What is the principle behind Magnetic Particle Testing (MPT)?

The principle behind Magnetic Particle Testing (MPT) involves inducing a magnetic field within a ferromagnetic material, allowing for the detection of surface and near-surface discontinuities. When the magnetic field is applied, any defects present, such as cracks or voids, can disrupt the magnetic field lines, creating a leakage field.

When ferromagnetic particles that are often coated in a dye or fluorescent material are applied to the surface, they will be attracted to these leakage fields and accumulate at the locations of the defects. This accumulation creates a visible indication of the flaws, allowing inspectors to identify and assess them. The sensitivity of MPT makes it particularly effective for detecting small surface defects that may not be visible to the naked eye.

The other methods mentioned—chemical reactions, ultrasonic waves, and heat—represent different non-destructive testing techniques, each with its distinct principle and application. However, they do not involve the magnetic field characteristics that are central to the functioning of Magnetic Particle Testing, which is why the correct answer is focused on inducing a magnetic field.