How does penetrant testing (PT) work?

Penetrant testing (PT) is one of the oldest methods of non-destructive testing (NDT). As early as the late 19th century, it was used in the railway sector, for example, to inspect wheels for cracks. At that time, railway wheels were immersed in large tanks containing heavy oil diluted with paraffin. After the wheels were removed from the tank, their surfaces were carefully cleaned and then coated with a suspension of chalk and alcohol.

Once the alcohol had evaporated, the wheels were made to vibrate by striking them with a hammer. This forced the heavy oil out of surface openings, such as cracks, and into the chalk layer. Defects then appeared as dark indications against a white background. This method was used until the 1940s and was later largely replaced by magnetic particle testing.

The basic principle of modern penetrant testing

Modern colour contrast penetrant testing (in daylight) and fluorescent penetrant testing (in a darkened environment for greater sensitivity) no longer use heavy oil and chalk, but follow the same basic principle. As in the past, three essential components are required: a cleaner (formerly paraffin, for example), a penetrant (formerly heavy oil) and a developer (formerly chalk).

Preparing the surface

Every penetrant test begins with careful preparation of the component surface. Reliable testing is only possible if the penetrant can enter any existing cracks without obstruction. This preparation serves several purposes:

• Removing deposits and dirt so that the penetrant can enter any cracks
• Reducing surface tension so that the penetrant wets the surface effectively
• Removing contamination to prevent false indications

False indications occur when penetrant collects on dirt or residues and produces a misleading indication. Car care provides a useful illustration of surface tension: after waxing, water should wet the surface as little as possible and form beads – in penetrant testing, the exact opposite is required.

Penetration of the testing medium

After cleaning, the penetrant is applied to the component surface. Capillary action draws it into existing surface openings. The narrower the opening, the stronger this effect and the greater the driving force for the penetrant to enter. However, the rate of penetration depends on the viscosity of the penetrant. The higher the viscosity, the slower the penetration and the longer the required penetration time. This is why using honey as a penetrant would not be a good idea, for example.

Removal of excess penetrant

In the next step, penetrant is removed from the component surface while leaving it in the cracks. This process is known as excess penetrant removal. The aim is to remove excess penetrant without washing the testing medium out of the defects. For this reason, brushes or high-pressure cleaners, for example, are unsuitable for this step.

The role of the developer

The developer is then applied to the surface in as thin a layer as possible. It performs several important functions.

• First, it provides contrast. A coloured indication is much easier to see against a light background than on a dark surface.
• Second, the developer helps draw the penetrant out of the crack. This is necessary because capillary action initially holds the penetrant in the crack. A greater force is needed to draw it out. This force is again generated by capillary action – this time within the developer layer. The developer consists of very fine particles with small gaps and pores between them. These are narrower than the crack openings and therefore generate a stronger capillary force. As a result, the penetrant is drawn out of the crack and into the developer layer.
• Third, the developer enlarges the visible indication. The penetrant spreads not only upwards but also sideways within the developer layer. As a result, the indication usually appears slightly larger than the actual crack opening.

The importance of indication development over time

Observing how an indication develops over time provides particularly useful information about a surface defect. The shape, intensity and growth of the indication can give further clues about the type and size of a defect.

Advantages and limitations of penetrant testing

Penetrant testing has several characteristic advantages and disadvantages compared with other non-destructive testing methods. Its advantages include its almost universal applicability to many materials and its adaptability to complex component geometries.

Its disadvantages include a relatively time-consuming testing procedure and the fact that only surface defects can be detected – and only those that are open to the surface.

Penetrant testing: conclusion

Penetrant testing (PT) is often underestimated and sometimes reduced to handling three spray cans: cleaner, penetrant and developer. In fact, it is a powerful inspection method. When applied correctly, penetrant testing – particularly with emulsifiable fluorescent penetrants – can detect surface defects with very high sensitivity and reliability.