The penetrant materials used today are much more
sophisticated than the kerosene and whiting first used by railroad
inspectors near the turn of the 20th century. Today's penetrants
are carefully formulated to produce the level of sensitivity desired
by the inspector. To perform well, a penetrant must possess a
number of important characteristics. A penetrant must:
- spread easily over the surface of the material being inspected
to provide complete and even coverage.
- be drawn into surface breaking defects by capillary action.
- remain in the defect but remove easily from the surface of
- remain fluid so it can be drawn back to the surface of the
part through the drying and developing steps.
- be highly visible or fluoresce brightly to produce easy to
- not be harmful to the material being tested or the inspector.
penetrant materials do not perform the same and are not designed
to perform the same. Penetrant manufactures have developed different
formulations to address a variety of inspection applications.
Some applications call for the detection of the smallest defects
possible and have smooth surfaces where the penetrant is easy to
remove. In other applications, the rejectable defect size may be
larger and a penetrant formulated to find larger flaws can be
used. The penetrants that are used to detect the smallest defect
will also produce the largest amount of irrelevant indications.
Penetrant materials are classified in the various industry and
government specifications by their physical characteristics and
their performance. Aerospace Material Specification (AMS) 2644,
Inspection Material, Penetrant, is now the primary specification
used in the USA to control penetrant materials. Historically,
Military Standard 25135, Inspection Materials, Penetrants, has
been the primary document for specifying penetrants but this document
is slowly being phased out and replaced by AMS 2644. Other specifications
such as ASTM 1417, Standard Practice for Liquid Penetrant Examinations,
may also contain information on the classification of penetrant
materials but they are generally referred back to MIL-I-25135
or AMS 2644.
Penetrant materials come in two basic types. These types are
- Type 1 - Fluorescent Penetrants
- Type 2 - Visible Penetrants
penetrants contain a dye or several dyes that fluoresce when exposed
to ultraviolet radiation. Visible penetrants contain a red dye
that provides high contrast against the white developer background.
Fluorescent penetrant systems are more sensitive than visible
penetrant systems because the eye is drawn to the glow of the
fluorescing indication. However, visible penetrants do not require
a darkened area and an ultraviolet light in order to make an inspection.
Visible penetrants are also less vulnerable to contamination from
things such as cleaning fluid that can significantly reduce the
strength of a fluorescent indication.
Penetrants are then classified by the method used to remove the
excess penetrant from the part. The four methods are listed below:
- Method A - Water Washable
- Method B - Post-Emulsifiable, Lipophilic
- Method C - Solvent Removable
- Method D - Post-Emulsifiable, Hydrophilic
Water washable (Method A) penetrants can be removed from the
part by rinsing with water alone. These penetrants contain an
emulsifying agent (detergent) that makes it possible to wash the
penetrant from the part surface with water alone. Water washable
penetrants are sometimes referred to as self-emulsifying systems.
Post-emulsifiable penetrants come in two varieties, lipophilic
and hydrophilic. In post-emulsifiers, lipophilic systems (Method
B), the penetrant is oil soluble and interacts with the oil-based
emulsifier to make removal possible. Post-emulsifiable, hydrophilic
systems (Method D), use an emulsifier that is a water soluble
detergent which lifts the excess penetrant from the surface of
the part with a water wash. Solvent removable penetrants require
the use of a solvent to remove the penetrant from the part.
Penetrants are then classified based on the strength or detectability
of the indication that is produced for a number of very small
and tight fatigue cracks. The five sensitivity levels are shown
- Level ½ - Ultra Low Sensitivity
- Level 1 - Low Sensitivity
- Level 2 - Medium Sensitivity
- Level 3 - High Sensitivity
- Level 4 - Ultra-High Sensitivity
The major US government and industry specifications currently
rely on the US Air Force Materials Laboratory at Wright-Patterson
Air Force Base to classify penetrants into one of the five sensitivity
levels. This procedure uses titanium and Inconel specimens with
small surface cracks produced in low cycle fatigue bending to
classify penetrant systems. The brightness of the indication produced
is measured using a photometer. The sensitivity levels and the
test procedure used can be found in Military Specification MIL-I-25135
and Aerospace Material Specification 2644, Penetrant Inspection
An interesting note about the sensitivity levels is that only
four levels were originally planned. However, when some penetrants were
judged to have sensitivities significantly less than most others
in the level 1 category, the ½ level was created. An excellent
historical summary of the development of test specimens for evaluating
the performance of penetrant materials can be found in the following
Flaherty, J. J., History of Penetrants: The First 20 Years, 1941-61,
Materials Evaluation, Vol. 44, No. 12, November 1986, pp. 1371-1374,
1376, 1378, 1380, 1382