In liquid penetrant testing, what physical action reveals cracks within the material?

In liquid penetrant testing, the physical action that reveals cracks within the material is capillary action. This method involves applying a penetrant dye to the surface of a material and allowing it to seep into any surface-breaking cracks or flaws. The capillary action draws the penetrant into these openings by utilizing the surface tension and adhesion properties of the liquid.

Once the penetrant has been given sufficient time to penetrate, a developer is applied to the surface. The developer draws the penetrant out of the cracks due to its absorbent nature, causing the penetrant to form a visible indication against the background of the surface. This process is crucial because it not only highlights the presence of flaws but also allows the inspector to evaluate their size and shape.

Understanding capillary action is essential because it influences how effectively the penetrant can be drawn into very small cracks, which is a key factor in the success of this nondestructive testing method.