be more demanding. For ferromagnetic iron and steel, MPI typically offers higher sensitivity to surface defects and quicker, more practical inspections, which is why it surpasses LPI in these materials. Radiography and ultrasonics target internal flaws and are not as efficient for surface-breaking cracks on ferromagnetic metals, and eddy current testing, while useful, has different limitations in coverage and defect types compared to MPI.