Liquid Penetrant Testing Level 1 (PT-1) Practice Exam

How is the penetrant applied to the surface of a test specimen drawn into discontinuities?

1. By suction

2. By pressure

3. By capillary action

4. By evaporation

The correct answer is: By capillary action

The correct answer is that the penetrant is drawn into discontinuities by capillary action. This phenomenon occurs because the penetrant, which is a liquid, can fill small voids, cracks, or pores in the surface of the test specimen due to the forces of adhesion and cohesion that take place at the liquid-solid interface. When penetrants are applied to a surface, they spread out and can be absorbed into tight spaces within the material. This absorption happens because the surface tension of the penetrant interacts favorably with the material, allowing it to be 'pulled' into the discontinuities. Capillary action is crucial for the effectiveness of liquid penetrant testing, as it ensures that the penetrant can infiltrate all accessible defects, making them visible during the inspection process after excess penetrant is removed and a developer is applied. Understanding capillary action is important for technicians to ensure they apply the penetrant correctly and observe the proper dwell times, which allow sufficient time for the penetrant to flow into any existing discontinuities.