Liquid penetrant inspection is effective in distinguishing defects which are open on surfaces such as grinding cracks, forging cracks and bursts, fatigue, overload and impact fractures or laps. The basic steps to be followed are:
Pre – Cleaning
The test surface is cleaned off from any dirt, paint, oil or anything that can keep away the penetrant from any defect causing unnecessary or misleading indications. Cleaning procedures includes alkaline cleaning steps, media blasting, solvents or vapour degreasing. This step aims to clean the surface where existing defects are open to the surface, dry and free from contaminants. If media blasting is used it might over do the work on small cracks on the surface part which will require an etching bath as an after bath treatment.
The surface of the element being test is applied with a dye penetrant allowing the dye penetrant to soak into any damages usually called the dwell time. Generally, it takes 5 to 30 mintues depending on the penetrant used, material tested and the size of damage. Smaller damages requires more penetration time. Due to natural incompatibilities you must not apply solvent – based penetrant to surfaces being inspected with water – washable penetrant.
Removing Excess Penetrant
Excess penetrant should be removed from the surface. Removal method is dependent on the type of penetrant used such as solvent – removable or water – washable. The most sensitive are the emulsifiers interacting chemically with the oily dye penetrant making it removable with sprayed with water. If you are using a solvent – remover with lint – free cloth make sure you do not spray the solvent directly on the test surface to avoid the dye penetrant being removed from the flaws. Excess penetrant should be properly removed before the developer is applied because this may leave evidences in the developed area masking the indications or damages. Moreover, it may produce false indications which will severely hinder the ability to do the inspection properly.
Application of Developer
Once the excess penetrant is removed a white developer is applied on the sample. Different types of developers are available for use such as dry powder, water soluble or water suspendable. Your choice of developer is controlled by the compatibility of penetrant and conditions of inspection.
Bleed out is when the developer has drawn out the penetrant from the defects on the surface and forms a visible indication. Areas that bleed out will tell you the location, orientation and probable types of defects on the surface.
The inspector will use a visible light with appropriate intensity either 100 foot candles or 1100 lux which is typically for evidence of dye penetrant. Ultraviolet (UV-A) radiation of appropriate intensity with low ambient light levels is used for fluorescent penetrant testing. Inspection should take place after a 10 minute development time on the test surface allowing the blotting action to happen. In using the visible dye penetrant sample of indication formation may be observed by the inspector.
After inspection and noting down of defects the test surface should be cleaned.
Different types of liquid penetrant testing can be used in detecting defects on open surfaces or cracks. Red dye or fluorescent penetrant can also be used in different types of wet and dry developers.