Which type of part cannot be tested using the liquid penetrant method?

Liquid penetrant testing (PT) is a nondestructive testing method primarily used to detect surface-breaking defects in non-porous materials. The technique relies on the ability of a liquid penetrant to seep into cracks or pores, which are typically found in solid materials.

A part made from porous material presents a significant limitation for this testing method because the penetrant can travel through the pores, leading to a misinterpretation of results. With porous materials, the penetrant cannot remain localized in a defect and instead is absorbed throughout the material, which can mask the presence of any actual surface-breaking defects. This characteristic effectively renders liquid penetrant testing unsuitable for porous materials, as it does not provide accurate or reliable indications of flaws.

In contrast, parts made of plastic, aluminum, or steel are generally suitable for testing using this method, provided they do not have porous characteristics and the surface condition is appropriate. Thus, the distinction of porous materials makes them incompatible with liquid penetrant testing, emphasizing the importance of material properties in the choice of testing method.