Flaw entrapment efficiency describes the ability of a penetrant to form an indication large enough to be detected. Some of the factors influencing flaw entrapment efficiency are:
- Volume of a defect
The larger the discontinuity in terms of depth and width, the more penetrant it will hold and the more penetrant there will be present to form the indication in the developer.
- Length
The length of a defect, whilst influencing the volume of the penetrant present, has a strong influence upon the ability of the human eye to detect the indication. Very fine indications such as those formed by fine fatigue or stress corrosion cracks have insufficient width to be detected visually and can only be located when they are of sufficient length.
- Contaminants
Penetrating fluids will enter fine, clean discontinuities more readily than wide and contaminated ones. Contaminants such as oil, water and corrosion products can both reduce the volume available to the penetrant and adversely influence the contact angle of the penetrant.
Highly acidic or alkaline contaminants also cause fading of the dye present within the penetrant.
- Type of dye
The most obvious influence of dye type is seen when one changes between colour contrast and fluorescent penetrant dyes, the latter giving much higher sensitivity than the former.
- Concentration of dye
The higher the dye concentration the more intense is the indication. The dip and drain method of processing allows more volatile constituents of a penetrant to evaporate during the dwell time and thus increases the concentration of the dye within the remaining penetrant.
- Processing
Components that are cleaned until there is no background coloration present offer a high degree of contrast for any penetrant indications, but this may also be indicative of over-emulsification and over-washing, with the risk that penetrant may have been removed from defects. A small degree of background shows, therefore, that over-washing has not occurred.
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