Which of the following parts cannot be tested using the liquid penetrant method?

The liquid penetrant method is a non-destructive testing technique commonly used to detect surface-breaking flaws on non-porous materials. This method works by applying a liquid penetrant to the surface of the material, allowing it to seep into any cracks or voids. After a specified dwell time, the excess penetrant is removed, and a developer is applied to draw out the trapped penetrant from the flaws, making them visible under UV or white light.

In the context of the options presented, parts made from porous plastic materials cannot be effectively tested using the liquid penetrant method because the porosity of the material leads to penetrants being absorbed into the structure rather than remaining on the surface. This absorption can obscure flaws instead of revealing them, rendering the method ineffective for such materials.

Metallic components, alloys, and ceramics, on the other hand, are typically non-porous and compatible with the liquid penetrant method, making them suitable for detecting surface defects.