What is it?
Liquid (or dye) penetrant inspection is an extension of visual inspection and is used for detecting surface-breaking flaws, such as cracks, laps and folds, on any non-absorbent material’s surface.
How does it work?
The basic stages of liquid penetrant inspection are shown below. Firstly, the surface to be inspected is cleaned thoroughly to remove all traces of dirt and grease. A brightly coloured or fluorescent liquid is then applied liberally to the component surface and allowed to penetrate any surface-breaking cracks or cavities. The time the liquid is allowed to soak into the material’s surface is normally about 20 minutes. After soaking, the excess liquid penetrant is wiped from the surface and a developer applied. The developer is usually a dry white powder, which draws penetrant out of any cracks by reverse capillary action to produce indications on the surface. These (coloured) indications are broader than the actual flaw and are therefore more easily visible.
- Sample before testing.
- Liquid penetrant applied.
- Surplus wiped off leaving penetrant in crack.
- Developer powder applied, dye soaks into powder.
- View coloured indications, or UV lamp shows up fluorescent indications.
A number of different liquid penetrant systems are used in industry. Fluorescent penetrants are normally used when the maximum flaw sensitivity is required. However, these penetrants must be viewed under darkened conditions with a UV lamp, which may not be practical. The most commonly used systems are solvent removable, or water washable, red dye systems, which typically comprise three aerosol cans – cleaning fluid, penetrant and developer. These systems are often used to check weld quality during fabrication.