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 several 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 the part.
- 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 see indications.
- Not be harmful to the material being tested or the inspector.
All 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 number 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 listed below:
- Type 1 – Fluorescent Penetrants
- Type 2 – Visible Penetrants
Fluorescent 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 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 several very small and tight fatigue cracks. The five sensitivity levels are shown below:
- 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 Materials.
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 reference.