Liquid Penetrant Inspection
Detecting Surface-breaking Flaws
Liquid Penetrant Inspection (LPI or PT) is very effective in detecting flaws that are open to the surface such as fatigue, quench or grinding cracks; forging cracks and bursts; overload and impact fractures; porosity; laps and seams. LPI is commonly used on materials such as metals, glass, plastics and fired ceramics. This method relies on the penetrant seeping into the flaw, then forming a surface indication that is observed under special lighting after the penetrant is given time to “bleed out” from the flaw.
The first step in Liquid Penetrant Inspection is to thoroughly clean the surface to be inspected to be sure it is free of oil, grease, water, heat-treat scale, paint, plating and other contaminants that may prevent penetrant from entering flaws. The part may also require etching if mechanical operations such as machining, sanding, or grit blasting have been performed because they can smear metal over the flaw opening and prevent the penetrant from entering.
Next, a red visible or fluorescent dye penetrant is applied to the part by spraying, brushing or immersing in a penetrant bath. The penetrant is left on the surface for a sufficient time to allow as much penetrant as possible to seep into a defect. Penetrant dwell time is the total time that the penetrant is in contact with the part’s surface.
After the dwell time has elapsed, the excess penetrant is removed from the surface. Depending on the penetrant inspection method used, this step may involve cleaning with a solvent (solvent-removable), direct rinsing with water (water-washable), or first treating the part with an emulsifier (post-emulsifiable) before rinsing with water. When using the water-washable or post-emulsifiable methods, the part is placed in a low-temperature oven and allowed time to dry after rinsing.
A thin layer of developer is applied to the part to assist in drawing penetrant trapped in flaws back to the surface where it will be visible as indications. Developers may be applied by dusting (dry powder) or spraying (wet developers). These indications are larger than the actual flaw, and therefore, are more visible. When using fluorescent penetrants, indications must be viewed under darkened conditions with a high-intensity UV lamp.
The final step in the process is to thoroughly clean the surface to remove any residues.
Inspections at LTI
Our priority is meeting customer needs with accurate results, timely turnaround and quality service. Inspections are performed according to customer specifications and industry requirements by LTI certified technicians. Their certifications include ASNT SNT-TC-1A, NAS410/MIL-STD-410, Pratt & Whitney PWA-NDTQ and MIL-STD-2132.
Large-volume orders and products of all shapes and sizes are efficiently inspected in our 2,500 sq. ft. LPI area. We are equipped with three large penetrant dip tanks, a 25 ft. dwell tank for pipe, tubing, bar, etc., and a large inspection room with three high-intensity black lights. All orders are processed with only the highest quality Magnaflux inspection materials.
Advantages and Limitations
Liquid Penetrant Inspection offers a fast and relatively inexpensive means of surface inspection since large areas and quantities of parts or materials can be inspected quickly. The process is flexible for inspecting parts of almost any shape and for most materials that are not extremely rough or porous. Indications are produced directly on the surface of the part and provide a visual representation of the flaw. LPI is highly sensitive to small surface discontinuities.
One of the major limitations of a penetrant inspection is that flaws must be open to the surface. Also, surface finish and roughness can affect inspection sensitivity. Pre-cleaning of parts is critical since contaminants can mask defects, and post-cleaning is required to remove residues.