QUALITY AND PROCESS CONTROL IN LIQUID PENETRANT TEST

Quality & Process Control

Quality control of the penetrant inspection process is essential to get good and consistent results. Since several steps and materials are involved in the inspection process, there are quality control procedures for each of them.

Temperature Control

The temperature of the penetrant materials and the part being inspected can have an effect on the results. Temperatures from 27 to 49°C are reported in the literature to produce optimal results. Many specifications allow testing in the range of 4 to 52°C. Raising the temperature beyond this level will significantly raise the speed of evaporation of penetrants causing them to dry out quickly.

Since the surface tension of most materials decrease as the temperature increases, raising the temperature of the penetrant will increase the wetting of the surface and the capillary forces. Of course, the opposite is also true, so lowering the temperature will have a negative effect on the flow characteristics.

Penetrant Quality Control

The quality of a penetrant inspection is highly dependent on the quality of the penetrant materials used. 

Only products meeting the requirements of an industry specification, such as AMS 2644, should be used. Deterioration of new penetrants primarily results from aging and contamination. Virtually all organic dyes deteriorate over time, resulting in a loss of color or fluorescent response, but deterioration can be slowed with proper storage. 

When possible, keep the materials in a closed container and protect from freezing and exposure to high heat.

Contamination can occur during storage and use. Of course, open tank systems are much more susceptible to contamination than are spray systems. Regular checks must be performed to ensure that the material performance has not degraded. 

When the penetrant is first received from the manufacturer, a sample of the fresh solution should be collected and stored as a standard for future comparison. The standard specimen should be stored in a sealed, opaque glass or metal container.

 Penetrants that are in-use should be compared regularly to the standard specimen to detect any changes in properties or performance.

Dwell Quality Control

Dwell times are usually recommended by the penetrant producer or required by the specification being followed. The only real quality control required in the dwell step of the process is to ensure that a minimum dwell time is reached. There is no harm in allowing a penetrant to dwell longer than the minimum time as long as the penetrant is not allowed to dry on the part.

Emulsifier Path Quality Control

Quality control of the emulsifier bath is important and it should be performed per the requirements of the applicable specification.

Lipophilic Emulsifiers

Lipophilic emulsifiers mix with penetrants but when the concentration of penetrant contamination in the emulsifier becomes too great, the mixture will not function effectively as a remover. Standards require that lipophilic emulsifiers be capable of 20% penetrant contamination without a reduction in performance. When the cleaning action of the emulsifier becomes less than that of new material, it should be replaced.

Hydrophilic Emulsifiers

Hydrophilic emulsifiers have less tolerance for penetrant contamination. The penetrant tolerance varies with emulsifier concentration and the type of contaminating penetrant. In some cases, as little as 1% (by volume) penetrant contamination can seriously affect the performance of an emulsifier.

Emulsifier Concentration and Contact Time

The optimal emulsifier contact time is dependent on a number of variables that include the emulsifier used, the emulsifier concentration, the surface roughness of the part being inspected, and other factors. Usually some experimentation is required to select the proper emulsifier contact time.

Wash Quality Control

The wash temperature, pressure and time are three parameters that are typically controlled in penetrant inspection process specification. A coarse spray or an immersion wash tank with air agitation is often used

. When the spray method is used, the water pressure is usually limited to 276 kPa. The temperature range of the water isusually specified as a wide range (e.g., 10 to 38°C). The wash time should only be as long as necessary to decrease the background to an acceptable level. Frequent visual checks of the part should be made to determine when the part has been adequately rinsed.

Drying Process Quality Control

The temperature used to dry parts after the application of an aqueous wet developer or prior to the application of a dry powder or a nonaqueous wet developer, must be controlled to prevent drying in the penetrant in the flaw. To prevent harming the penetrant material, drying temperature should be kept to less than 71°C. Also, the drying time should be limited to the minimum length necessary to thoroughly dry the component being inspected.

Developer Quality Control

The function of the developer is very important in a penetrant inspection. In order to accomplish its functions, a developer must adhere to the part surface and result in a uniform, highly porous layer with many paths for the penetrant to be moved due to capillary action. Developers are either applied wet or dry, but the desired end result is always a uniform, highly porous, surface layer. Since the quality control requirements for each of the developer types is slightly different, they will be covered individually.

Dry Powder Developer

A dry powder developer should be checked daily to ensure that it is fluffy and not caked. It should be similar to fresh powdered sugar and not granulated like powdered soap. It should also be relatively free from specks of fluorescent penetrant material from previous inspection. This check is performed by spreading a sample of the developer out and examining it under UV light.

When using the developer, a light coat is applied by immersing the test component or dusting the surface. After the development time, excessive powder can be removed by gently blowing on the surface with air not exceeding 35 kPa.

Wet Soluble/Suspendable Developer

Wet soluble developer must be completely dissolved in the water and wet suspendable developer must be thoroughly mixed prior to application. The concentration of powder in the carrier solution must be controlled in these developers.

The concentration should be checked at least weekly using a hydrometer to make sure it meets the manufacturer's specification. To check for contamination, the solution should be examined weekly using both white light and UV light. Some specifications require that a clean aluminum panel be dipped in the developer, dried, and examined for indications of contamination by fluorescent penetrant materials.

These developers are applied by spraying, flowing or immersing the component. They should never be applied with a brush. Care should be taken to avoid a heavy accumulation of the developer solution in crevices and recesses.

Solvent Suspendable

Solvent suspendable developers are typically supplied in sealed aerosol spray cans. Since the developer solution is in a sealed vessel, direct check of the solution is not possible. However, the way that the developer is dispensed must be monitored. The spray developer should produce a fine, even coating on the surface of the part. Make sure the can is well shaken and apply a thin coating to a test article. If the spray produces spatters or an uneven coating, the can should be discarded.

When applying a solvent suspendable developer, it is up to the inspector to control the thickness of the coating. With a visible penetrant system, the developer coating must be thick enough to provide a white contrasting background but not heavy enough to mask indications. When using a fluorescent penetrant system, a very light coating should be used. The developer should be applied under white light and should appear evenly transparent.

Development Time

Parts should be allowed to develop for a minimum of 10 minutes and no more than 2 hours before inspecting.

Lighting Quality Control

Proper lighting is of great importance when visually inspecting a surface for a penetrant indication. Obviously, the lighting requirements are different for an inspection conducted using a visible dye penetrant than they are for an inspection conducted using a fluorescent dye penetrant.

Lighting for Visible Dye Penetrant Inspections

When using a visible penetrant, the intensity of the white light is of principal importance. Inspections can be conducted using natural lighting or artificial lighting.

However, since natural daylight changes from time to time, the use of artificial lighting is recommended to get better uniformity. Artificial lighting should be white whenever possible (halogen lamps are most commonly used). The light intensity is required to be 100 foot-candles at the surface being inspected.

Lighting for Fluorescent Penetrant Inspections

Fluorescent penetrant dyes are excited by UV light of 365nm wavelength and emit visible light somewhere in the green-yellow range between 520 and 580nm. The source of ultraviolet light is often a mercury arc lamp with a filter. The lamps emit many wavelengths and a filter is used to remove all but the UV and a small amount of visible light between 310 and 410nm. Visible light of wavelengths above 410nm interferes with contrast, and UV emissions below 310nm include some hazardous wavelengths.

Standards and procedures require verification of filter condition and light intensity. The black light filter should be clean and the light should never be used with a cracked filter. Most UV light must be warmed up prior to use and should be on for at least 15 minutes before beginning an inspection. Since fluorescent brightness is linear with respect to ultraviolet excitation, a change in the intensity of the light (from age or damage) and a change in the distance of the light source from the surface being inspected will have a direct impact on the inspection. For UV lights used in component evaluations, the normally accepted intensity is 1000 μW/cm2 at 38cm distance from the filter face. The required check should be performed when a new bulb is installed, at startup of the inspection cycle, if a change in intensity is noticed, or every eight hours of continuous use.

When performing a fluorescent penetrant inspection, it is important to keep white light to a minimum as it will significantly reduce the inspector’s ability to detect fluorescent indications. Light levels of less than 2 foot-candles are required by most procedures. When checking black light intensity a reading of the white light produced by the black light may be required to verify white light is being removed by the filter.

Light Measurement

Light intensity measurements are made using a radiometer (an instrument that transfers light energy into an electrical current). Some radiometers have the ability to measure both black and white light, while others require a separate sensor for each measurement

. Whichever type is used, the sensing area should be clean and free of any materials that could reduce or obstruct light reaching the sensor. 

Radiometers arerelatively unstable instruments and readings often change considerable over time. Therefore, they should be calibrated at least every six months.

System Performance Check

A system performance check is typically required daily, at the reactivation of a system after maintenance or repairs, or any time the system is suspected of being out of control. System performance checks involve processing a test specimen with known defects to determine if the process will reveal discontinuities of the size required. The specimen must be processed following the same procedure used to process production parts. The ideal specimen is a production item that has natural defects of the minimum acceptable size. As with penetrant inspections in general, results are directly dependent on the skill of the operator and, therefore, each operator should process a test specimen.

There are some universal test specimens that can be used if a standard part is not available. The most commonly used test specimen is the TAM or PSM panel which is used for fluorescent penetrant systems. 

These panels are usually made of stainless steel that has been chrome plated on one half and surfaced finished on the other half to produce the desired roughness. The chrome plated section is impacted from the back side to produce a starburst set of cracks in the chrome. There are five impacted areas with a range of different crack sizes corresponding to the five levels of sensitivity.

Care of system performance check specimens is critical. Specimens should be handled carefully to avoid damage. They should be cleaned thoroughly between uses and storage in a solvent is generally recommended. Before processing a specimen, it should be inspected under UV light to make sure that it is clean and not already producing an indication.

Nature of the Defect

The nature of the defect can have a large effect on sensitivity of a liquid penetrant inspection. Sensitivity is defined as the smallest defect that can be detected with a high degree of reliability. Typically, the crack length at the sample surface is used to definesize of the defect. However, the crack length alone does not determine whether a flaw will be seen or go undetected. The volume of the defect is likely to be the more important feature. 

The flaw must be of sufficient volume so that enough penetrant will bleed back out to a size that is detectable by the eye or that will satisfy the dimensional thresholds of fluorescence. The figure shows an example of fluorescent penetrant inspection probability of detection (POD) curve as a function of crack length.

In general, penetrant testing is more effective at finding:

 Small round defects than small linear defects.

 Deeper flaws than shallow flaws.

 Flaws with a narrow opening at the surface than wide open flaws.

 Flaws on smooth surfaces than on rough surfaces.

 Flaws with rough fracture surfaces than smooth fracture surfaces.

 Flaws under tensile or no loading than flaws under compression loading.

Health and Safety Precautions

When proper health and safety precautions are followed, liquid penetrant inspection operations can be completed without harm to inspection personnel. However, there is a number of health and safety related issues that need to be taken in consideration. The most common of those are discussed here.

Chemical Safety

Whenever chemicals must be handled, certain precautions must be taken. Before working with a chemical of any kind, it is highly recommended that the material safety data sheets (MSDS) be reviewed so that proper chemical safety and hygiene practices can be followed. Some of the penetrant materials are flammable and, therefore, should be used and stored in small quantities. They should only be used in a well ventilated area and ignition sources avoided. Eye protection should always be worn to prevent contact of the chemicals with the eyes. Gloves and other protective clothing should be worn to limit contact with the chemicals.

Ultraviolet Light Safety

Ultraviolet (UV) light has wavelengths ranging from 180 to 400 nanometers. These wavelengths place UV light in the invisible part of the electromagnetic spectrum between visible light and X-rays. The most familiar source of UV radiation is the sun and is necessary in small doses for certain chemical processes to occur in the body. However, too much exposure can be harmful to the skin and eyes.

 The greatest threat with UV light exposure is that the individual is generally unaware that the damage is occurring. There is usually no pain associated with the injury until several hours after the exposure. Skin and eye damage occurs at wavelengths around 320 nm and shorter which is well below the 365 nm wavelength, where penetrants are designed to fluoresce. 

Therefore, UV lamps sold for use in penetrant testing are almost always filtered to remove the harmful UV wavelengths. The lamps produce radiation at the harmful wavelengths so it is essential that they be used with the proper filter in place and in good condition.