Non-Destructive Testing (NDT)

Liquid Penetrant
Testing (PT)

What is Liquid Penetrant Testing?

Penetrant or liquid penetrant testing (PT) refers to a surface type non-destructive testing method that allows a liquid (containing dye to increase visibility) to fill an empty void, which is open to the surface, due to the capillary action of the liquid.

Once the excess penetrant is removed from the surface a developer is applied to the surface which allows the penetrant to be absorbed into a contrasting medium (normally white powder) revealing the presence of a discontinuity. Penetrant testing can effectively be applied to any type of material, as long as the discontinuity sought, is open to the surface.

The PT method offers the following advantages

  • Can be used to test a variety of different materials whether Magnetic or Non Magnetic.
  • Detects defects that are open to the surface and most detrimental to the strength of the part, e.g. Fatigue cracking.
  • Tests are quick, easily applied and relatively inexpensive.
  • Penetrant testing is very sensitive to fine surface cracks.
  • Easy to apply to and test components of complex geometry.
  • Flaws can be easily seen and remain so until wiped off, thus facilitating accurate repair.


1. Precleaning

Contaminants such as scale, rust, oil, grease, paint and water shall be removed if necessary using mechanical or chemical methods, or a combination of these. Precleaning shall ensure that the test surface is free from residues and that it allows the penetrant to enter any discontinuity.

2. Application of penetrant

The penetrant can be applied to the part to be tested by spraying, brushing, flooding, dipping or immersion. The penetrant shall remain on the test surface throughout the entire penetration time.

3. Penetration time

The appropriate penetration time depends on the properties of the penetrant, the application temperature, the material of the part to be tested and the discontinuities to be detected. It is usually between 5 – 60 minutes.

4. Excess penetrant removal

The excess penetrant is removed from the test surface with water or approved cleaner. The excess penetrant removal shall be such that penetrant remains in the discontinuities. Control under UV light (≥ 100 μW/cm² und < 100 lx) respectively daylight or white light (≥ 350 lx).

5. Developing

The developer shall be maintained in a uniform condition during use and shall be evenly applied to the test surface. The development times should usually be between 10 – 30 minutes. It begins immediately after application, when dry developer is applied and immediately after drying of the developer layer, when a wet developer is applied.

6. Inspection

Indications are produced during the developing. Inspection shall be carried out when the development time has elapsed. The indications become visible when using fluorescent penetrants under UV light (≥ 1000 μW/cm² und < 20 lx) or when color contrast penetrants are used under daylight / white light (≥ 500 lx). Evaluation and documentation may be done by any adequate method.


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