Penetrant Testing

Penetrant testing is a non-destructive testing method used to detect discontinuities open to the testing surface which includes, but is not limited to, cracks, porosity, pipes, seams, forging laps and fusion defects. It is suitable for use on a wide variety of non-absorbent materials such as metals, ceramics and plastics however, porous or overly rough materials are generally unsuitable for penetrant testing.

Penetrant testing is achieved by applying a penetrant liquid to the test specimen’s surface. This liquid is drawn into any surface breaking discontinuity by Capillary Action. After a suitable amount of time, the excess penetrant is removed from the inspection surface and a finely divided developer powder is applied. The developer draws residual penetrant from surface discontinuities thereby providing a visual indication of the discontinuity’s location. 

There are six steps to the penetrant process:

1. Pre-clean
2. Apply penetrant
3. Remove excess surface penetrant
4. Apply developer
5. Inspect and evaluate
6. Post clean

Penetrant systems are available in colour contrast, UV fluorescent and dual mode (contrast and fluorescent) types. The type of test is determined by the testing environment, type and size of anticipated discontinuity and cost.

Materials suitable for penetrant testing include:

• Metals (aluminium, copper, titanium, brass, cast iron, stainless steel)
• Glass
• Plastics
• Glazed ceramics

Materials not suitable for penetrant testing include:

• Porous materials
• Rough materials
• Fibrous material, such as wood and unsealed composites (fibre glass)
• Absorbent plastics
• Liquid or gaseous oxygen systems
• Painted or coated material where discontinuities on the surface are closed

Advantages of Penetrant Testing

Penetrant testing is a highly sensitive, non-destructive method that has incredible capacity for detecting extremely fine flaws and the smallest surface discontinuities. Penetrant testing also has the ability to inspect small objects even if awkwardly shaped, allowing them to be batch tested for improved efficiency.

Discontinuities capable of detection by penetrant testing include:

• Rolled products: cracks, seams and surface laminations.
• Castings: cold shuts, hot tears, porosity, blowholes and shrinkage defects.
• Forgings: cracks, laps and surface bursts.
• Welds: cracks, surface porosity, undercut, overlap, lack of fusion if open to the surface, and lack of penetration if testing from root side.

Limitations of Penetrant Testing

The main limitation of the penetrant method is the requirement for the discontinuity to be open to the inspection surface. The penetrant must be able to enter the discontinuity and be drawn out again, by the developer, or the method is ineffective. All contaminants and coatings on the test surface must also be removed before testing.

Where fluorescent penetrant techniques are employed the area must be darkened to below 20 lux to allow adequate viewing conditions for inspection under ultraviolet light.

A further limitation is elevated temperatures, which can dry out the penetrant preventing it from being drawn out. Care must also be taken to ensure the penetrant system chemicals are compatible with the material under test. Penetrants containing chlorinated or halogenated molecules can cause serious corrosion in certain metals. Some polymers and rubbers can become permanently stained and “crazed” by contact with penetrant chemicals. 

Further to the above limitations, some penetrant system chemicals are flammable so extreme caution needs to be exercised when using around sources of ignition.