Liquid pentetrants seep into various types of minute surface openings by capillary action. Because of this, the process is well suited to the detection of all types of surface cracks, laps, porosity, shrinkage areas, laminations, and similar discontinuities.

1. The process of liquid penetrant testing:

- Test component is coated with a liquid that contains a visible or fluorescent dye, liquid penetrants the surface flaws due to the capillary effect.

Picture 1: Apply liquid on surface of test component.

- Excess liquid is removed from the surface but is left in the surface breaking discontinuities.

Picture 2: After we remove the liquid on surface, we will apply the developer on test component.

- The developer is applied to draw the penetrant out of the discontinuities. With fluorescent dyes, the bleed - out can be seen under the UV light. With visible dyes, vivid colour contrast against white developer can be seen under the daylight or white light.

Picture 3: Discontinuities with visible dyes in white light.

Picture 4: Discontinuities with fluorescent dyes in UV light.

2. Advantages and Disadvantages of Liquid penetrant testing:

- Advantages:

• Applicable to non - ferromagnetics.
• Able to test large part with a portable kit.
• Batch testing.
• Applicable to small parts with complex geometry.
• Simple, cheap, easy to interpret.
• Sensitivity.

- Disadvantages:

• Will only detect defects open to the surface.
• Careful surface preparation required.
• Not applicable to porous materials.
• Temperature dependent.
• Cannot retest indefinitely.
• Compatibility of chemicals.