Why we widely use Liquid Penetration Testing (LPT) in industries?
Evaluating the quality of a test specimen is an important procedure done in every company. As we all know, NDT is the frequent technique used for it. All the NDT equipment are very expensive and purchasing of such equipment is not at all a practical solution. Among all the NDT technique Liquid Penetrant Testing (LPT) is one of the technique that a company can own reasonably.
LPT has a long term legacy. A heavy oil available in railway workshop is poured in a tank along with kerosene to make the penetrant. Heavy locomotive parts are then submerged in this solution for inspection. The submerged locomotive part is then cleaned and a layer of alcohol with a fine suspension of chalk powder is applied on its surface. After some times the alcohol gets evaporated and a white layer will appear on the surface. Finally, this part is vibrated by the hard bang of a hammer, causing the oil in discontinuity to sweep out and give a contrast with the white background
This testing method is used to find surface defects of an object by applying some immigrants like the cleaner, penetrants, developers, etc. By the application of these immigrants, the flaws will bleed out and provide an excellent indication of surface defect.
Types of LPT
- Fluorescent Dye Penetrant Testing
- Visible Dye Penetrant Testing
Components used in LPT
The cleaner is used for the pre-cleaning purpose. by means of a cleaner, we can remove dust particle, paint oil, grease, etc. The main goal employed to the cleaner is to provide a clean surface to bring out the defect open to the surface.
If there is a presence of a defect on the surface of test specimen the applied penetrant will get drawn into the surface breaking defect by capillary action the penetrant will remain in the defect but remove easily from the surface of the part.
Sweep out the penetrant trapped in the discontinuity and project it out for defect evaluation. Developer provides a contrast base with the penetrant to enhance the defect identified
Defect inspected in LPT
The sensitivity of LPT is defined as the reliability in detecting the surface defect. The crack length at the sample surface will probably define the size of the crack. If the volume of the flaw is large, the larger will be the bleed out of penetrant. Small round defects, deeper flaw, narrow opening flaw, defects on a smooth surface, flaws due to tensile load, etc are some of the main defect inspected under the guidance of LPT.