Liquid penetrant testing is primarily used to detect what type of discontinuities?

Liquid penetrant testing is specifically designed to identify discontinuities that are open to the surface, such as cracks, seams, and other surface flaws. The technique relies on the ability of a penetrant fluid to seep into these surface-breaking discontinuities. Once the penetrant is applied, it fills the defects, and after excess penetrant is removed, a developer is applied. This causes the penetrant trapped in the cracks to bleed out and create visible indications that can be readily inspected.

In contrast, subsurface defects cannot be detected by liquid penetrant testing, as this method only evaluates surface conditions. While it can also identify weld defects, the primary characteristic of this testing method is its capability to reveal surface-discontinuities rather than limiting detection to specific types of defects. Invisible cracks would not be effectively detected unless they are actually open to the surface as implied in option B, as the penetrant needs access to the flaw to be able to fill it and show a visible indicator.