Delusions about Dye Penetrant Testing (DPT)

Liquid penetrant inspection (LPI), also known as dye penetrant inspection (DPI) or penetrant testing (PT), was first developed in the early 1940s to detect flaws on the surface of materials. Dye is drawn into any surface breaking defects which are then highlighted by the application of a developer which draws the dye back out of the defect.

Let us now look at some of the misconceptions about DPT

Misconception: Penetrant with high sensitivity is always better

Its not always necessary to use the penetrant which has high sensitivity. While choosing the sensitivity we need to consider surface finish of the job & work procedure for the same. A high sensitivity level fluorescent penetrant is appropriate for smooth, highly machined surfaces whereas lower sensitivity fluorescent penetrant is a better choice for rough surfaces.

Misconception: All the discontinuities obtained are defects

Firstly, we need to understand the difference between a discontinuity & defect. Discontinuity is an objective lack of material, an interruption in the physical consistence of a part while a Defect is an excessive condition, outside the acceptance limits, which risks to compromise the stability or the functionality of the job. So, a defect or discontinuity can only be defined by the design or manufacturing team as what is acceptable and what should be rejected.

Misconception: DPT is only done on non-ferrous metals

DPT can be done on any kind of material weather ferrous or nonferrous the only material on which it cannot be performed are porous material.

Misconception: Water washable penetrants are water based

Not all the water washable penetrants are water based, it’s just that water washable penetrants contain surfactants which help in removing the penetrant from the surface regardless of penetrant being a water or oil based.

Misconception: Only penetrant is needed to perform DPT

One will at least need a set of penetrants, developer & cleaner when performing a DPT.

Misconception: UV light or black light is always required for DPT

Visible dye penetrant test can be done in day light or white light with at least 100 fc (1000 lx) of visible light when measured at the examination surface. Whereas Fluorescent dye penetrant requires a UV light with a minimum of 1000 µW/cm2 at the examination surface in a dark room.

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Misconception: Post cleaning is not much important

Components shall be cleaned after examination to remove developers and other examination material residues if these are detrimental to subsequent operations, or the components intended function.

Misconception: One needs to have an inspection booth setup for DPT

Penetrant testing is easy to perform & does not require any special setup, as DPT material is also available in aerosol cans for convenience and portability.

Depth of discontinuity is related to the speed of bleeding

Depth of surface discontinuities may be correlated with the richness of color and speed of bleeding. Wiping the first layer of developer and immediately spraying the second coat may help in judging the depth of flaw by observing speed & intensity of second bleeding.

Penetrant inspection can take place at any point in the manufacturing process

Penetrant inspection is useful immediately after any manufacturing process which is known to cause discontinuities. This makes it easy to rework on the part or discard it which help to save time & cost. Penetrant inspection can be performed more than once during the manufacturing of the part. However, care must be taken to perform penetrant inspection prior to mechanical operations such as shot blasting, peening or grinding that will smear the metal surface these may close the discontinuity.

We have tried covering a few misconceptions here, if you know any more, please feel free to revert on this mail. Also, do visit our website for more such interesting blogs.

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