What is the primary limitation of Liquid Penetrant Testing?

The primary limitation of Liquid Penetrant Testing (LPT) is that it cannot detect subsurface defects. This testing method is based on the principle of capillary action, where a liquid penetrant is applied to the surface of a test material. If there are any surface-breaking defects, the penetrant will seep into these openings and can later be revealed through a developer that draws the penetrant out of the defect, making it visible for inspection.

However, since LPT is designed to identify discontinuities that are open to the surface, it inherently cannot detect flaws that lie beneath the surface. This limitation is significant because many critical failures in materials can originate deeper within the material structure, such as fatigue cracks or inclusions that do not reach the surface. Therefore, while LPT is a valuable tool for detecting surface flaws, it does not provide any insight into the internal integrity of a material, making it less effective for identifying certain types of defects.

This is why other techniques, such as ultrasonic testing or radiographic testing, may be preferred in situations where subsurface integrity is of particular concern.