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Principle of Penetrant Testing

Principle of PT

So called PT (Penetrant Testing) or LPT (Liquid Penetrant Testing), this method can be applied to the surface flaw inspection of almost every materials and products irrespective of their stuff. When the "penetrant", vivid colored or fluorescent, is applied to the surface to be inspected, it penetrates into the surface breaking flaw and the flaw is filled with the penetrant. Then filled penetrant will be taken up from the flaw by the "developer" applied and will spread around the flaw on the surface.
Inspector easily finds the flaw observing the taken up penetrant on the surface.

Procedure of PT

Procedure of PT is usually
 "Pre-cleaning" - "Penetrating" - "Penetrant Removal (Washing)" - "Developing" - "Observation" - "Post-cleaning".

Dust, oil or other liquid on the surface to be inspected, that will prevent perfect penetration of penetrant into the flaw, is removed by petroleum solvent, organic solvent etc. before the penetrating process.

Penetrating is based on the rule called Capillary Phenomenon. Visible dye penetrant (vivid colored) or fluorescent penetrant is used for this process, that will penetrate into the surface breaking flaw.

(3)Penetrant removal (Washing)
Excess penetrant on the surface will be taken off by the remover. The method of the penetrant removal varies depending on the type of the penetrant used.

The penetrant kept in the flaw is taken up by the developer and forms flaw indication on the surface.

The surface is visually observed and the flaw indication is identified.

Penetrant flaw detection materials

Penetrant (P), Developer (D) and Remover (R) are usually used for PT.
There are two types of PT method, those are Dye penetrant PT using visible dye penetrant flaw detection material and Fluorescent PT using fluorescent penetrant flaw detection material.

Penetrant (P)

Following penetrant, of which chemical characteristics are different, is used for PT.

(1)Water washable penetrant
Since almost penetrant is oil based, it is difficult to wash it off by water. The water washable penetrant is emulsifier (surface-active agent) added penetrant in order to be water washable.

(2)Solvent removable penetrant
This is oil based penetrant and the organic solvent is used for removing excess penetrant.

(3)Post emulsifiable penetrant
This penetrant does not contain emulsifier. After penetrating, the emulsifier is applied to remove excess penetrant for water washing.


The principal content of the developer is inorganic fine powder. Three kinds of developer are used, those are Dry developer (powder type), Aqueous developer (water suspension type) and Solvent developer (aerosol, quick drying type).