Liquid Penetrant Inspection is a method of NDT that was developed in the 1940s. Although times have changed today and newer and more innovative methods have been introduced to make it more efficient and accurate, the basic principles remain the same.
Because Liquid Penetrant Inspection is non-destructive, meaning it does not harm the inspected samples, it has proven highly effective in detecting flaws such as porosity, cracks, seams, fractures and laps. These flaws can be caused by fatigue, impact, overload, shrinkage, machining, quenching, grinding, forging or bursting. This makes Liquid Penetrant Inspection have many applications, such as being used for machined parts, weldments, castings, forgings, among other manufactured products that are put into service.
NDT is the acronym for Non Destructive Testing, which is a branch of engineering that is associated with methods of testing for, detecting and evaluating material flaws. NDT is important because these flaws are the primary reason why the material or the structure fails or becomes less serviceable. They can be cracks, inclusions or structural property variations. Through NDT, safety, quality control and longer plant life can be guaranteed.
It is important to note that Non-Destructive Testing is used for in-service inspection, component measurement and physical property measurements. NDT works well because it has no impactful effects on the structure or material being tested. It can be carried out in various ways, including visual examination, radiography, ultrasonic tests, positron annihilation and magnetic particle crack detection. Furthermore, it can be automated or used in localised problematic areas.
NDT is simple and cost-effective, but it can only detect surface breaking flaws like cracks, laps and porosity. To be able to detect it, the flaw must be relatively close to the surface or reach the surface itself.
Liquid Penetrant Testing is a method of NDT that improves upon visual inspection. It is faster, covers a larger area, and is relatively cheap.
When carrying out Liquid Penetrant Testing, the process occurs in 3 stages:
1. Surface cleaning
This is the stage where you clean the surface that needs to be tested in preparation for the test. You want to remove all the oil, dirt, grease, liquid, paint, scale or any other contaminant that may stand in the way preventing the penetrant from seeping into the flaws. The surface is de-greased, washed, wiped clean, or chemically cleaned among other cleaning methods. If the surface to be tested has been machined or sanded, it may need to be further etched to remove any contaminants. Surface cleaning is an essential step in the process of liquid penetrant testing. It can be done manually, semi-automatically or completely automatically.
2. Application of Penetrant Liquid
The penetrant liquid is then applied onto the surface. This can be done by dipping, spraying or brushing. You can also choose to dip the surface or component in a penetrant bath. The choice of application technique depends on:
– Material surface finish
– Compatibility of the penetrant and the material
– The degree of desired sensitivity
– The size of the surface to be tested
– The shape and accessibility of the surface to be tested
– The use or function of the component being inspected
– The equipment available for testing
Penetrant liquids can be categorized into three groups. The first group is the water-soluble penetrants, which, as the name suggests, are soluble in water. The next group contains all the penetrants that are post emulsifiable with water rinsing. Finally, the third group are the penetrants that are solvent removable. As you may have noticed, the primary difference between these three groups is the method used to remove excess penetrant.
3. Removal of excess penetrant liquid
After a specified period of time, the excess penetrant liquid is removed. The dwell time of the penetrant is usually a matter of preference, as long as it is not too long or too short. The type of penetrant used can also dictate how much time is needed. The surface finish, specifications of the test, and the type of material will also influence dwell time requirements. When removing the excess penetrant liquid, depending on the type of penetrant used, this is usually done using water or an acceptable solvent.
4. Developer application
The developer is then applied, which helps highlight problematic areas. This is the stage where the flaws will be revealed. Developers are typically dry powders, but there are some that are liquid solutions. Dry powders may be dusted onto the surface while wet developers can be lightly sprayed. Again, the type and method of developer application depend on:
– Material surface finish- Compatibility of the developer, the penetrant and the material- The degree of desired sensitivity- The size of the surface to be tested- The shape and accessibility of the surface to be tested- The use or function of the component being inspected
5. Surface Inspection
Once the developer has been applied, you will have to wait for a short amount of time before the surface needs to be inspected. This can be done visually or using cameras and other visual assistance equipment. The recording will then be replayed and problematic areas identified and marked out.
During the inspection, cracks, laps or bursts appear as continuous lines or broken dotted line marks. On the other hand, porosity in the material, shrinkage, improper bonding and leakages will appear as random dots or large localised areas of colour.
It is important to note that some instances require the inspection to be done by trained and certified professionals. They will carry out their visual inspections under a black light, UV lamp or white light, depending on the penetrant and the developer used. After the inspection is complete, the sample can either be rejected or accepted depending on the requirements. If the sample is rejected, the inspector needs to determine the cause of the flaw wherever possible.
Always verify that the inspectors you are using are qualified and certified to do the job. The laboratory you use also needs to have all the required certifications, and you need to establish whether there are any needed customer or end user approvals.
6. Post Inspection
After the inspection has been completed, cleaning of the surface needs to be done. Anti-corrosion solutions may be required at this stage.
The penetrant solution used usually contains a dye that makes the indication visible under white light. It may also contain a fluorescent material that lights up under ultraviolet light, typically UV-A. Fluorescent penetrants are much more costly, though, and should only be used when the maximum flaw sensitivity is required.
Similarly, if you require extremely high sensitivity, you can use radioactive tracers. However, these are highly dangerous and require the use of specialized testing teams and very stringent application and testing procedures. This, the use of radioactive tracers in Liquid Penetrant Testing is extremely rare.
Liquid Penetrant Testing works under a very simple principle. The liquid penetrant is first drawn into the breaches or cracks on the surface of the material through capillarity. The excess penetrant is then removed, leaving the bit trapped in the problem areas. When a developer is added, it draws out the penetrant in the cracks and produces a surface indication of the problem. This is usually in the form of glowing lines that show up under special lights such as UV lights. The cracks on these surfaces can sometimes be very tiny, sometimes merely 150 nanometers wide. However, the indicators show up as much wider lines, which makes the flaws more visible and easier to identify.
Liquid Penetrant Testing can be applied to almost any non-porous surface. It does not work if the surface is porous or absorbs the penetrant. Similarly, the surface has to be clean. It can either be metallic or non-metallic, such as glass, plastics and ceramics, as long as it is not dirty or very rough. Very rough surfaces tend to trap the penetrant in the little fissures on their surfaces, giving false readings.
Liquid Penetrant Testing can also be automated. There are production lines where the units are cleaned, dipped in penetrant, washed, dried, and developed automatically. A recent development has also been the introduction of robotic inspectors, where robots are programmed to scan the surface of the material using cameras and through pattern recognition, identify flaws on the surface. Similarly, image enhancement techniques for the convenience of human inspectors have also been introduced.
Here are a few pro tips:
1. Always check the required specifications for your industry before you begin inspection
2. If you can, buy all your penetrant products in kits, This will massively ease portability and convenience.
2. Application of penetrant and penetrant dwell time
3. Penetrant removal
4. Application of developer and developer dwell time
1. Do Fluorescent Liquid Penetrants Have Different Sensitivities?
Yes. Fluorescent liquid penetrants are classified into four categories (1-4) depending on sensitivity. Category 1 fluorescent liquid penetrants are the least sensitive of the group, while category 4 fluorescent liquid penetrants are the most sensitive, being able to detect even the smallest flaws.