LIQUID PENETRANT TEST (LPT)

Liquid Penetration Test (LPT)

      It detects flaws that are open to the surface. e.g. crack, lack of bond, porosity, cold shuts etc., it can be effectively used not only in the case of ferrous metal but it is especially useful for non-ferrous product and on non metallic such as ceramic, plastic and glass.

     The principle of LPT is that the liquid used to enter small opening such as crack or porosities by capillary action.  The rate and extent of this action are depended upon such properties as surface tension, cohesion, adhesion and viscosity.  They are also influenced by factors such as the condition of the surface of material and intensity of discontinuity.

     For liquid to penetrate effectively the surface of the material must be thoroughly cleaned of all material that would obstruct the entrance of the liquid into the defect.  After cleaning, the liquid penetrate is applied evenly over the surface and allow to remain long enough to permit penetration into possible discontinuities.

     The liquid is then completely removed from the surface of component and either the wet or a dry developer is applied.  The liquid that has penetrated the defect will then bleed out on to the surface and the developer will help delineate them.

     This will show the location and general nature and magnitude of any defect present.  To hasten this action, the part may be struck sharply to produce vibration to force the liquid out of the defect.

Liquid Penetrant Test Result

Fig 14.17

Fluorescent penetrant inspection [Zyglo process]

     This method is sensitive to small surface discontinuity such has crack shrinking and porosity open to the surface which tend to retain penetrant in spite of the Vince.  Smooth or machine job surface provide more satisfactory condition for test.

Operational Step:-

1. Clean the surface of the object to be inspected for cracks etc.
2. Apply fluorescent penetrant on the surface by either dipping, spraying or brushing.  Allow a penetration time of one hour.  The fluorescent penetrant is drawn into crackle by capillary action
3. Wash the surface with water spray to remove penetrant form surface but not form crack
4.  Apply the developer. Developer acts like a blotter should draw a penetration indication.
5. The  surface  is  viewed  under black light having the wavelength 3650 A^o units [1Ao=10^-8cm] which is between the visible and ultraviolet in the spectrum. Blacklight causes penetrant to glow in dark.