Non destructive methods do not permanently change the item being inspected.
This makes NDT a really valuable technique for testing items at defined stages during manufacture.
Liquid or Dye Penetrant Testing is just one such technique in the NDT collection. It has the advantage over other methods when looking for defects in castings or welded work in that it is fast and low cost. Not only is it at home verifying that castings or welds are of good quality but also valuable in looking for fatigue cracks or fractures in items that are routinely subjected to stress such as aircraft landing gear.
In its simplest form a red penetrant dye is used. For more sensitive materials a fluorescent penetrant dye is used. The basic method is the same however. Using a lint free cloth the test surface is cleaned to remove any grease, dirt or loose particles that could give rise to false results. The penetrant dye is then applied and left for up to half an hour allowing time to soak into any hairline cracks or flaws. Raising the temperature during this stage using a suitable industrial oven can assist in dye penetration especially when looking for extremely fine hairline stress fractures.
Excess dye is then removed taking care not to remove the penetrant dye from any flaws. If using a white powder developer the inspection piece must be thoroughly dried using the oven at a temperature typically around 140°C. The required temperature can vary depending on the penetrant dye being used.
The developer is white in colour and can be applied as a powder or water based. The developer draws the dye out of any hairline cracks in a process known as bleed out. Areas of bleed out can take between 10 and 30 minutes to develop and are indicative of surface cracks in the product. If a red penetrant dye is used the inspector will use visible light of approximately 1100 lux. Fluorescent dye inspection takes place in a darkened room using a lamp with a UV light source.
The advantage of UV dye penetration inspection is speed and low cost. However this method can only detect surface flaws so other NDT methods of looking for material fractures may also need to be used.