Liquid Penetrant Inspection
Penetrant testing can be performed on magnetic and non-magnetic materials, but does not work well on materials that are porous. Penetrants may be ''visible'', meaning they can be seen in ambient light, or fluorescent, requiring the use of a ''black'' light. Talon Test Laboratories offers a comprehensive range of Liquid Penetrant inspections.
Liquid Penetrant Inspection Methods
Fluorescent Penetrants (Type 1) Water Washable (Method A) Medium Sensitivity (Level 2)
Fluorescent Penetrants (Type 1) Water Washable (Method A) High Sensitivity (Level 3)
Fluorescent Penetrants (Type 1) Post-Emulsifiable, Hydrophilic (Method D) Ultra-High Sensitivity (Level 4)
Advantages of Liquid Penetrant Inspection
- Large surface areas or large volumes of parts/materials can be inspected rapidly and at low cost
- Parts with complex geometry are routinely inspected
- Indications are produced directly on surface of the part providing a visual image of the discontinuity
- Equipment investment is minimal
Fluorescent Penetrants (Type 1) 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.
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.
Water-washable penetrants are most often applied by dipping the part in a penetrant tank, but the penetrant may be applied to large parts by spraying or brushing. Once the part is fully covered with penetrant, the part is placed on a drain board for the penetrant dwell time, and then taken to a rinse station where it is washed with a course water spray to remove the excess penetrant.
Once the excess penetrant has been removed, the part may be placed in a warm air dryer or in front of a gentle fan until the water has been removed. The part can then be placed in a dry developer tank and coated with developer, or allowed to sit for the remaining dwell time then inspected.
Post-Emulsifiable, Hydrophilic (Method D) systems 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.
Post-emulsifiable penetrants are penetrants that do not have an emulsifier included in its chemical make-up like water-washable penetrants. Post-emulsifiable penetrants are applied in a similar manner, but prior to the water-washing step, emulsifier is applied to the surface for a prescribed period of time (emulsifier dwell) to remove the excess penetrant.
When the emulsifier dwell time has elapsed, the part is subjected to the same water wash and developing process used for water-washable penetrants. Emulsifiers can be lipophilic (oil-based) or hydrophilic (water-based).
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 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.