Liquid penetrant testing (LPT)
This is a method which can be employed for the detection of open-to-surface discontinuities in any industrial product which is made from a non-porous material. This method is widely used for testing of non-magnetic materials. In this method a liquid penetrant is applied to the surface of the product for a certain predetermined time, after which the excess penetrant is removed from the surface. The surface is then dried and a developer is applied to it. The penetrant which remains in the discontinuity is absorbed by the developer to indicate the presence as well as the location, size and nature of the discontinuity. The process is illustrated in Figure 1.2.
Penetrants used are either visible dye penetrant or fluorescent dye penetrant. The inspection for the presence of visible dye indications is made under white light while inspection of presence of indications by fluorescent dye penetrant is made under ultraviolet (or black) light under darkened conditions. The liquid penetrant processes are further sub-divided according to the method of washing of the specimen. The penetrants can be:
(i) water-washable, (ii) postemulsifiable,
i.e. an emulsifier is added to the excess penetrant on surface of the specimen to make it water-washable, and (iii) solvent removable, i.e. the excess penetrant is needed to be dissolved in a solvent to remove it from the test specimen surface. In order of decreasing sensitivity and decreasing cost, the liquid penetrant processes can be listed as:
(1) Post emulsifiable fluorescent dye penetrant.
(2) Solvent removable fluorescent dye penetrant.
(3) Water washable fluorescent dye penetrant.
(4) Post emulsifiable visible dye penetrant.
(5) Solvent removable visible dye penetrant.
(6) Water washable visible dye penetrant.
Some of the advantages of liquid penetrant testing are as follows:
(1) Relatively low cost.
(2) Highly portable NDT method.
(3) Highly sensitive to fine, tight discontinuities.
(4) Fairly simple method.
(5) Can be used on a variety of materials.
(6) All surface discontinuities are detected in one operation, regardless of orientation.
Some of the limitations of liquid penetrant testing are as follows:
(1) Test surface must be free of all contaminants (dirt, oil, grease, paint, rust, etc.).
(2) Detects surface discontinuities only.
(3) Cannot be used on porous specimens and is difficult to use on very rough surfaces.
(4) Removal of all penetrant materials, following the test, is often required.
(5) There is no easy method to produce permanent record.