Surface flaw detection using fluorescent and visible methods
Penetrant testing is a simple non-destructive testing (NDT) method used to locate surface-breaking discontinuities in metals and many non-metals using a penetrating liquid.
- By its very nature, penetrant testing is a simple straightforward testing method; however, it is also very easy to make a mistake during the process and so invalidate the test. Penetrant testing has its roots in the rail industry with a test then called the oil and whiting method. This method involved immersing a cleaned component into dirty crankcase oil diluted with kerosene (dirty oil worked better than fresh unused oil) and, after the component had been allowed to drain, it was then cleaned with solvent.
- The component was then covered with whiting, which is powdered chalk. Any oil trapped in the discontinuities would bleed back into the whiting, giving an indication. Penetrant testing has not changed too much over the years, penetrants are more sensitive now but in essence the process is the same.
- There are two distinct categories of penetrant and the difference is in the type of dye used to make it. One type is called color-contrast penetrant and is made with an intense red dye – inspection is conducted using white light. The second type is called fluorescent penetrant and is made with a fluorescent dye.