Which NDT method primarily involves the use of magnetic fields?

Magnetic particle testing is the NDT method that primarily involves the use of magnetic fields. This technique is used to detect surface and near-surface discontinuities in ferromagnetic materials. During the testing process, the component is magnetized, creating a magnetic field. When magnetic particles are applied to the surface, they will accumulate at areas of discontinuity, such as cracks or inclusions, due to the disturbance of the magnetic field. This accumulation makes the discontinuities visible under proper lighting conditions, allowing for effective evaluation of the material's integrity.

The other methods listed do not rely on magnetic fields: ultrasonic testing utilizes high-frequency sound waves to detect flaws within materials; dye penetrant testing employs penetrant dyes that reveal surface flaws through capillary action; and radiographic testing uses X-rays or gamma rays to inspect the internal structure of components. Each of these methods has its unique principles and applications, but magnetic particle testing stands out for its specific reliance on magnetic fields.