Penetrant testing (PT) or inspection is one of many non-destructive testing (NDT) techniques available. Inspectors and manufacturers can use a non-destructive test to see if a raw material, process job or welded part has any discontinuities, such as hairline fractures, indents, or other irregularities that could affect the material’s functionality.
Non-destructive testing is an excellent approach to assess materials without compromising their integrity or composition. Liquid penetrant testing is a low-cost approach for detecting inconsistencies, faults, and damage in nonporous materials, such as metals, polymers, and ceramics.
Penetrant testing is also called liquid penetrant testing (LPT), dye penetrant testing or inspection (DPT or DPI) or fluorescent penetrant testing or inspection (FPT or FPI). The key term here is ‘penetrant’ as the test or inspection done is so labelled based on the type of penetrant (specialised liquid) used – whether it’s dye (usually red in colour) or a fluorescent material.
PT is an NDT technique extensively used in inspecting various components used in the mining industry, such as ball and rod mills, conveyances, crushers, furnaces, hoists, process piping, pressure vessels and tanks.
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What Is Penetrant Testing?
The dye penetrant testing method employs certain liquids of a specific viscosity and contact angle to discover discontinuities in corrosion tests or check for material surface cracks, fractures, porosity, overlaps, and joint flaws. These discontinuities may be caused by fatigue or processes like grinding and quenching.
Liquid penetrants have the property of capillary action (aka wicking, capillarity, capillary effect or capillary motion), so they can enter fine surface discontinuities or narrow gaps (without the aid of external forces like gravity), and remain there even when the excess liquid penetrant is removed. This means that even after wiping off the extra penetrant, any liquid that has already entered a discontinuity (no matter how tight or narrow) will remain there.
A developer is then applied (for dye, non-fluorescent or visible penetrant testing, a white developer is used) to reveal the retained penetrant (e.g. red colour in dye penetrant testing) indicating the defects existing on the test surface.
The developer aids in the extraction of penetrant from the fault or flaw, allowing the inspector to see otherwise invisible (to the naked eye) gaps or discontinuities in the material being tested. Depending on the type of dye employed (whether fluorescent or non-fluorescent), inspection is done using ultraviolet or white light (visible).
[View of the fluorescence penetrant test highlighting cracks]
Penetrant Testing Steps and Procedure
To conduct penetrant testing, you will need the following materials:
- Test surface or material
- Cleaner (e.g. solvent remover, detergent, vapour degreaser, etc.)
- Penetrant (red for non-fluorescent dye penetrant testing or fluorescent)
- Cleaner or water to be used for removing any excess penetrant
- Several clean, lint-free rags or cloths
- White (for non-fluorescent dye penetrant testing) or UV (for fluorescent dye penetrant testing) light
- PPE or gloves for protection
The liquid penetrant testing technique involves applying a penetrant to a nonporous surface and determining whether the excess penetrant reveals a flaw in the material.
Below are the general liquid penetrant testing steps:
Step 1: Clean the surface of the test material
Any debris, paint, oil, grease, or loose scale that could keep the penetrant from entering a defect or which could lead to irrelevant or erroneous results should be eliminated from the test surface.
Needless to say, it’s crucial to clean the surface thoroughly so it is devoid of contaminants. This is essential because the liquid penetrant test depends on capillary action; that is, for the liquid to flow through even the narrowest of spaces. The removal of dust, rust, paint or debris helps to ensure the penetrant is able to flow freely.
Make sure you clean the surface with a lint-free cloth to avoid accidentally adding more foreign particles or contaminants onto the surface of the material being tested.
Solvents, alkaline washing, steam degreasing, and sandblasting are all examples of cleaning options.
Step 2: Apply the liquid penetrant
The next step in the penetrant inspection process involves applying the liquid penetrant. Make sure to do this only when the material to be tested has been cleaned and dried completely.
The testing proper can then begin with the application of the liquid penetrant on the material. Among the most common application methods are:
- Applying the liquid penetrant with a brush
- Using an aerosol spray
- Dipping the material in the liquid penetrant
Once applied, the red-coloured liquid used in non-fluorescent dye penetrant testing will enter fractures and other discontinuities on the surface of the material, including imperfections that are imperceptible to the naked eye.
The inspector must calculate the penetration time or the time the material should sit with the liquid penetrant after it has been applied. The timeframe usually ranges from 10 to 15 minutes, depending on the type of material and the size of the flaw. In general, dwell time typically lasts between five and 30 minutes.
Step 3: Remove the excess penetrant
The removal of excess penetrant is the third of the dye penetrant testing steps.
However, this can vary depending on the type of penetrant utilised. The more common options include washing the material with water, solvent, or by adding a lipophilic or hydrophilic post-emulsifier. The simplest way to remove excess liquid penetrant dye is by using a dry, clean, lint-free cloth.
If you’ll be using a solvent spray cleaning material, do not spray it directly on the surface. More importantly, do not do anything that could agitate or displace the liquid penetrant from the cracks or defects.
Step 4: Apply the developer
After the excess penetrant has been removed completely, it’s time to apply the developer.
Developers come in various forms. These include non-aqueous wet, dry powder, water-suspendable, and water-soluble developers. The developer used depends primarily on two factors: penetrant compatibility and inspection conditions.
The developer can be applied through different means, namely via air suspension, immersing the test material, electrostatic spraying and powder application.
When employing a non-aqueous wet developer or dry powder, for example, make sure the sample material is dried thoroughly before developer application. Meanwhile, soluble or suspendible developers need to be applied while the test piece is still damp or wet.
The developer works to create a visual indication of the discontinuities or surface inconsistencies by bringing the liquid penetrant from the faults to the surface. The dislodged sections can then be used to determine the position, orientation, and possible types of surface cracks or discontinuities.
Because of their strong contrast, red and white are the most popular colours for penetrants and developers, respectively. Penetrants used in fluorescent penetrant testing, on the other hand, contain pigments that light up when exposed to ultraviolet light.
Step 5: Inspect the test material
At this stage, the inspector checks the material to evaluate how or whether the penetrant has leaked out, thereby suggesting surface cracks, faults, or breaks. They merely clean up the residual penetrant and developer after the evaluation.
For visible dye penetrant testing, the inspector uses a visible or white light at the right intensity (e.g. (100 foot-candles or 1100 lux). When it comes to fluorescent penetrant inspections, ultraviolet radiation (UV-A) of sufficient intensity (e.g. 1,000 micro-watts per square centimetre) and low ambient lighting are used.
The inspection phase should ideally take place after 10 to 30 minutes of development time, depending on the material being tested as this allows for the blotting action to take place.
When using visible dye, make sure you observe the sample for indication formation. It is considered good practice to make a note of indications as they form because the characteristics of the bleed out are a significant part of the interpretation characterisation of discontinuities.
Step 6: Clean the test material
After interpreting, recording and evaluating the test indications, the inspector cleans the test material immediately since chemical penetrants can contaminate or harm the test object surface if the chemical stays on indefinitely.
During this step, you can use the materials you used for pretest cleaning. Ensure you use a clean and dry lint-free cloth for wiping off the cleaner.
[View cracks on a welded pipe after liquid penetrant test]
Applications of Penetrant Testing
Any solid, non-porous items, such as metals, ceramics, organic and composite materials, can be tested through liquid penetrant testing. PT is often used to detect discontinuities in materials and to find defects in castings, forgings, and welded parts, such as during corrosion tests.
Penetrant testing or inspection is extensively used to evaluate and detect surface discontinuities on components used in a range of industries, including energy or power generation, mining, aviation, aerospace engineering, manufacturing, military, petrochemicals, and construction.
Materials that can be examined with penetrant testing
Liquid penetrant testing can be used effectively on nonporous materials that are generally smooth. These include metals, glass, plastics and fired ceramics.
Materials that can’t be examined with penetrant testing
PT cannot be applied to porous materials since the pores on the surface could have the same (or even greater) depth, size or intensity of luminescence (as in the case of fluorescent PT) as the defects themselves.
The presence of pores in the test material will render the applicability and reliability of the PT defects detection process useless or futile.
Advantages of Penetrant Testing
While the application of PT is limited to the detection of surface discontinuities, this NDT testing technique also offers other advantages:
- It is highly sensitive, thereby allowing for the detection of even minute discontinuities on the material surface.
- It can be used on a wide range of non-porous materials, including metallic and non-metallic, magnetic and non-magnetic, and conductive and non-conductive materials.
- It allows for the convenient and rapid inspection of large volumes of materials.
- It can be used even on parts with irregular or complex shapes.
- It enables the inspector to observe indications directly on the surface of the part being tested.
- It is a portable NDT method where the materials needed are also readily available in aerosol form.
- The materials and equipment needed to conduct PT are low cost.
Disadvantages of Penetrant Testing
Penetrant testing is a popular NDT technique due to its affordability, portability and wide applications. However, it also comes with a few disadvantages:
- It can only be used for detecting surface-breaking defects.
- Its application is limited to non-porous materials or surfaces.
- The pre-cleaning step is highly critical as improper cleaning can leave contaminants that can mask material defects.
- All metal smearing resulting from machining, grinding, and grit or vapour blasting must be removed completely.
- Direct access to the surface of the material being inspected is required.
- The finish and roughness of the material surface can affect the accuracy of the inspection results.
- To achieve an accurate outcome, all steps in the PT process must be done sequentially and precisely.
- It is temperature sensitive and can be messy to conduct
- Pre- and post-cleaning of test parts or materials is necessary.
- It poses environmental concerns as it involves the use of chemicals.
Australian Standards for Penetrant Testing – AS ISO 3452.1:2020
The AS ISO 3452.1:2020 section of ISO 3452 describes a technique of penetrant testing designed to detect discontinuities that are open to the surface of the test material. These imperfections include cracks, laps, folds, porosity, and the absence of fusion.
It also provides standards for process and control testing, but it is not meant to be used as acceptance criteria. It does not include information on the viability of various test systems for specific applications or test equipment requirements.
Penetrant Testing and Other NDT Services by AME
Companies, industries and other establishments that require root cause analysis and NDT testing services in Perth on pressure equipment and other materials can depend on the extensive experience and expertise of Asset Management Engineers (AME).
Whether it’s penetration testing, eddy current testing, ultrasonic testing or magnetic particle testing you require, you can achieve accurate results with AME because we only use innovative top-of-the-line equipment and materials.
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