Liquid penetrant testing is a widely utilized non-destructive inspection method effective on various materials, including metals and plastics. The process involves cleaning the test object, applying penetrant, allowing dwell time for defect infiltration, and then using a developer to reveal defects. This method is employed across multiple industries, including aerospace and automotive, to ensure the integrity of components.

1. Module II
Introduction to Liquid Penetrant Testing
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Hareesh K Nair
Assistant Professor
Department of Mechanical Engineering

2. Liquid Penetrant Testing
• Liquid penetrant inspection
is one of the oldest and
most widely used non
destructive testing methods.
It is also called as dye
penetrant inspection.
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3. Liquid Penetrant Testing
(continue)
• Penetrant testing can be applied to most of materials
including metallic and non metallic objects
• Metallic materials include aluminum, magnesium, titanium, cast iron,
stainless steel, powdered metal products, copper, brass, bronze, etc.
• Non metallic materials include ceramics, plastic, molded rubber,
composites, glass, etc.
• Penetrant testing can’t be applied on a porous surface
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4. Penetrant Testing History
• Penetrant first used in 1900s to detect cracks in locomotive parts
• The method was called oil and whiting
• The method used dirty lubricating oil that was thinned with kerosene followed
by application of chalk coating which absorbed oil from the cracks
• In 1940s fluorescent or visible dye was added to the oil
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5. • Penetrants
• The penetrants are mixtures
of organic solvents, which are
characterized their ability to
wet materials, spread rapidly
and penetrate into minute.
• Developer
• Developer is good absorbent
material capable of drawing
traces trapped penetrants from
the discontinues back into
surfaces. It also provide a
light background to increase
contrast when visual penetrant
is used
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6. Principles
• Clean & dry test object
• Apply penetrant and let it penetrate
• Remove excess penetrant
• Apply developer
• Evaluate indications
• Post clean test object
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7. Types of Penetrants
• Basically Penetrants are classified as Based on
1. Physical Properties
i) Fluorescent Penetrants
ii) Visual penetrants
i) Fluorescent penetrants are generally green in colour
and they contain dye that glow brightly when exposed
to UV lights
ii) Visual penetrants are usually red in colour that provide
high contrast against the white developer background
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8. 2. Based on removal technique
i) Solvent removable
Ii)Water washable
Iii)Post emulsifiable
i) Solvent Removable
In this method , Penetrants are removed by wiping
with a cloth dampened with solvent
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10. ii) Water Washable / WW
• Have a built in
emulsifier, easily
removed by water rinse
• Typical rinse procedure
are (refer to manual):
Pressure < 40 psi
Temperature 10o - 38o C
Angle 45o - 75o
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11. iii) Post Emulsifiable / PE
• Not contain emulsifying agent, not
soluble in water
• Emulsifier applied to remove excess
penetrant by dipping, spraying, or
flowing
• Emulsifier dwell time is 1 - 4
minutes
• Penetrant – emulsifier mixture
isremoved by water rinse
• There are 2 types of emulsifier,
lipophilic and hydrophilic
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12. Lipophilic Emulsifier
• Mechanism of lipophilic emulsification is by diffusion
• Both molecules of emulsifier and penetrant penetrate each other to
form a washable mixture
• Rate of diffusion increases as concentration and temperature raised
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13. Hydrophilic Emulsifier
• Is a water based solution, mixture of chemical called surfactant
• Works based on principle of peeling the penetrant
• Need pre-rinsing prior to emulsifier application to help remove 60 –
80 % penetrant and provide an even surface
• Applied by immersion or spraying
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14. 3 Based on Strength Indication
• i) Level ½- Ultra Low sensitivity
• ii) Level 1- Low Sensitivity
• iii) Level 2- Medium Sensitivity
• Iv) Level 3- High Sensitivity
• V) Level 4- Ultra High Sensitivity
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15. Properties of Good penetrant
1. Capillarity
It is the ability of a liquid to flow in narrow
spaces without the assistance of external forces. This
property helps the penetrant to fill a void.
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16. Contact Angle
Angle formed by the solid-liquid interface and liquid
– vapour interface measured from the side of a liquid
is called contact angle.
Mostly the liquid penetrant has contact angle of 0deg
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17. 3. Viscosity
• Viscosity is the internal resistance of a liquid to
flow. The penetrants with less viscosity fill the
cracks in time
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18. 4. Surface Tension
• Surface tension is nothing but wetting capability of
a liquid. Penetrant should have good wetting
capability to spread over the surface
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19. 5. Cohesiveness and Adhesiveness
• The attractive force between like molecules and
unlike molecules called cohesiveness and Adhesive
forces respectively
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20. 6. Specific Gravity
• Specific gravity is the ratio of the density of a liquid
to the density of the standard liquid. Penetrant have
Sp Gravity less than one
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21. 7.Non-Flammable
8.Volatility
Tendency of a substance to evaporate at normal
temperature is called volatility
9. Removability
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22. Characteristics of Good Penetrants
• It should spread easily over the surface being inspected
• It should be easily drawn into discontinuities by
capillary action
• It should not be harmful to the inspector or the material
being tested
• It should possess high indicating capability
• It should remain in fluid state
• It should Possess less dwell time
• It should not affect the physical and chemical properties
of the testing material
• It should be cohesive, Adhesive and relatively low in
cost
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23. Properties of Good developer
• It should have good absorption characteristics
• It should have a good contrast background
• It should be able to uniformly cover the surface with
the thin smooth Coating
• It should be chemically inert with test material
• It should provide a good contrast background for
bright and clean indications
• It should be non toxic
• It should be easy to remove after inspection
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24. Types of Developers
1. Dry Powder Developers
2. Water Suspendable developers
3. Water soluble developers
4. Non-aqueous developers
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25. 1 Dry Powder Developers
• It is a mix of light feathery powder that clumps
where penetrant bleeds back to the surface in order
to produce very defined indications
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26. 2 Water- Suspendable Developer
• Water suspendable developers consist of insoluble
developers particles suspended in water. Water
suspendable developers require frequent stirring to
keep the particle from setting out of suspension
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27. 3. Water Soluble developer
• Water soluble developer is a crystalline powder that
forms a clear solution when mixed with water.
• The solution recrystallizes on the suraface when the
water is drained
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28. 4. Non Aqueous
• Non Aqueous developer suspend the developer in a
volatile solvent.
• It is supplied by a spray gun or aerosol can and it Is
most sensitive developer for inspecting small areas
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29. Basic Processing Steps of a Liquid
Penetrant Inspection
1. Pre-clean part.
This can range from grinding and wire brushing to merely wiping the part
with a rag moistened with the cleaner/ remover. The surface needs to be
free of dirt, rust, scale, paint, oil, and grease, and be smooth enough to wipe
off the penetrant without leaving residue
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30. 2. Penetrant Application:
Once the surface has been thoroughly cleaned and dried, the
penetrant material is applied by spraying, brushing, or
immersing the part in a penetrant bath
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31. 3. Penetrant Dwell:
The penetrant is left on the surface for a sufficient time to allow as much
penetrant as possible to be drawn from or to seep into a defect. Penetrant
dwell time is the total time that the penetrant is in contact with the part
surface. Dwell times are usually recommended by the penetrant producers
or required by the specification being followed. The times vary depending
on the application, penetrant materials used, the material, the form of the
material being inspected, and the type of defect being inspected for.
Minimum dwell times typically range from five to 60 minutes. Generally,
there is no harm in using a longer penetrant dwell time as long as the
penetrant is not allowed to dry. The ideal dwell time is often determined by
experimentation and may be very specific to a particular application.
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32. 4. Excess Penetrant Removal:
This is the most delicate part of the inspection procedure because the
excess penetrant must be removed from the surface of the sample while
removing as little penetrant as possible from defects. Depending on the
penetrant system used, this step may involve cleaning with a solvent, direct
rinsing with water, or first treating the part with an emulsifier and then
rinsing with water
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33. 5.Developer Application:
A thin layer of developer is then applied to the sample to draw
penetrant trapped in flaws back to the surface where it will be visible.
Developers come in a variety of forms that may be applied by dusting
(dry powdered), dipping, or spraying (wet developers).
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34. 6.Evaluate indications.
It is critical to examine the part within the time frame designated in the written
procedure. Length of an indication can grow over time as penetrant bleeds out,
causing an acceptable indication to be a rejectable defect. Length of indication
is measured for evaluation, not length of the flaw. Here, the two linear
indications are rejectable defects. The round indication is non relevant.
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35. 7. Post-clean part.
The part needs to be cleaned to remove all developer
after it has been evaluated.
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36. Penetrant Test Methods
Commonly used penetrant testing methods are
1) Water Washable Method
2) Post Emulsifiable Method
3)Solvent Removable Method
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37. Interpretation of Results
SL
NO
Defect Descripton
1 Casting Porosity Spherical surface Indications
2 Casting Cold Shut Dotted lines
3 Cracks Staight continuous surface lines
4 Hot tears Ragged line of variable width
5 Heat treat Cracks Multiple irregular lines
6 Thermal cracks interconnecting lines
7 Lack of fusion Broken line of varying width
8 Fatigue cracks continues line in parts
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38. False Indications
A false indication is an accumulation of penetrant caused by a
drop of penetrant left on the workpiece.
However, there are two conditions which may create
accumulations of penetrant that are sometimes confused with
the true crack and discontinuities
The first includes indications caused by poor washing and the
other one may created where parts press fir to each other.
If a wheel press fit onto a shaft, penetrant will show an
indication at the fit line
But these types of indications are easy to identify since they
are regular in form and shape
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39. Safety precautions in LPI
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40. Typical applications of Liquid penetrant
Inspection
1. Aerospace :Typical Components that are checked by this method
include Turbine, rotor disc, blades, aircraft wheels, Casting, forged
parts and welded assemblies
2. Automobiles: Many automotive parts particularly aluminium
castings and forging including pistons and cylinder heads are
subjected to this form of quality checks before assembly
3. Railways: LPI to detect fatigue cracking is also used for the
regular in service examination of the bogie frames of railway
locomotive and the rolling stock
4. Tool and dies: field drilling rays, drill pipes, castings and drilling
equipment's inspected by this methods
5.Inspection on reactors and tank: Tanks, vessels, reactors, piping,
dyers in the chemical, petro-chemical industries.
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