Liquid Penetrant inspection utilizes the natural accumulation of a fluid around a discontinuity to create recognizable indications of a crack or any other surface opening. Capillary action attracts a fluid to the discontinuity as compared to the surroundings. In order to locate the area of excess fluid the background area must be of sufficient contrast thus leading to distinct detection of the defect on the surface.
Penetrant inspection depends on the ability of a liquid to wet the surface of a solid work piece or specimen, and flow over that surface to form a continuous and reasonably uniform coating, thus penetrating into cavities that are open to the surface. The ability of a liquid to flow over a surface and enter surface cavities depends on surface tension and capillary action. The cohesive forces between the molecules of a liquid cause surface tension. Capillary action is the phenomenon of the rising or depression of liquid in narrow cavities.
Visible light or ultraviolet light is required for inspection of penetrant indications. In penetrant inspection, when particles in the fluid are struck by the incident ultraviolet light they are excited to a higher energy level. After being excited each particle returns to its original unexcited level with the emission of light having a wavelength longer than the original source. This phenomenon is known as fluorescence. Thus, the emitted light is in the visible spectrum. Ultraviolet light is not normally seen by the human eye and is known as black light. When inspection is performed in a very subdued light, regions clear of the penetrant appear black. Due to this effect, the visibility of small indications is greatly enhanced.
Figure 3. Depeiction of cracks after die penetration
Penetrant inspection is achieved by following the sequence of operations below:
Cleaning: Initial cleaning of the surface to be inspected is very important. Scales, flakes, paint, dirt, grease and other chemicals that are not cleaned from the surface, tend to accumulate on penetrant. This leads to masking of real indications or creation of defect indications where none exist. The cleaned surface must be adequately dried before the penetrant is applied, as presence of any excess cleaning fluid would dilute the penetrant and diminish the brilliance of the indication.
Penetrant Application: Penetrant fluid should be applied to the cleaned surface. The fluid should spread freely and evenly over the surface and move into cracks. Application of penetrant may be achieved by dipping the component in a bath of penetrant liquid or by spraying or brushing. A minimum dwell time of 20 to 30 minutes must be allowed after penetrant application.
Removal of excess penetrant: This is the most important step in the entire process. The desired result is that the surface is completely clear of the penetrant. The crack or defect retains all the penetrant that entered into it. Excess cleaning may remove penetrant from the upper region of the defect with the result that no defect is indicated. Insufficient cleaning will leave a background of penetrant on the surface. The difference in contrast as a consequence may not be sufficient for the defect to be recognized. Care must be exercised so that neither insufficient nor excess cleaning is done.
Application of developer: After removing the excess penetrant a thin coating of developer is applied over the surface to draw the penetrant out of the crack and increase its visibility. Another important function of the developer is that it covers the surface with a color that provides good visual contrast to the penetrant. This increases visibility of the defect.
Inspection and Evaluation: The last step in the process is scanning the surface for indications. The scanning may be carried out under visible light conditions or with ultraviolet or laser beams. Defect recognition can also be made with the human eye or with automated optical scanners.
Penetrating testing methods
Water washable method: In this method all materials used are water-soluble. After the initial cleaning and drying, the penetrant used is a water-based fluid. When aqueous developers are used there is no need for drying prior to the application of the developer. If non-aqueous developers are used the part is dried before developer application.
Post Emulsification method: This method is a combination of water and solvent based inspections. The differences between this method and the water washable method are (1) the penetrant used and (2) the need for an emulsifier. If a solvent-based penetrant is used and followed by an emulsifier application the rest of the process can follow the water washable path.
Solvent removable method: This process is an oil based inspection process. Penetrant removal must be accompanied by hand wiping the part with a rag dampened with solvent. This technique is used for inspection of pipe welds and pressure vessel welds.
Liquid penetrant test equipment varies from highly portable kits containing aerosol cans to large, high-speed stationary production units, to highly specialized units for testing of custom parts.
Portable inspection materials consist of visible or fluorescent dye penetrant, a penetrant remover and a developer. Complete portable inspection kits also provide a storage case, application materials, cleaning rags and suitable set of instructions. When fluorescent dye penetrant is supplied, a small portable UV lamp may also be included.
Applications and Limitations
Some proven applications of liquid penetrant testing are:
Inspection of tools and dies.
Inspection of tank vessels, reactors, piping, driers and pumps in chemical, food and paper industries.
Inspection of oil field drilling rigs, drill pipe, castings and drilling equipment.
Inspection of aircraft engine parts, propellers, wing fittings, castings etc.
Inspection of diesel locomotive truck and bus parts.
Liquid penetrant cannot be applied to porous materials. In case of ferromagnetic materials, MPT is preferred, as it will detect subsurface discontinuities, defects filled with oxides, and those covered by paint films.