Surface cracks and pinholes that are not visible to the naked eye can be located by liquid penetrant inspection. It is widely used to locate leaks in welds and can be applied with austenitic steels and nonferrous materials where magnetic particle inspection would be useless.
Liquid penetrant inspection is often referred to as an extension of the visual inspection method. Many standards, such as the AWS D1.1 Code, say that “welds subject to liquid penetrant testing … shall be evaluated on the basis of the requirements for visual inspection.”
This examination process is useful for detection of open to surface discontinuities only. Applicable to varieties of non porous materials. Produces enlarged indication for easy detection. Relative volume of a flaw is indicated by the amount of bleed out.