Which type of discontinuity can NOT be detected by liquid penetrant examination?

Liquid penetrant examination is a non-destructive testing method primarily used to detect surface defects in materials. It works by applying a penetrant dye to the surface of the material, which seeps into any surface-breaking defects. This method is effective for identifying various types of discontinuities visible at or near the surface.

Subsurface porosity, however, cannot be detected using this method because it occurs beneath the surface of the material, and the penetrant dye cannot reach these subsurface defects. The liquid penetrant techniques are designed just for surface-level detection and are not capable of revealing defects that lie beneath the surface.

Surface cracks are directly detectable since they break the surface layer, making them visible to the liquid penetrant. Weld defects, which can affect the surface of a weld bead or the surrounding material, are also detectable because they create openings for the penetrant to flow into. Corrosion pits, similar to surface cracks, will be present at the surface and thus can be identified by this testing method.

This differentiation is critical for ensuring the integrity of materials and structures, emphasizing the limitations of liquid penetrant examination when it comes to identifying subsurface issues.