Which of the following can a penetrant inspection NOT find?

Penetrant inspection is a widely used non-destructive testing method employed to identify surface-breaking defects in solid materials. This technique involves applying a liquid penetrant to the surface of the test material. The penetrant seeps into any surface cracks or defects, which can then be revealed using a developer.

The method is particularly effective for detecting surface cracks, surface pores, and various external features, as these imperfections are accessible to the penetrant. However, it is important to note that penetrant inspection is not capable of detecting internal cavities. Internal features are beyond the reach of the penetrant liquid, meaning that any defect situated within the structure, not exposed at the surface, will remain undetected. Therefore, while the inspection method excels at revealing surface-related issues, it falls short when it comes to assessing internal cavity defects, making this the correct answer.