Which type of discontinuity is impossible to detect using a liquid penetrant test?

Liquid penetrant testing is a widely used method for detecting surface-breaking defects in non-porous materials. It involves applying a liquid penetrant to the surface of a part, allowing it to seep into any cracks or defects, and then removing the excess penetrant before applying a developer to make the penetrant in the defects visible.

The reason why non-metallic internal inclusions are difficult, if not impossible, to detect using liquid penetrant testing lies in the nature of the test itself. This method is exclusively effective on defects that are open to the surface. Internal inclusions occur within the material’s body, making them inaccessible to the penetrant. Since the penetrant requires a pathway to escape the defect for it to be indicated on the surface, any inclusions that are internal and not exposed do not allow for this visibility.

In contrast, surface cracks, weld porosity, and corrosion pits are all surface-breaking defects that can be adequately evaluated using liquid penetrant testing. These kinds of defects present an opening at the surface that allows the penetrant to penetrate and later be revealed. Thus, the testing method is specifically designed to find such surface defects rather than internal ones.