Liquid penetrant testing for surface breaking defects

A low viscosity penetrant is applied to a clean, dry surface and left for a dwell time so it can be drawn into any opening by capillary action. The excess is removed from the surface without washing it out of the defects. A thin developer then draws the trapped penetrant back out, forming an indication wider than the defect itself. The technician inspects, interprets each indication and evaluates it against the acceptance criteria. The physics is simple; the result depends on discipline in cleaning, timing and lighting.

1. Precleaning: remove oil, paint, scale and smeared metal; machined soft alloys often need etching first.
2. Penetrant application and dwell, commonly at least 10 minutes and longer for tight fatigue cracks.
3. Excess removal by water wash, emulsifier or solvent wipe, depending on the method.
4. Developer application and development time.
5. Inspection under UV-A light in a darkened area for fluorescent, or good white light for visible dye.
6. Evaluation, marking, postcleaning and the report.

Fluorescent penetrant (Type I) glows under UV-A light and is the more sensitive choice; it is standard on aerospace production lines and engine overhaul. Visible red dye penetrant (Type II) needs only white light and a portable kit, which makes it practical for welds, field repairs and parts where a darkened booth is not possible. Penetrant systems are also grouped by removal method: water washable, post emulsifiable (lipophilic or hydrophilic) and solvent removable. Post emulsifiable fluorescent systems give the best control on fine cracks; solvent removable visible kits are the field workhorse.

Fluorescent penetrant materials are qualified to AMS 2644, which lists them by type, removal method and sensitivity level, from level 1/2 (ultralow) through levels 1, 2, 3 and 4 (ultrahigh). Higher sensitivity finds tighter cracks but also holds more background on rough surfaces, so the specification or the engine manual usually names the level. Rough castings often run at a lower level than smooth machined or polished parts. If your drawing only says "FPI", ask the lab which level they propose and why.

PT works on aluminum, magnesium, titanium, copper alloys, austenitic stainless steels, nickel superalloys, carbon steels, many plastics, glass and glazed or dense ceramics. Aerospace uses fluorescent penetrant on castings, forgings, turbine blades and machined structural parts; power generation and petrochemical plants use visible dye on welds and repairs; automotive and general manufacturing use it on aluminum castings and safety parts. See aerospace NDT inspection and casting inspection services for industry specific requirements.

• ASTM E1417, standard practice for liquid penetrant testing, the usual call out in aerospace and defense
• AMS 2644 for penetrant materials, with the qualified products list
• ASTM E165 as the general practice, ASME Section V Article 6 for pressure equipment, AWS D1.1 for structural welds
• Engine manufacturer and prime contractor specifications where they apply
• Technician qualification to SNT-TC-1A, CP-189 or NAS 410, and NADCAP NDT accreditation when the customer requires it

Ask how the lab checks its process: penetrant contamination and brightness checks, known defect panels, UV-A intensity and ambient light readings, and wash water temperature and pressure.

Cost depends on part size, quantity, fluorescent or visible, sensitivity level, etching, masking and reporting requirements. Production lines price per part; field work prices per hour.

Turnaround for lab batches is often 1 to 3 business days. See NDT inspection cost for what moves prices across methods.