Which of the following materials cannot be tested by the usual liquid penetrant tests?

The reason unglazed porous ceramic cannot be tested by typical liquid penetrant tests lies in the nature of its porosity. Liquid penetrant testing is designed to detect surface-breaking defects by using a liquid that can seep into these flaws. However, unglazed porous ceramic has a structure that is highly absorbent and can lead to the penetrant being absorbed into the material rather than remaining on the surface where it can be observed. This characteristic makes it difficult to achieve accurate readings since the penetrant can also be trapped within the pores, giving false indications or making it impossible to clean adequately for verification.

On the other hand, materials such as aluminum, steel, and glass generally exhibit more suitable surface characteristics for penetrant testing. These materials have less porosity and can provide clearer indications of defects when tested, allowing for effective evaluation of surface integrity. The properties of these materials facilitate the liquid staying on the surface, thereby making it possible for the dye to highlight any discontinuities present.