What is the primary limitation of liquid penetrant testing?

The primary limitation of liquid penetrant testing lies in its inability to detect subsurface discontinuities. This method is designed to reveal surface-breaking defects, such as cracks, porosity, and other imperfections that exist on or near the surface of materials. Liquid penetrant testing operates by applying a liquid penetrant to the surface of the test object, allowing it to seep into any defects. After a dwell time, the excess penetrant is removed, and a developer is applied to draw out the penetrant from the defects, making them visible.

However, because this testing method relies on surface interaction, it cannot identify issues that lie beneath the surface, such as internal fractures or voids that do not reach the outer layer of the material. This limitation is significant in applications where the integrity of subsurface structures is critical to performance and safety.

The other options are not primary limitations. Liquid penetrant testing can be relatively affordable compared to more complex methods (though this may depend on specific penetrants used), may work on painted surfaces if compatible materials are chosen, and while it may require appropriate lighting for best results, this does not fundamentally hinder its effectiveness or application.