and History of
penetrant inspection is a method that is used to reveal surface
breaking flaws by bleedout 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 "black
light") rendering indications from cracks and
other surface flaws more readily visible to inspectors.