NDT – Liquid Penetrant Testing


One of the most popular NDT training courses currently offered at the SAIW is Liquid Penetrant Testing. This dye-penetrant surface technique is often the ‘first step’ for NDT personnel in unlocking the numerous career opportunities in the exciting field of NDT. The method is offered at Level 1, 2 or 3 depending on the technician’s experience level and/or company’s need.

Since 1935, due to the growing use of light alloys and stainless steels, which could not be subjected to magnetic particle testing, the dye penetrant inspection method was developed. Continual research has perfected reliable and effective penetrant inspection products.

General Description

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

Basic principles of dye penetrant inspection

Step 1 – Preparation of the surface which consists in making the defect emerge and/or freeing it from any bodies that may prevent the infiltration of the penetrant product. (E.g. pickling, removing grease, etc.)


Step 2 – Application of the penetrant onto the surface to be inspected for a duration that is long enough to allow the penetrant product to infiltrate into the defects.

Step 3 – Elimination of the excess penetrant from the surface to be inspected. This operation should not remove the penetrant from the defects.


Step 4 – Application of developer made up of capillary agents which make the penetrant bleed out of the orifices


Step 5 – Examination of dye penetrant inspection results.

Step 6 – Cleaning the surfaces post-test and surface protection. The surfaces are cleaned usually using the same process as pre cleaning.

Step 7 – If necessary, a protective cover is applied to store the parts.

For more information on this course or any other NDT Training courses please feel free to contact the NDT Training Manager Mark Digby on 011 298 2100.