Which type of defect is least likely to be detected in magnetic particle testing?

Magnetic particle testing (MPT) is particularly effective for detecting surface and near-surface discontinuities due to the nature of how the testing works. In MPT, a magnetic field is induced in the test material, and ferromagnetic particles are applied. These particles are attracted to areas where there are discontinuities in the material that disrupt the magnetic field.

Subsurface voids are less likely to be detected by this method because MPT primarily reveals defects that are open to the surface. If a void is located beneath the surface, the magnetic field may not be disrupted in a way that causes the magnetic particles to cluster at the site of the void. Consequently, surface defects such as cracks, laminations, and weld flaws are much more visible and can be readily detected.

Surface cracks, laminations, and weld defects typically produce surface or near-surface indications that allow the magnetic particles to accumulate and form a visible indication. This makes them much easier to detect compared to subsurface voids, which may remain undetected during the testing process. Therefore, the limited ability of magnetic particle testing to identify subsurface flaws highlights why this type of defect is considered the least detectable using this method.