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Introduction to Penetrant Testing

Improving Detection
—Visual Acuity
—Contrast Sensitivity
—Eye's Response to Light

Steps for Liquid PI
Common Uses for PI
Pros and Cons of PI

PT Materials
Penetrant Testing Matl's
—Surface Energy
—Specific Gravity
—Color and Fluorescence
   —Why things Fluoresce
—Dimensional Threshold
—Stability of Penetrants

Methods & Techniques
—Cleaning Methods
—Metal Smear
Technique Selection
Application Technique
Penetrant Removal
Selecting Developer

Quality & Process Control
System Performance Check

Other Considerations
Defect Nature
Health & Safety



Quality Control of Developer

The function of the developer is very important in a penetrant inspection. It must draw out of the discontinuity a sufficient amount of penetrant to form an indication, and it must spread the penetrant out on the surface to produce a visible indication. In a fluorescent penetrant inspection, the amount of penetrant brought to the surface must exceed the dye's thin film threshold of fluorescence, or the indication will not fluoresce. Additionally, the developer makes fluorescent indications appear brighter than indications produced with the same amount of dye but without the developer.

In order to accomplish these 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. If there are ten or more fluorescent specks in a 10 cm diameter area, the batch should be discarded.

Apply a light coat of the developer 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 or 5 psi.

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. If a scum is present or the solution fluoresces, it should be replaced. 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 immediately after the final wash. A uniform coating should be 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. Prolonged contact of the component with the developer solution should be avoided in order to minimize dilution or removal of the penetrant from discontinuities.

Solvent Suspendable (AKA Nonaqueous Wet)

Solvent suspendable developers are typically supplied in an sealed aerosol spray can. 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.