Liquid Penetrant Inspection also is known as Dye penetrant is an inspection method used for checking cracks or leakage in non-porous materials. This method is commonly used since it is cheaper. Liquid penetrant inspection is used with both ferrous and non-ferrous materials such as plastic, ceramics, and metals. This method detects defects such as cracks, leakage, forging and casting.

How it works

The process uses the principle of capillary action. The penetrant is applied to a test component then it is given enough time for penetration. The penetrant will flow through the cracks and then the excess component is removed. A developer is applied to make the penetrant visible on the test component. Inspection is done on the test component either via white light or ultraviolet depending on the penetrant used.

The Inspection Process

Cleaning – This is the first step in the inspection process. It involves the removal of paint, dirt or grease that might hinder observation during the inspection process. Cleaning can be done using solvents, media blasting or vapour degreasing.

Application of penetrant – This should be done carefully on the surface of the test item. Give the penetrant enough time for enough penetration. It should take 5-30 minutes for the penetrant to penetrate the item being tested. This will differ in various situations depending on the material and the penetrant being used. 

Excess penetrant removal – The excess penetrant should be removed to prevent altering of the inspection results. There are different methods of removing the excess penetrant from the item being tested. Each method will depend on the penetrant used. Emulsifiers are the common choice since they will work with many penetrants. Types of penetrants include lipophilic, solvent removable, water-washable and hydrophilic.

Application of developer – Examples of developers include water-suspendable, dry powder, water-soluble and non-aqueous wet developer. The choice of the developer will depend on its compatibility with the penetrant. This is due to the differences in solubility of penetrants. The main role of the developer is to draw penetrant from the defects into the surface. This forms visible indications called bleed-out which are used to indicate the type of defect mostly when using an LPI penetrant line.

Inspection -Fluorescent penetrant examinations will require ultraviolet light for carrying out the inspection. Dye penetrant uses visible light that has an adequate intensity of about 1100 Aux. Other than this the bleed-outs should be used to interpret the possible defects types on the tested material. 

Post cleaning – This is the last step that should follow after the recording of test findings. The tested material should be thoroughly cleaned after the process to facilitate efficient future liquid penetrant inspections.

There are many advantages of using DP inspection one being quicker testing times and this method has similar attributes with NDT supplies [non-destructive testing]. Contact ATH NDT on 01282 842624 to discuss your requirements.