Which of the following materials cannot be effectively tested using liquid penetrants?

Liquid penetrant testing is a nondestructive testing method used to detect surface-breaking defects in solid materials. This method relies on the ability of a liquid to penetrate into small cracks or voids on a surface. While effective for many materials, certain materials are not as suitable for this testing method.

Unglazed ceramics, unlike metals, plastics, and glass, typically have a porous and irregular surface structure. This porosity can cause issues with the penetrant material as it may be absorbed into the body of the ceramic rather than remaining on the surface for adequate detection of flaws. The effectiveness of liquid penetrant testing is diminished in materials that do not allow a clear visual indication of penetrant presence, which is crucial for identifying defects.

Conversely, metals, plastics, and glass generally have smoother surfaces that enable the penetrant to pool in any surface defects, making them suitable candidates for this type of testing.