Basics of LPI

8/12/2019 Basics of LPI

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Basics of NDT


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What is Nondestructive Testing?

Nondestructive testing (NDT) is the process ofinspecting, testing, or evaluating materials,

components or assemblies for discontinuities,or differences in characteristics withoutdestroying the serviceability of the part or

system. In other words, when the inspectionor test is completed the part can still be used.


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Major types of NDT Detection of surface flaws

Visual Magnetic Particle Inspection Fluorescent Dye Penetrant Inspection

Detection of internal flaws Radiography Ultrasonic Testing


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Visual Testing

visual inspection is one of the most commonand most powerful means of non-destructivetesting. Visual testing requires adequateillumination of the test surface and propereye-sight of the tester. - See more at:http://ndtservices.blogspot.in/2013/03/ndt-

method-visual-inspection.html#sthash.BrktA9mN.dpuf


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Advantages & DisadvantagesAdvantages

Simple

Less training requiredRelatively less costly equipment

DisadvantagesSlowInspector Fatigue is moreSurrounding environmental factors are important


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Principle LPT is based upon capillary action, where low

surface tension fluid penetrates into clean and drysurface-breaking discontinuities. Penetrant may beapplied to the test component by dipping, spraying,or brushing.

After adequate penetration time has been allowed,the excess penetrant is removed and a developer isapplied.

The developer helps to draw penetrant out of theflaw so that an invisible indication becomes visibleto the inspector.

Inspection is performed under ultraviolet or whitelight, depending on the type of dye used -

fluorescent or non-fluorescent (visible).


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Eight steps of Dye penetrant Inspection

1. Surface Preparation2. Penetrant Application

3. Penetrant Dwell Time4. Excess Penetrant Removal5. Developer Application

6. Indication Development Time7. Evaluation / Interpretation8. Post cleaning


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1. Surface Preparation One of the most critical steps of a liquid

penetrant inspection is the surfacepreparation.

The surface must be free of oil, grease, water,or other contaminants that may preventpenetrant from entering flaws.


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2. Penetrant Application Once the surface has been thoroughly cleaned

and dried, the penetrant material is applied byspraying, brushing, or immersing the part in a

penetrant bath.


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3. Penetrant Dwell Time The penetrant is left on the surface for a sufficient time

to allow as much penetrant as possible to be drawnfrom 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 specificationbeing followed.

The times vary depending on the application,penetrant materials used, the material, the form of thematerial being inspected, and the type of defect beinginspected for


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4. Excess Penetrant Removal This is the most delicate part of the inspection

procedure because the excess penetrant mustbe removed from the surface of the sample

while removing as little penetrant as possiblefrom defects.


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5. Developer Application A thin layer of developer is then applied to the

sample to draw penetrant trapped in flawsback to the surface where it will be visible.Developers may be applied by dipping orspraying (wet developers).


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Photo Courtesy of Contesco


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6. Indication Development Time The developer is allowed to stand on the part surface

for a period of time sufficient to permit theextraction of the trapped penetrant out of anysurface flaws. This development time is usually aminimum of 10 minutes. Significantly longer times

may be necessary for tight cracks.


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7. Evaluation and Interpretation Inspection is then performed under

appropriate lighting to detect indications fromany flaws which may be present.


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8. Clean Surface

The final step in the process is to thoroughlyclean the part surface to remove thedeveloper from the parts that were found tobe acceptable.


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Advantages of LPI

Applicable to all non-porous materials Able to test large parts with a portable kit Batch testing

Applicable to small parts with complexgeometry Simple,cheap, easy to interpret Good sensitivity Training requirements typically less than for

other NDT methods.


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Disadvantages of LPI

Will only detect defects open to the surface Careful surface preparation required

Not applicable to porous materials Temperature dependent Compatibility of chemicals

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Basics of LPI