What Is Dye Penetrant Testing?
Dye penetrant is a dye used by inspectors in dye penetrant inspections, an inspection method in which inspectors apply a dye or liquid to a surface to identify defects in it.
Dye penetrant is also commonly called liquid penetrant or liquid dye penetrant.
Also, dye penetrant testing is also commonly called dye penetrant inspection (DPI), liquid penetrant inspection (LPI), liquid dye penetrant testing, liquid dye penetrant inspection, liquid penetrant testing (LPT), or simply penetrant testing (PT).
In this guide, we will use the terms dye penetrant and liquid dye penetrant interchangeably, following the alternate terms listed above.
Dye penetrant is one of the most commonly used inspection methods. It falls into the non-destructive testing category of inspection methods, because inspectors can use it without permanently altering or damaging the object they’re inspecting.
[Dye penetrant NDT is just one of the non-destructive testing (NDT) methods that inspectors use. Learn more about NDT in this in-depth guide.]
Here is a menu to help you navigate this guide:
- What Is Dye Penetrant Inspection?
- Procedure for a Dye Penetrant Test
- Dye Penetrant Standards and Codes
What Is Dye Penetrant Testing?
Inspectors use liquid dye penetrant testing to look for cracks on the surface of assets and materials.
After flowing the dye penetrant onto the surface they want to inspect, inspectors will then draw the liquid out using a chalk-like developer, and thus reveal any material defects that might be present on the surface.
The procedure for a dye penetrant test relies on the principle of capillary action, which describes how fluids penetrate into cracks (or discontinuities) on the surface of a material.
Liquid dye penetrant testing is commonly used to inspect the surfaces of non-porous assets made out of ceramics, plastics, and metals, where inspectors will be looking at:
In these materials, inspectors will be using dye penetrant to look for defects like:
- Joint flaws
- Porosity on the surface of the materials
- Cracks (cracks from fatigue, hairline cracks, or grinding and quenching cracks)
- Incomplete fusion
Dye penetration inspections were first performed in the railroad industry in the early 20th century.
These early tests followed the oil and whiting method, in which a person first cleans a surface with an oil solvent and then applies chalk to the area (or another “whiting” material). The whiting would absorb oil from cracks present in the material, revealing their presence so that railroad personnel could identify and fix them.
Other industries began to adopt the liquid dye penetrant inspection method for inspections, eventually adding dye to make cracks more visible.
The Pros and Cons of Dye Penetrant Examination
Penetrant testing is a popular NDT method for inspectors because it is inexpensive and fairly easy to learn how to do.
But those are just two of its benefits—here’s a list of pros and cons for dye penetrant testing:
- Easy to perform, even with complicated surfaces/shapes
- Inexpensive—no expensive cameras or equipment are required to perform DPI
- Can be used to inspect large areas quickly
- Findings (i.e., defects identified by this method) can be seen visually on the surface of the materials and can show the dimensions of the defect
- Material flexibility—can be used on a variety of materials, including ferrous/non-ferrous, conductive/non-conductive, and magnetic/non-magnetic
- Limited findings—only detects cracks on the surface (or “open” to the surface)
- Porous materials can’t be inspected with dye penetrant
- Dirty surfaces can’t be inspected with dye penetrant—DP won’t work on surfaces that contain paint, oil, dirt, rust, or any other similar kind of obstruction
- Direct access to the material is required
- There are several steps in the inspection process, each of which could impact the quality of the findings
- Cleaning is required both before and after the inspection (before to prepare the surface for the penetrant and after to clean the penetrant off the surface)
- Chemicals are involved—inspectors must follow protocol to handle and get rid of them, and these chemicals could produce hazardous or flammable fumes
Procedure for a Dye Penetrant Test
In dye penetrant testing, inspectors generally follow these six steps:
1. Clean the surface
First, inspectors clean the surface they plan to test so that the surface is open and any defects it contains will be exposed, instead of remaining hidden underneath dirt or other foreign elements.
Cleaning processes inspectors commonly follow could include less invasive methods, like vapor degreasing, the use of solvents, or just wiping it with a wet rag, or more invasive methods, like grinding or wire brushing.
2. Apply the dye penetrant
The penetrant that inspectors use is made just for this purpose, and it’s typically sprayed or wiped onto the surface with a brush. After applying the penetrant, inspectors wait for a “dwell period” of five to twenty minutes to allow it to dry. (The right amount of time should be indicated on the label of the specific penetrant being used.)
3. Remove extra penetrant and apply remover
Remove any excess penetrant with a dry rag.
After cleaning off extra penetrant, apply a remover to the surface and rub it dry with a fresh clean, dry rag.
4. Apply developer
After cleaning and removing the dye penetrant, apply a white developer to the surface. The developer will draw the penetrant from the flaws or cracks on the surface of the material and make them visible.
At this point, cracks and other types of defects will be visible either to the naked eye or using white or ultraviolet light, depending on the type of penetrant that was used.
Now that the defects have been made visible, inspectors can conduct a visual inspection to identify any flaws that are present.
The difference between visible dye penetrant and fluorescent dye penetrant:
- For visible dye penetrants, regular lighting conditions will allow defects to be visible.
- For fluorescent penetrants, U.V. lights at low ambient lighting will allow defects to be visible.
6. Clean the surface
After the inspection inspectors typically clean the surface that was inspected to return it to its original condition.
Choosing Penetrants, Removers, and Developers
Inspectors have a few options to choose from for the penetrant, remover, and developer in the penetrant testing process.
Each of these options can be matched with any other (except for dry powder, which can’t be used with color contrast penetrant) allowing inspectors to create their own combination to suit their needs.
Here are the different options for each step:
- Color contrast
- Combination (both color contrast and fluorescent)
- Post-emulsifiable remover
- Dry powder
- Non-aqueous developer
Inspectors choose between each of these options based on factors like the type of surface they’re inspecting, the size of the surface, and complexity of the surface.
Dye Penetrant Standards and Codes
Given how inexpensive it is and how relatively easy it is to do, dye penetrant is often used by inspectors for informational purposes—that is, for inspections that do not have to comply with a specific code or set of standards.
But penetrant testing is also commonly used for code-based inspections.
For these inspections, inspectors must follow specific steps in how they conduct the test, including the requirement that they follow a written procedure and that the person conducting the test is certified to do so by the relevant standards body.
Here are some of the more widely used dye penetrant testing codes:
ASTM (American Society of Testing and Materials)
- ASTM E 165: Standard Practice for Liquid Penetrant Examination for General Industry
- ASTM E 1417: Standard Practice for Liquid Penetrant Testing
- ASME BPVC, Section V, Article 6: Liquid Penetrant Examination
- ASME BPVC, Section V, Article 24: Standard Test Method for Liquid Penetrant Examination SE-165 (this standard is identical to ASTM E-165)
ISO (International Standards Organization)
- ISO 3452-1: Non-destructive testing - Penetrant testing - Part 1. General principles
- ISO 3452-2 : Non-destructive testing - Penetrant testing - Part 2: Testing of penetrant materials
- ISO 3452-3: Non-destructive testing - Penetrant testing - Part 3: Reference test blocks
- ISO 3452-4 : Non-destructive testing - Penetrant testing - Part 4: Equipment
- ISO 3452-5 : Non-destructive testing - Penetrant testing - Part 5: Penetrant testing at temperatures higher than 50
- ISO 3452-6 : Non-destructive testing - Penetrant testing - Part 6: Penetrant testing at temperatures lower than 10
- ISO 3059: Non-destructive testing - Penetrant testing and magnetic particle testing - Viewing conditions
- ISO 12706: Non-destructive testing - Penetrant testing - Vocabulary
- ISO 23277: Non-destructive testing of welds - Penetrant testing of welds - Acceptance levels
CEN (European Committee for Standardization)
- EN 1371-1: Founding - Liquid penetrant inspection - Part 1: Sand, gravity die and low pressure die castings
- EN 1371-2: Founding - Liquid penetrant inspection - Part 2: Investment castings
- EN 10228-2: Non-destructive testing of steel forgings - Part 2: Penetrant testing
- EN 10246-11: Non-destructive testing of steel tubes - Part 11: Liquid penetrant testing of seamless and welded steel tubes for the detection of surface imperfections
Follow these links to read more about penetrant testing on the websites of internationally recognized standards organizations:
- American Society of Mechanical Engineers (ASME)
- American Society for Testing and Materials (ASTM)
- The National Board of Boiler and Pressure Vessel Inspectors (NBIC)
- International Standards Organization (ISO)
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