Introduction to Liquid Penetrant Testing

Introduction to 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.

•  Penetrant testing can be applied to most of materials including metallic and non metallic objectsMetallic materials include aluminum, magnesium, titanium, castiron,stainless steel, powdered metal products, copper, brass, bronze,etc.

• Nonmetallic materials include ceramics, plastic, moldedrubber, composites, glass, etc. Penetrant testing can’t be applied on a porous surface

Penetrant Testing History

• Penetrant first used in 1900 s 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 1940 s fluorescent or visible dye was added to the oil

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 intosurfaces. It also provide a light background to increase contrast when visual penetrant is used

Principles

• Clean & dry test object
• Apply penetrant and let it penetrate
• Remove excess penetrant
• Apply developer
• Evaluate indications
• Post clean test object

Types of Penetrants

1.Basically Penetrants are classified as Based on Physical Properties

• i) Fluorescent Penetrants
• ii) Visual penetrants

Fluorescent penetrants are generally green in colour and they contain dye that glow brightly when exposed to UV lights

Visual penetrants are usually red in colour that provide high contrast against the white developer background

2. Based on removal technique

• i) Solvent removable
• ii)Water washable
• iii)Post emulsifiable

Solvent Removable In this method , Penetrants are removed by wiping with a cloth dampened with  solvent

Water Washable / WW Have a built in emulsifier, easily removed by water rinse

 Post Emulsifiable / PE Not contain emulsifying agent, not soluble in water Emulsifier applied to r     remove excess penetrant by dipping, spraying, or flowing Emulsifier dwell time is 1 - 4 minutes

 Penetrant-emulsifier mixture is removed by water

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

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

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.

2.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 edeg

3. Viscosity

Viscosity is the internal resistance of a liquid to flow. The penetrants with less viscosity fill the cracks in time

4. Surface Tension

Surface tension is nothing but wetting capability of a liquid. Penetrant should have good wetting capability to spread over the surface

5.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

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

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

Types of Developers

1. Dry Powder Developers
2. Water Spendable developers
3. Water soluble developers
4. Non-aqueous developers

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

2 Water- Suspend able Developer

Water suspend-able developers consist of insoluble developers particles suspended in

water. Water suspend-able developers require frequent stirring to keep the particle from setting out of suspension

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

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

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

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

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.

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

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).

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 reject-able defect.

Length of indication is measured for evaluation, not length of the flaw. Here, the two linear  indications are reject-able defects. The round indication is non relevant.

7. Post-clean part.

The part needs to be cleaned to remove all developer after it has been evaluated.

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 fit 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

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 railwaylocomotive and the rolling stock
4. Tool and die s: field drilling rays, drill pipes, castings anddrilling equipment's inspected by this methods
5. Inspection on reactors and tank : Tanks, vessels, reactors,piping, dyers in the chemical, petro-chemical industries.