Which type of discontinuity cannot be detected by the liquid penetrant test method?

The liquid penetrant test method is specifically designed to detect surface discontinuities by applying a liquid penetrant to the surface of a material, allowing it to enter any surface-breaking defects, and then drawing it out to highlight these defects. This method is effective for identifying surface cracks, weld defects, and porosity.

Internal forging bursts, however, are subsurface discontinuities. They occur within the material and do not reach the surface. Since the liquid penetrant method focuses only on defect visibility at or near the surface, it does not have the capability to detect discontinuities that are not open to the external surface. Therefore, internal forging bursts cannot be identified by liquid penetrant testing, making it the correct choice in this context.