Penetrant Testing

This type of testing uses the forces of capillary action to detect surface breaking defects. It is impossible to detect defects which do not break the surface with this method, but it can be used on both magnetic and non-magnetic materials providing they are non-porous.

There are several types of penetrant systems, this includes the following which are shown in a descending order of flaw detection sensitivity:

• Post-emulsifable – fluorescent
• Solvent based – fluorescent
• Water based – fluorescent
• Post emulsifiable colour contrast
• Solvent based – colour contrast
• Water based – colour contrast

Fluorescent penetrants require the use of an ultraviolet (UV-A) light to view indications, whilst colour contrast penetrants are viewed with the naked eye.

One of the most common site used penetrant systems uses solvent based colour contrast penetrants in aerosols. A typical sequence of operations on a steel test item is as follows:

1. Clean area using wire brush, cloth and solvent. On aluminiums, other soft alloys and plastics, wire brushing should not be used, as there is a danger that surface breaking defects may be closed.

2. Apply penetrant – leave for typically 15 minutes. Colour contrast penetrants are normally red in colour and should remain on the part long enough to be draw into any surface discontinuities. This time can vary from about ten minutes to several hours depending on the type of material and size/type of defect sought.

3. Remove surface penetrant using cloth and solvent. Apply solvent to the cloth and not directly on to the work piece. Clean thoroughly.

4. Apply developer – leave for typically 15 minutes. The developer draws any penetrant remaining in any surface breaking discontinuities with a blotting action.

5. Interpret area. Any discontinuities are indicated by a red mark, e.g. line or dot against a white background. Fluorescent penetrants would show green-yellow when viewed with an ultraviolet (UV-A) light