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Penetrant types


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Introduction This module is intended to provide an introduction to the NDT method of penetrant testing. Penetrant Testing, or PT, is a nondestructive testing method that builds on the principle of Visual Inspection. PT increases the “seeability” of small discontinuities that the human eye might not be able to detect alone.


Outline General Introduction Penetrant Materials and Considerations Basic Steps in Penetrant Testing Common Equipment Advantages and Limitations Summary Glossary of Terms

How Does PT Work?:

How Does PT Work? In penetrant testing, a liquid with high surface wetting characteristics is applied to the surface of a component under test. The penetrant “penetrates” into surface breaking discontinuities via capillary action and other mechanisms. Excess penetrant is removed from the surface and a developer is applied to pull trapped penetrant back the surface. With good inspection technique, visual indications of any discontinuities present become apparent.

What Makes PT Work?:

What Makes PT Work? Every step of the penetrant process is done to promote capillary action. This is the phenomenon of a liquid rising or climbing when confined to small openings due to surface wetting properties of the liquid. Some examples: Plants and trees draw water up from the ground to their branches and leaves to supply their nourishment. The human body has miles of capillaries that carry life sustaining blood to our entire body.

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Basic Process of PT 1) Clean & Dry Component 4) Apply Developer 2) Apply Penetrant 3) Remove Excess 5) Visual Inspection 6) Post Clean Component

What Can Be Inspected Via PT?:

What Can Be Inspected Via PT? Almost any material that has a relatively smooth, non-porous surface on which discontinuities or defects are suspected.

What Can NOT be Inspected Via PT?:

What Can NOT be Inspected Via PT? Components with rough surfaces, such as sand castings, that trap and hold penetrant. Porous ceramics Wood and other fibrous materials. Plastic parts that absorb or react with the penetrant materials. Components with coatings that prevent penetrants from entering defects. Defect indications become less distinguishable as the background “noise” level increases.

What Types of Discontinuities Can Be Detected Via PT?:

What Types of Discontinuities Can Be Detected Via PT? All defects that are open to the surface. Rolled products-- cracks, seams, laminations. Castings--cold shuts, hot tears, porosity, blow holes, shrinkage. Forgings– cracks, laps, external bursts. Welds– cracks, porosity, undercut, overlap, lack of fusion, lack of penetration.

Choices of Penetrant Materials:

Choices of Penetrant Materials Penetrant Type I Fluorescent II Visible Method A Water Washable B Postemulsifiable - Lipophilic C Solvent Removable D Postemulsifiable - Hydrophilic Developer Form Dry Powder Wet, Water Soluble Wet, Water Suspendable Wet, Non-Aqueous

Penetrant Materials:

Penetrant Materials Penetrants are formulated to possess a number of important characteristics. To perform well, a penetrant must: Spread easily over the surface being inspected. Be drawn into surface breaking defects by capillary action or other mechanisms. Remain in the defect but remove easily from the surface of the part. Remain fluid through the drying and developing steps so it can be drawn back to the surface. Be highly visible or fluoresce brightly to produce easy to see indications. Not be harmful to the inspector or to the material being tested.

Sensitivity Levels:

Sensitivity Levels Penetrants are also formulated to produce a variety of sensitivity levels. The higher the sensitivity level, the smaller the defect that the penetrant system is capable of detecting. The five sensitivity levels are: Level 4 - Ultra-High Sensitivity Level 3 - High Sensitivity Level 2 - Medium Sensitivity Level 1 - Low Sensitivity As the sensitivity level increases, so does the number of non relevant indications. Therefore, a penetrant needs to be selected that will find the defects of interest but not produce too many non relevant indications.

Visible Vs Fluorescent PT:

Visible Vs Fluorescent PT Inspection can be performed using visible (or red dye) or fluorescent penetrant materials. Visible Pt is performed under white light while fluorescent PT must be performed using an ultraviolet light in a darkened area. All are all in the level 1 sensitivity range. Fluorescent PT is more sensitive than visible PT because the eye is more sensitive to a bright indication on a dark background. Sensitivity ranges from 1 to 4. Photo Courtesy of Contesco

Why is Visible Penetrant Red and Fluorescent Penetrant Green?:

Why is Visible Penetrant Red and Fluorescent Penetrant Green ? Visible penetrant is usually red because red stands out and provides a high level of contrast against a light background Fluorescent penetrant is green because the eye is most sensitive to the color green due to the number and arrangement of the cones (the color receptors) in the eye.

Penetrant Removal Method:

Penetrant Removal Method Penetrants are also classified by the method of removing the excess penetrant . Solvent Removable penetrants are removed by wiping with a cloth dampened with solvent. They are supplied in aerosol cans for portability and are primarily used for spot checks. Water Washable penetrants are removed with a course spray of water. They are the easiest to employ and most cost effective when inspecting large areas. Post-Emulsifiable penetrants are water-washable only after they have reacted with an emulsifier solution. A post-emulsifiable system is used when washing the penetrant out of the defect is a concern. The emulsifier is given time to reacts with the penetrant on the surface but not the penetrant trapped in the flaw.


Developers The role of the developer is to pull trapped penetrant out of defects and to spread it out on the surface so that it can be seen. Also provides a light background to increase contrast when visible penetrant is used. Developer materials are available in several different forms Dry Powder is a mix of light fluffy powder that clumps together where penetrant bleeds back to the surface to produces very defined indications. Wet, Water Suspendable is a powder that is suspended in a water that covers the surface with a relatively uniform layer of developer when the water is evaporated. The solution is somewhat difficult to maintain as the powder settles out over time. Wet, Water Soluble is a crystalline powder that forms a clear solution when mixed with water. The solution recrystallizes on the surface when the water is driven off. Indications sometimes lack definition and look milky. Not recommended for use with water-washable penetrants. Wet, Non-Aqueous - is supplied in a spray can and is the most sensitive developer for inspecting small areas. It is too costly and difficult to apply to large areas.

6 Steps of Penetrant Testing:

6 Steps of Penetrant Testing 1. Pre-Clean 2. Penetrant Application 3. Excess Penetrant Removal 4. Developer Application 5. Inspect/Evaluate 6. Post-clean

Pre-cleaning – Step 1:

Pre-cleaning – Step 1 Pre-cleaning is the most important step in the PT process!!! Parts must be free of dirt, rust, scale, oil, grease, etc. to perform a reliable inspection. The cleaning process must remove contaminants from the surfaces of the part and defects, and must not plug any of the defects.

Caution About Metal Smearing:

Caution About Metal Smearing Some machining, surface finishing and cleaning operations can cause a thin layer of metal to smear on the surface and prevent penetrant from entering any flaws that may be present. Etching of the surface prior to inspection is sometimes required. Before Sanding After Sanding After Etching

Penetrant Application – Step 2:

Penetrant Application – Step 2 Many methods of application are possible such as: Brushing Spraying Dipping/ Immersing Flow-on And more

Dwell Time:

Dwell Time The penetrant solution must be allowed to “dwell” on the surface of the part to allow the penetrant time to fill any defects present. The dwell time vary according to penetrant type, temperature, material type and surface finish.

Excess Penetrant Removal – Step 3:

Excess Penetrant Removal – Step 3 The removal technique depends upon the type of penetrant used, as stated earlier… Solvent Removable Water Washable Post Emulsifiable

Excess Penetrant Removal – Step 3 (cont.):

Excess Penetrant Removal – Step 3 (cont.) Water Washable A coarse water spray is used to remove the excess penetrant. The procedure used as a guideline for the inspection will specify water temperature (typically 50-100°F) and pressure (typically not more than 40 psi), etc.

Excess Penetrant Removal – Step 3 (cont.):

Solvent Removable The part is wiped with a clean dry cloth to remove the bulk of the excess penetrant. Then, a cloth lightly dampened with solvent is used to remove any remaining penetrant on the surface. Excess Penetrant Removal – Step 3 (cont.)

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Solvent Removable (cont.) Any time a solvent is used in the penetrant inspection process, a suitable flash time is required to allow excess solvent to evaporate. Excess Penetrant Removal – Step 3 (cont.)

Excess Penetrant Removal – Step 3 (cont.):

Excess Penetrant Removal – Step 3 (cont.) Post Emulsifiable When there is concern about removing much of the penetrant from the defect, a post emulsifiable system is used. This involves an additional step in which an emulsifier is applied to the surface of the part after the penetrant dwell time. The emulsifier is given just enough time to react with the penetrant on the surface to render it water washable but not enough time to diffuse into the penetrant trapped in the defects.

Developer Application – Step 4:

Developer Application – Step 4 The method of developer application is is dependent on the type of developer used. The primary methods for the following main developer types will be covered in the following slides. Dry Wet Nonaqueous Wet

Developer Application – Step 4 (cont.):

Developer Application – Step 4 (cont.) Dry Powder Developer Prior to applying a dry powder developer, the component must be thoroughly dried. Drying is usually accomplished in a hot air circulating oven. The developer is then applied by immersing the part in the powder or by dusting of the part with the powder. The part can also be placed in a developer dust cloud chamber.

Developer Application – Step 4 (cont.):

Developer Application – Step 4 (cont.) Wet developers are applied by immersing or spraying the part while it is still wet from the penetrant removal process. The part is completely coated and the excess liquid allowed to drain to prevent pooling The part is then dried in a hot air circulating oven. Wet Developer (water- suspended and water- soluble)

Developer Application – Step 4 (cont.):

Developer Application – Step 4 (cont.) Nonaqueous Developer (AKA Solvent-Suspended) Nonaqueous developer is applied by a aerosol spray to a thoroughly dried and cooled part. A thin even coating should be applied. The coating should be white but still slightly transparent when performing a visible dye penetrant inspection, and even thinner when performing a fluorescent penetrant inspection.

Inspection/Evaluation – Step 5:

Inspection/Evaluation – Step 5 In this step the inspector evaluates the penetrant indications against specified accept/reject criteria and attempts to determine the origin of the indication. The indications are judged to be either relevant, non-relevant or false. Non-relevant weld geometry indications Relevant crack indications from an abusive drilling process

Inspection/Evaluation – Step 5:

Inspection/Evaluation – Step 5 A very important step of evaluation is to document findings on an inspection report form or other record keeping form. This may be supported with drawings or photos of indications, etc.

Post Clean – Step 6:

Post Clean – Step 6 The final step in the penetrant inspection process is to thoroughly clean the part that has been tested to remove all penetrant processing materials. The residual materials could possibly affect the performance of the part or affect its visual appeal.

Penetrant Inspection Systems:

Penetrant Inspection Systems Penetrant systems can be highly portable or stationary. Portable Penetrant System Stationary Penetrant System Image courtesy of Nebraska Army National Guard

Verification of Penetrant System Performance:

Verification of Penetrant System Performance Since penetrant testing involves multiple processing steps, the performance of the materials and the processes should be routinely checked using performance verification tools, which include: TAM Panels Crack Sensitivity Panels Run Check Panels

PowerPoint Presentation:

Advantages of Penetrant Testing Relative ease of use. Can be used on a wide range of material types. Large areas or large volumes of parts/materials can be inspected rapidly and at low cost. Parts with complex geometries are routinely inspected. Indications are produced directly on surface of the part providing a visual image of the discontinuity. Initial equipment investment is low. Aerosol spray cans can make equipment very portable.

PowerPoint Presentation:

Limitations of Penetrant Testing Only detects surface breaking defects. Requires relatively smooth nonporous material. Precleaning is critical. Contaminants can mask defects. Requires multiple operations under controlled conditions. Chemical handling precautions necessary (toxicity, fire, waste). Metal smearing from machining, grinding and other operations inhibits detection. Materials may need to be etched prior to inspection. Post cleaning is necessary to remove chemicals.


Summary Penetrant testing (PT) is one of the most widely used nondestructive testing methods. Its popularity can be attributed to two main factors, which are its relative ease of use and its flexibility. However, PT involves a number of processing steps that must be closely control to achieve optimal sensitivity.

Glossary of Terms:

Glossary of Terms Capillary Action - the tendency of certain liquids to travel or climb when exposed to small openings. Contrast - the relative amount of light emitted or reflected between and indication and its background. Defect - a discontinuity that effects the usefulness of a part or specimen. Developer - a finely divided material applied over the surface of a part to help promote reverse capillary action and thus bring out a penetrant indication.

Glossary of Terms:

Glossary of Terms Discontinuity - any interruption in the normal physical structure of a part or weld. It may or may not affect the usefulness of a part. Dwell Time - the period of time that a penetrant or developer must remain in contact with the surface of a part under test. Emulsification Time - the time allowed for the emulsifier to render the penetrant water washable and thus allow the part to be washed. Emulsifier - a material applied over a film of penetrant that renders it water washable.

Glossary of Terms:

Glossary of Terms Evaluation - the process of deciding as to the severity of the condition after an indication has been interpreted. False Indication - an indication caused by improper processing; not caused by a relevant or non-relevant condition. Flash Time - the time required for the solvent to evaporate from the surface of a part when used to preclean or remove excess penetrant. Fluorescent Dye - a dye which becomes fluorescent (gives off light) when exposed to short wave radiation such as ultraviolet light.

Glossary of Terms:

Glossary of Terms Indication - the visible evidence or penetrant bleed-out on the surface of the specimen Interpretation - the process of evaluating an indication in an attempt to determine the cause and nature of the discontinuity. Non-Aqueous Developer - a developer in which developing powder is applied as a suspension in a quick drying solvent Penetrant - a liquid used in fluorescent or visible dye penetrant inspection to penetrate into the surface openings of parts inspected via these methods

Glossary of Terms:

Glossary of Terms Relevant Indication - an indication that has been determined not to be false or non-relevant - and actual discontinuity Seeability - the characteristic of an indication that enables it to be seen against the adverse conditions of background, outside light, etc. Sensitivity - the ability of a penetrant to detect surface openings. Higher sensitivity indicates smaller discontinuities can be detected Ultraviolet Light (or Black Light) - light energy just below the visible range of violet light (356 nanometers).

Glossary of Terms:

Glossary of Terms Viscosity - the resistance of a fluid to the motion of its particles Washability - the property of a penetrant which permits it to be cleaned from the surface of a part by washing with water

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