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

The choice of non-metallic internal inclusions as the type of discontinuity that is impossible to detect using a liquid penetrant test is accurate because liquid penetrant testing is primarily designed to identify surface-breaking defects. The liquid penetrant method involves applying a colored or fluorescent dye to the surface of a test specimen and allowing it to seep into any surface cracks or discontinuities. After a certain dwell time, excess penetrant is removed, and a developer is applied to draw out any penetrant trapped in the flaws, making them visible.

Non-metallic internal inclusions, such as oxides or sulfides embedded within the material rather than breaking the surface, do not interact with the penetrant. Since these inclusions do not extend to the exterior of the material, they remain undetected by liquid penetrant testing, which solely detects flaws accessible on the surface.

In contrast, defects like surface cracks, weld porosity, and corrosion pits are examples of flaws that do extend to the surface, making them detectable by liquid penetrant testing. This method is effective for identifying these types of discontinuities since they allow for the penetrant material to enter the flaws and be revealed through the subsequent processing steps.