Amongst all the non-destructive testing methods, there are only a few that are suitable for all kinds of applications. For example, on the most complex components, electromagnetic testing methods might not be useful, whereas using radiographic testing procedures might help. Similarly, ultrasounds are adjustable, nonetheless, they require proper coupling and straight contact with the surface being tested, which might be possible every time. Further, we will look at one of the simplest and easiest non-destructive testing methods is Liquid penetrant testing (LPT).
Liquid penetrant testing (LPT) which is also called liquid penetrant inspection (LPI) or dye penetrant inspection is a popular and cost-effective solution. LPT test is one of the oldest and simplest methods used in the non-destructive testing technique.
The initial use of LPT testing was documented in the railroad industry. Cast railroad wheels were immersed in used oil, then it was dried up. After that, the cast railroad wheel was layered with powdered chalk or dissolution of chalk in alcohol. As soon as the wheels were dry if any oil that remained in the fault would draw out into the chalk and the defect would be identified.
LPT testing methods make use of surface tension which is the capacity of the liquid to move into narrow gaps without any aid, all the more so in opposition to, outer forces like gravity, to spot surface-breaking flaws. LPT test methods can be manual, semi-automatic, or fully automated.
LPT test is used to identify any surface-connected gaps like fissures from weariness, grinding, and quenching, in addition to porosity, faults in joints, unfinished fusion, and fractures. LPT testing method is used in the aerospace industry, metalworking shops, power generation, and in any procedure that utilizes any kind of metal linking technique like casting. It is applicable in the petrochemical industry to prove weldments and to examine piping.
Let’s understand the basic principles of LPT testing.
Basic Principles of LPT Testing
LPT testing is based on the principle of capillary action. In the LPT test, a specifically prepared liquid enters into a clean and dry surface gap. The penetrant is put on the component to be tested by either spraying, dipping, or brushing. Subsequently, some time is given for the penetrant to disseminate.
After a while, the excessive penetrant is wiped off and the developer is spread over the component. The developer assists in dragging out any penetrant that is left in the fault so that it is easily visible to the LPT test inspector. LPT testing is executed under ultraviolet light but it can also be carried out under white light, based on the kind of dye used and sensitivity needed.
After understanding the few basic principles of the LPT test. We now would want to analyze it. LPT testing although being one of the popular testing methods in non-destructive testing has its own set of advantages and disadvantages. In the coming section, we will evaluate both its benefits and limitations.
Steps involved in LPT testing
Based on the basic principles of LPT testing, it can be divided into six-stage inspection processes. This includes:
1. Cleaning the surface
2. Application of the liquid penetrant (using brush, spray, or dipping it)
3. Taking off extra penetrant liquid using solvent or water
4. Applying developer
5. Examining the surface being tested with the help of visual presentation
6. Cleaning the surface after inspection.
Advantages of LPT test
● The use of the LPT testing method is cost-effective that is the equipment and material required are comparatively reasonable.
● LPT test provides a rapid solution to conduct an inspection of components with greater areas and volumes.
● Even the slightest discontinuities can be identified using LPT testing as it has high sensitivity.
● LPT test is preferable for a component that has complex shapes.
● The devices and equipment used in the LPT testing method are extremely portable as they are also offered in aerosol spray cans.
● LPT testing method is economical, adaptable, and necessitates a nominal level of training as compared to other non-destructive testing methods. There is a high range of materials that can be inspected with the help of LPT testing like magnetic and non-magnetic materials, conductive and non-conductive materials, and metallic and non-metallic materials.
● Indications of the defect are directly observable on the surface of the components and comprise a visual depiction of the defect.
● LPT tests can be easily conducted without any hassles.
● Further inspection of indication can be done.
Disadvantages of LPT test
● Using the LPT testing method, only surface-related defects can be located.
● The inspection is limited to only a few kinds of materials specifically with the comparatively nonporous surface.
● The pre-requisite for LPT testing is that the components should be cleaned before the inspection as pollutants can cover up the faults.
● Metal coating from grinding, grit, or vapor blasting and machining should be taken out.
● The surface that is being tested using the LPT test should be directly accessible.
● As LPT tests are highly sensitive, any surface roughness or finish can lead to fault inspection.
● It is essential to execute and regulate numerous process operations.
● The parts that have undergone testing and are found to be intact are required to be cleaned after the inspection is complete.
● LPT testing technique requires proper handling of chemicals and their disposal.
● There is a possibility of a faulty interpretation of the flaws.
● Even though the LPT testing methods are less costly, require cost-friendly material, and less training, it acts as a screening tool. The operator can find the length and identify the flaws nonetheless, using the LPT test it is not possible to assess the development of the defect or know the severity of its depth.
● The method is restricted to only pass or fails evaluation that can bring about the removal of acceptable parts and saving defective parts that can lead to a drastic increase in the expenses.
● Fumes can be dangerous and combustible without proper aeration.
It is necessary to keep in mind that the penetrant used for testing is an extremely thin liquid created to leak into even a very small fissure. Subsequently, if any machinery has sewed weld or material that is not sealed by a weld, the penetrant will move at the bottom of the welds and between coats of unfused material. As a result, it will become almost impossible to extract penetrants from these places. The stuck penetrant will more likely lead to other flaws in the welds that will require further welding to be implemented or it might draw out over time and pollute the process and paint solutions.
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