Liquid penetrant testing (LPT) is one of the most widely used non-destructive testing methods. LPT is based on capillarity or capillary attraction, where a liquid is able to flow into narrow spaces without the assistance of – or even in opposition to – external forces such as gravity. The materials processes and procedures used in liquid penetrant testing are designed to make the results of this capillary action visible and capable of interpretation. Liquid penetrant testing is an effective means of locating and determining the severity of surface discontinuities in materials, including those that are not visible to the naked eye.
NDT penetrant testing includes fluorescent dye penetrant, which is typically green and uses a white developer to draw the dye back to the surface from inside the discontinuities by 'wicking' or capillary action. The fluorescent penetrant is characterised by its ability to emit visible radiation when excited by UV-A light and may be used on a variety of materials. Fluorescent penetrant is typically more sensitive than visible dye and is ranked into 4 levels of sensitivity. It does, however, require special lighting conditions: NDT penetrant technicians must have UV-A lamps, blackout shades and power generators in the field. Fluorescent-dye testing is easily performed under laboratory conditions.
Visible dye penetrant is a method of NDT penetrant testing, which is typically red and uses a white developer to draw the dye back to the surface from inside the discontinuities by 'wicking' or capillary action. This is often referred to as the 'colour contrast' method. The colour contrast created in the dye penetration test is highly visible under natural light and may be used on a variety of materials. It is generally not as sensitive to very small discontinuities as the fluorescent penetrant testing method. However, it does not require special lighting conditions, therefore NDT technicians do not have to carry UV-A Lamps, blackout shades or power generators.