NON- DESTRUCTIVE TESTING

B. Tech VI Semester

BY

Mr. Vishnu Pratap Singh, Assistant Professor

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DEPARTMENT OF MECHANICAL ENGINEERING

BUDDHA INSTITUTE OF TECHNOLOGY

GIDA GORAKHPUR

LIQUID PENETRANT TESTING

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UNIT-II

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• Liquid pene_.trant testing is one of the oldest and simplest NDT methods where its earliest versions (using kerosene and oil mixture) dates back to the 19th century.
• This method is used to reveal surface discontinuities by bleed out of a colored or fluorescent dye from the flaw.
• This technique is based on the ability of a liquid to be drawn into a "clean" surface discontinuity by capillary action.
• After a period of time called the "dwell time", excess surface

penetrant is removed and a developer applied.

• This acts as a blotter that draws the penetrant from the

discontinuity to reveal its presence.

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• Penetrant inspection is a nondestructive test for defects

open to the surface in parts made of any nonporous material.

• It is used with equal success on such metals as

aluminum, magnesium, brass, copper, cast iron, stainless steel, and titanium.

• It may also be used on ceramics, plastics, molded

rubber, and glass.

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• A liquid penetrant is applied at the surface of

the specimen. The penetrant is drawn by the surface flaws due to capillary action and this is subsequently revealed by a developer, in addition with visual inspection.

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LIQUID PENETRANT INSPECTION

Liquid penetrant inspection is a method that is used to reveal surface breaking flaws by bleed out of a colored or fluorescent dye from the flaw.

The technique is based on the ability of a liquid to be drawn into a "clean" surface breaking flaw by capillary action.

After a period of time called the "dwell," excess surface penetrant is removed and a developer applied.It draws the penetrant from the flaw to reveal its presence.

Colored (contrast) penetrants require good white light while fluorescent penetrants need to be used in darkened conditions with an ultraviolet "black light".

Introduction

Penetrant Testing, or PT, is a nondestructive testing method that builds on the principle of Visual Inspection.

PT increases the “seeability” of small discontinuities that the human eye might not be able to detect alone.

How Does PT Work?

• In penetrant testing, a liquid with high surface wetting characteristics is applied to the surface of a component under test.
• The penetrant “penetrates” into surface breaking discontinuities via capillary action and other mechanisms.
• Excess penetrant is removed from the surface and a developer is applied to pull trapped penetrant back the surface.
• With good inspection technique, visual indications of any discontinuities present become apparent.

What Makes PT Work?

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Every step of the penetrant process is done to promote capillary action.

This is the phenomenon of a liquid rising or climbing when confined to small openings due to surface wetting properties of the liquid.

Some examples:

–Plants and trees draw water up from the ground to their branches and leaves to supply their nourishment.

–The human body has miles of capillaries that carry life sustaining blood to our entire body.

Basic Process of PT

1. Clean & Dry Component
2. Apply Penetrant
3. Remove Excess
4. Apply Developer
5. Visual Inspection
6. Post Clean Component

What Can Be Inspected Via PT?

Almost any material that has a relatively smooth, non-porous surface on which discontinuities or defects are suspected.

What Can NOT be Inspected Via PT?

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Components with rough surfaces, such as sand castings, that trap and hold penetrant.

Porous ceramics

Wood and other fibrous materials. Plastic parts that absorb or react with the penetrant materials.

Components with coatings that prevent penetrants from entering defects.

What Types of Discontinuities Can Be Detected Via PT?

All defects that are open to the surface.

–Rolled products-- cracks, seams, laminations.

–Castings--cold shuts, hot tears, porosity, blow holes, shrinkage.

–Forgings– cracks, laps, external bursts.

–Welds– cracks, porosity, undercut, overlap, lack of fusion, lack of penetration.

Choices of Penetrant Materials

Penetrant

Type

1. Fluorescent
2. Visible

Method

1. Water Washable
2. Postemulsifiable - Lipophilic
3. Solvent Removable
4. Postemulsifiable – Hydrophilic

Penetrant Materials

Penetrants are formulated to possess a number of important characteristics. To perform well, a penetrant must:

–Spread easily over the surface being inspected.

–Be drawn into surface breaking defects by capillary action or other mechanisms.

–Remain in the defect but remove easily from the surface of the part.

–Remain fluid through the drying and developing steps so it can be drawn back to the surface.

–Be highly visible or fluoresce brightly to produce easy to see indications.

–Not be harmful to the inspector or to the material being tested.

Sensitivity Levels

Penetrants are also formulated to produce a variety of sensitivity levels. The higher the sensitivity level, the smaller the defect that the penetrant system is capable of detecting.

The five sensitivity levels are:

–Level 4 - Ultra-High Sensitivity

–Level 3 - High Sensitivity

–Level 2 - Medium Sensitivity

–Level 1 - Low Sensitivity

As the sensitivity level increases, so does the number of non relevant indications. Therefore, a penetrant needs to be selected that will find the defects of interest but not produce too many non relevant indications.

Visible Vs Fluorescent PT

Inspection can be performed using visible (or red dye) or fluorescent penetrant materials.

Visible Pt is performed under white light while fluorescent PT must be performed using an ultraviolet light in a darkened area. All are all in the level 1 sensitivity range.

Fluorescent PT is more sensitive than visible PT because the eye is more sensitive to a bright indication on a dark background.

Sensitivity ranges from 1 to 4

Penetrant Removal Method

Penetrants are also classified by the method of removing the excess penetrant.

Solvent Removable penetrants are removed by wiping with a cloth dampened with solvent. They are supplied in aerosol cans for portability and are primarily used for spot checks.

Water Washable penetrants are removed with a course spray of water. They are the easiest to employ and most cost effective when inspecting large areas.

Post-Emulsifiable penetrants are water- washable only after they have reacted with an emulsifier solution. A post-emulsifiable system is used when washing the penetrant out of the defect is a concern. The emulsifier is given time to reacts with the penetrant on the surface but not the penetrant trapped in the

flaw

Choices of Developer

Developer Form

Dry Powder

Wet, Water Soluble

Wet, Water Suspendable Wet, Non-Aqueous

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Developers

The role of the developer is to pull trapped penetrant out of defects and to spread it out on the surface so that it can be seen. Also provides a light background to increase contrast when visible penetrant is used.

Developer materials are available in several different forms

–Dry Powder is a mix of light fluffy powder that clumps together where penetrant bleeds back to the surface to produces very defined indications.

–Wet, Water Suspendable is a powder that is suspended in a water that covers the surface with a relatively uniform layer of developer when the water is evaporated. The solution is somewhat difficult to maintain as the powder settles out over time.

–Wet, Water Soluble is a crystalline powder that forms a clear solution when mixed with water. The solution recrystallizes on the surface when the water is driven off. Indications sometimes lack definition and look milky. Not recommended for use with water-washable penetrants.

–Wet, Non-Aqueous - is supplied in a spray can and is the most sensitive developer for inspecting small areas. It is too costly and difficult to apply to large areas.

6 Steps of Penetrant Testing

1. Pre-Clean
2. Penetrant Application
3. Excess Penetrant Removal
4. Developer Application
5. Inspect/Evaluate
6. Post-clean

Pre-cleaning – Step 1

Parts must be free of dirt, rust, scale, oil, grease, etc. to perform a reliable inspection.

The cleaning process must remove contaminants from the surfaces of the part and defects, and must not plug any of the defects.

Pre-cleaning is the most important step in the PT process

Penetrant Application – Step 2

Many methods of application are possible such as:

` –Brushing

`–Spraying

–Dipping/ Immersing

–Flow-on

Dwell Time

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The penetrant solution must be allowed to “dwell” on the surface of the part to allow the penetrant time to fill any defects present.

The dwell time vary according to penetrant type, temperature, material type and surface finish.

Excess Penetrant Removal – Step 3

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The removal technique depends upon the type of penetrant used, as stated earlier…

–Solvent Removable

–Water Washable

–Post Emulsifiable

Excess Penetrant Removal – Step 3 (cont.)

Water Washable

A coarse water spray is used to remove the excess penetrant.

The procedure used as a guideline for the inspection will specify water temperature (typically 50-100°F) and pressure (typically not more than 40 psi), etc.

Excess Penetrant Removal – Step 3 (cont.)

The part is wiped with a clean dry cloth to remove the bulk of the excess penetrant.

Then, a cloth lightly dampened with solvent is used to remove any remaining penetrant on the surface.

Excess Penetrant Removal – Step 3 (cont.)

Solvent Removable (cont.)

Any time a solvent is used in the penetrant inspection process, a suitable flash time is required to allow excess solvent to evaporate.

Excess Penetrant Removal – Step 3 (cont.)

Post Emulsifiable

• When there is concern about removing much of the penetrant from the defect, a post emulsifiable system is used.
• This involves an additional step in which an emulsifier is applied to the surface of the part after the penetrant dwell time.
• The emulsifier is given just enough time to react with the penetrant on the surface to render it water washable but not enough time to diffuse into the penetrant trapped in the defects.

Developer Application – Step 4

The method of developer application is is dependent on the type of developer used. The primary methods for the following main developer types will be covered in the following slides.

–Dry

–Wet

–Nonaqueous Wet

Developer Application – Step 4 (cont.)

Dry Powder Developer

• Prior to applying a dry powder developer, the component must be thoroughly dried. Drying is usually accomplished in a hot air circulating oven.
• The developer is then applied by immersing the part in the powder or by dusti n

of the part with the powder.

• The part can also be placed

in a developer dust cloud cham

g

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ber.

Developer Application – Step 4 (cont.)

Wet Developer (water- suspended and water- soluble)

• Wet developers are applied by immersing or spraying the part while it is still wet from the penetrant removal process.
• The part is completely coated and the excess liquid allowed to drain to prevent pooling
• The part is then dried in a hot air circulating oven.

Developer Application – Step 4 (cont.)

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Nonaqueous Developer (AKA Solvent-Suspended)

• Nonaqueous developer is applied by a aerosol spray to a thoroughly dried and cooled part.
• A thin even coating should be applied. The coating should be white but still slightly transparent when performing a visible dye penetrant inspection, and even thinner when performing a fluorescent penetrant inspection.

Inspection/Evaluation – Step 5

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In this step the inspector evaluates the penetrant indications against specified accept/reject criteria and attempts to determine the origin of the indication.

The indications are judged to be either relevant, non-relevant or false.

Relevant crack indications from an abusive drilling process

Non-relevant weld geometry

indications

Inspection/Evaluation – Step 5

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A very important step of evaluation is to document findings on an inspection report form or other record keeping form.

This may be supported with drawings or photos of indications, etc.

Post Clean – Step 6

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The final step in the penetrant inspection process is to thoroughly clean the part that has been tested to remove all penetrant processing materials.

The residual materials could possibly affect the performance of the part or affect its visual appeal.

Advantages of Penetrant Testing

Relative ease of use.

Can be used on a wide range of material types. Large areas or large volumes of parts/materials can be inspected rapidly and at low cost.

Parts with complex geometries are routinely inspected.

Indications are produced directly on surface of the part providing a visual image of the discontinuity.

Initial equipment investment is low.

Aerosol spray cans can make equipment very

portable.

Limitations of Penetrant Testing

Only detects surface breaking defects. Requires relatively smooth nonporous material.

Precleaning is critical. Contaminants can mask defects.

Requires multiple operations under controlled conditions.

Chemical handling precautions necessary (toxicity, fire, waste).

Metal smearing from machining, grinding and other operations inhibits detection. Materials may need to be etched prior to inspection.

Post cleaning is necessary to remove chemicals.

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