What kind of defects can Liquid Penetrant Testing fail to detect?

Liquid Penetrant Testing (PT) is designed primarily to detect surface-breaking defects, such as cracks, laps, or porosity that are open to the surface. However, its effectiveness is limited to flaws that break the surface. Subsurface defects or volumetric flaws, which do not reach the surface, are not detectable using this method because the penetrant can only access the surfaces exposed to it.

This method relies on the ability of penetrants to seep into flaws that are open to the surface; thus, it cannot identify issues that lie beneath the surface of the material. Defects such as these might require other testing methods, such as ultrasonic or radiographic testing, to detect.

In contrast, surface-related defects—like finishing defects, which are typically superficial in nature—are within the detection capabilities of PT. Similarly, while color variations on the surface may indicate potential issues, they do not constitute detectable defects under the principles of Liquid Penetrant Testing, since they do not signify structural flaws. The essence of the method focuses on detecting discontinuities that penetrate through the material surface, making this particular choice the most accurate statement regarding the limitations of Liquid Penetrant Testing.