Which defect is least likely to be detected using surface crack testing methods?

Surface crack testing methods, such as dye penetrant testing or magnetic particle testing, are primarily designed to identify defects that are exposed on or just beneath the surface of a material.

Surface-breaking defects are directly visible and can easily be detected because these methods involve a fluid that penetrates cracks on the surface, making these defects readily identifiable. Similarly, linear near-surface defects, which may not reach the full depth of the material but still lie close to the exterior, can also be detected effectively, as the testing methods focus on the surface area.

Sub-surface inclusions, on the other hand, are located deeper within the material and may not extend to the surface at all. These inclusions may be microscopic voids or impurities trapped within the weld or base material, and since they do not communicate with the exterior of the material, standard surface crack testing techniques will not reveal their presence. As such, sub-surface inclusions are the least likely to be detected using these surface-focused methods.