Liquid penetrant inspection is a method that is used to show surface breaking flaws by bleed-out of a colored or fluorescent dye from the flaw. The technique is based on the ability of a liquid to be drawn into a “clean” surface breaking flaw by capillary action.
“After a period of time called the “dwell,” excess surface penetrant is removed and a developer applied. This acts as a blotter. It draws the penetrant from the flaw to reveal its presence. Colored (contrast) penetrants require good white light while fluorescent penetrants need to be used in darkened conditions with an ultraviolet “black light”.
A very early surface inspection technique involved the rubbing of carbon black on glazed pottery, whereby the carbon black would settle in surface cracks rendering them visible. Later, it became the practice in railway workshops to examine iron and steel components by the “oil and whiting” method. In this method, a heavy oil commonly available in railway workshops was diluted with kerosene in large tanks so that locomotive parts such as wheels could be submerged.
After removal and careful cleaning, the surface was then coated with a fine suspension of chalk in alcohol so that a white surface layer was formed once the alcohol had evaporated. The object was then vibrated by being struck with a hammer, causing the residual oil in any surface cracks to seep out and stain the white coating. This method was in use from the latter part of the 19th century to approximately 1940, when the magnetic particle method was introduced and found to be more sensitive for ferromagnetic iron and steels.
A different (though related) method was introduced in the 1940’s. The surface under examination was coated with a lacquer, and after drying, the sample was caused to vibrate by the tap of a hammer. The vibration causes the brittle lacquer layer to crack generally around surface defects. The brittle lacquer (stress coat) has been used primarily to show the distribution of stresses in a part and not for finding defects.
Many of these early developments were carried out by Magnaflux in Chicago, IL, USA in association with Switzer Bros., Cleveland, OH, USA. More effective penetrating oils containing highly visible (usually red) dyes were developed by Magnaflux to enhance flaw detection capability. This method, known as the visible or color contrast dye penetrant method, is still used quite extensively today. In 1942, Magnaflux introduced the Zyglo system of penetrant inspection where fluorescent dyes were added to the liquid penetrant. These dyes would then fluoresce when exposed to ultraviolet light (sometimes referred to as “blacklight“) rendering indications from cracks and other surface flaws more readily visible to inspectors.” Information from nde-ed.org