Chapter 2:- DPT Principal And Methods.

?Liquid penetrant testing is a nondestructive means of locating surface discontinuities based on capillarity or capillary action.

?In the liquid penetrant method, the liquid is applied to the surface of the specimen, and sufficient time is allowed for penetration of surface discontinuities. If the discontinuity is small or narrow, as in a crack or pinhole, capillarity assists the penetration.

?After sufficient time has passed for the penetrant to enter the discontinuity, the surface of the part is cleaned, capillary action is again employed to act as a blotter to draw penetrant from the discontinuity

.?To insure visibility, the liquid penetrant contains either a colored dye easily seen in white light, or a fluorescent dye visible under black (UV-A) light.

?Discontinuities that are subsurface in one stage of production could be open to the surface at another stage, such as after grinding or machining.

?Nonmetallic inclusions and porosity in the ingot may cause stringers, seams, forging laps, cold shuts, and the like as the billet or slab is processed in the manner shown below

:?Anything that could block the penetrant from entering the discontinuity must be removed.

?A list of contaminants that must be removed would include dirt, grease, rust, scale, acids, and even water.

?The cleaning solvent used must be volatile (readily vaporized) so that it easily evaporates out of the discontinuity and does not dilute the penetrant.

?Surface preparation by shot or sandblasting is not recommended.

?Discontinuities that were open to the surface may be closed by the shot or sandblasting.

?Historically, penetrant inspection was called the "oil and whiting method," as it used kerosene and a white powder for the inspection of railroad parts. However, in the past 40 years the process has been improved tremendously to the point where it is a reliable and accurate inspection technique.

?The liquid penetrants used in nondestructive testing can be categorized by the type of dye they contain,

1.Visible dye penetrants contain a colored (usually red) dye.

2.Fluorescent penetrants contain a fluorescent dye.

3.Dual sensitivity penetrants contain a combination of visible and fluorescent dyes.

?Penetrants can be further categorized by the processes used to remove the excess penetrant from the specimen.

1.Water-washable penetrants are either self-emulsifying or removable with plain water.

2.Post-emulsified penetrants require a separate emulsifier to make the penetrant water washable.

3.Solvent-removable penetrants must be removed with a solvent which is typical when using visible dye in pressurized spray cans.

?The flow chart below illustrates the processing sequence with visible dye and fluorescent penetrants.

?The selection of the best process, as listed on pages above both charts, depends upon:

1.Sensitivity required.

2.Number of articles to be tested.

3.Surface condition of part being inspected.

4.Configuration of test specimen.

5.Availability of water, electricity, compressed air, suitable testing area, etc.

?Penetrant testing is successfully used on metals such as aluminum, magnesium, brass, copper, cast iron, stainless steel, titanium, and most other common alloys.

?It can also be used to test other materials, including ceramics, plastics, molded rubber, powdered metal products, or glass.

?Penetrant testing is limited by its inability to test materials with discontinuities that are not open to the surface or having an extremely porous surface.

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