Factors which specify the quality of a penetrant and further influence the authenticity of inspection are:
Viscosity: The viscosity of penetrant should not be higher than 3.0 cSt at 38°C/100°F and not more than 5.0 cSt at the lowest temperature, as higher the viscosity slower rate of penetration & lower the viscosity penetrant drain away easily.
Capillarity: Capillarity is the ability of liquid to rise & fall in a narrow opening, hence cavities which offer narrow openings such as tight cracks or fatigue cracks of hairline type are best detected with penetrant test systems and penetrants easily into the narrow openings.
Fluidity: Ability of a liquid to flow is known as fluidity, the penetrant should have the ability to drain away from the component without dragging its original marks that revealed the defects.
Wettability: It is the ability of a penetrant to wet the surface of the material, it is the measure as the area of contact with the surface. The angle of contact for a penetrant should be less than 5 degrees.
Flash Point: Penetrant should have a high flash point as a matter of safety, flashpoint should be above 93.3°C/200°F.
Some other factors to be considered are chemical composition which should not react with the job surface and harm it, drying characteristics and specific gravity, etc.
The three major types of penetrants are Visible, Fluorescent and Dual (Fluorescent & Visible) type. The most appropriate penetrant is chosen on the basis of defect Sensitivity required, Job Dimensions, Material of the Job and Surface Condition.
Different types of Penetrants and Selection
Colour Contrast/Visible Dye Penetrant: Visible dye penetrant’s indication can be used in daylight or by artificial white light as it does not require a dark room setup. It is mainly used for large test parts which cannot be moved to a dark room setup and need to be tested remotely. Due to the simple application and no specific setup requirement, they are also cost effective as compared to other available penetrants.
Fluorescent Penetrant: These are mainly used for detection of tight/fine cracks which require fluorescent inspection and can be clearly visible under black/fluorescent light. These are mainly used in Aerospace industry for critical parts.
Dual Purpose Penetrant (Fluorescent/Colour Contrast): Dual purpose penetrant produces indications visible under both black light and natural or artificial white light. The larger flaws are visible under both black light and natural white light which provides two different level of sensitivity. The theory is that the visible non-fluorescent dyes are better able to detect large surface cracks while the fluorescent dyes are more useful in connection with the very fine surface cracks.
