Dye Penetrant Testing (DPI)
Liquid Penetrant Testing technique is a method that supplements visual inspection, revealing defects such as fine cracks or micro-porosity that would be invisible or difficult to detect by the naked eye in all non-porous materials (metals, plastics, or ceramics).
Liquid Penetrant Testing is a simple, cheap and easily portable inspection method that requires no
equipment apart from spray cans. It can detect surface breaking imperfections only and relies on a
coloured or fluorescent dye, sprayed on the surface and penetrating the imperfection. About 20 minutes
is generally specified to enable the dye to penetrate very fine imperfections (penetrant dwell time).
After the dwell time, the excess dye is cleaned off and the dye in the discontinuities is drawn to the
surface by spraying on a developer in the case of the colour contrast dye or exposing the surface to
ultra-violet light in the case of a fluorescent dye. The imperfections are then revealed by the dye
staining the developer or fluorescing. The fluorescent dye gives greater sensitivity than the colour
contrast dye and does not require the use of a developer.
It does, however, require the use of an ultra-violet light source and preferably a darkened room which
makes it a less portable inspection method than the colour contrast dye technique.
The dye used as a penetrant must be able to penetrate tight cracks, but must not be capable of being r
emoved from more open imperfections during the cleaning operation carried out prior to applying the
developer.
Although a simple inspection process to use, interpretation can be a problem if the surface
is naturally rough – coarsely ground or rough machined for example - or contains acceptable geometric
features that trap the dye. Training of personnel to recognise genuine and spurious indications is
therefore essential.
