BMFB 4283 NDT & FAILURE ANALYSIS

Lectures for Week 2 Prof. Qumrul Ahsan, PhD

Department of Engineering Materials Faculty of Manufacturing Engineering

Issues to address 2.0 Liquid Penetrant 2.1 Introduction 2.2 Fundamentals 2.3 Techniques 2.4 Applications

Introduction

•

This lecture is intended to provide an introduction to the NDT method of penetrant testing.

•

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.

PENETRANT TESTING (PT)

o

PT effectively requires:



Discontinuities open to the surface of the part

 

(

subsurface discontinuities or surface discontinuities not open to the surface aren’t detected

) Special cleaning of parts Good eyesight

What Can Be Inspected Via PT?

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

SURFACE BREAKING 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.

Principles of Penetrant Testing

•

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 (blotter) is applied to pull trapped penetrant back the surface.

•

Developer provides a contrasting background for visual indications of any discontinuities present become apparent.

Basic Process of PT

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

What Makes PT Work?

•

Surface Tension : An elastic force that acts tangential to the fluid surface to reduce the area is called surface tension The surface tension for a droplet of liquid is



= ½ (p o - p i ) r, where (p o - p i ) is the difference between the bubble’s exterior and interior pressure and r is the radius of curvature

•

The surface tension at interface of two mediums can be expressed as



lg

•

Wetting is the ability of a liquid from intermolecular

•

to maintain contact with a solid surface , resulting interactions when the two are brought together.

The degree of wetting (wettability) is determined by a force balance between adhesive and cohesive forces .

•

Adhesive forces between a liquid and solid cause a liquid drop to spread across the surface .

•

Cohesive forces within the liquid cause the drop to ball up and avoid contact with the surface

At the liquid-solid surface interface, if the molecules of the liquid have a stronger attraction to the molecules of the solid surface than to each other (the adhesive forces are stronger than the cohesive forces), wetting of the surface occurs. Alternately, if the liquid molecules are more strongly attracted to each other than the molecules of the solid surface (the cohesive forces are stronger than the adhesive forces), the liquid beads-up and does not wet the surface of the part.

Figure 2: Wetting of different fluids. A shows a fluid with very little wetting, while C shows a fluid with more wetting.

What Makes PT Work?

•

The contact angle is the angle formed by the solid/liquid interface and the liquid/vapor interface measured from the side of the liquid Contact angle Degree of wetting Strength of:

Sol./Liq.

interactions Liq./Liq.

interactions θ = 0 Perfect wetting strong weak 0 < θ < 90° 90° ≤ θ < 180° θ = 180° high wettability low wettability perfectly non-wetting strong weak weak weak strong weak strong strong

Contact angle of a liquid droplet wetted to a rigid solid surface

•

For a penetrant material to be effective, the contact angle should be as small as possible

•

Typical penetrant materials have contact angles on the order of 10 o

•

What Makes PT Work?

In liquid penetrant testing, two conditions must be met.

–

First, the surface energy of the solid-gas interface must be greater than the combined surface energies of the liquid-gas and the solid-liquid interfaces.

–

Second, the surface energy of the solid-gas interface must exceed the surface energy of the solid-liquid interface.

•

Capillary action : If a tube is sufficiently narrow and the liquid adhesion to its walls is sufficiently strong, surface tension can draw liquid up the tube. The height the column is lifted to is given by: where h is the height the liquid is lifted,



is the liquid-air surface tension,



is the density of the liquid, r is the radius of the capillary, g is the acceleration due to gravity, θ is the angle of contact described above.

Illustration of capillary rise and fall. Red=contact angle less than 90 ° ; blue=contact angle greater than 90 °

What Makes PT Work?

•

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.

•

Surface tension of liquid vs tube surface wetting

•

Surface tension – cohesive force

•

Tube surface wetting – adhesive force

•

Cohesive force> Adhesive force : Convex surface -> liq. fall below

•

Cohesive force< Adhesive force : Concave surface -> liq. rise up

Finer Tube, greater liquid rise finer defect (hairline) -> greater indication

Penetrability

•

For cylindrical volume capillary pressure P = 2



gl cosθ/r = 2Scosθ/r

–

Where S is the surface tension, r is the radius of the crack and θ is the contact angle.

–

Influenced by variables: surface condition, and type of test object, type of penetrant, temperature of test object and contamination.

•

Fluid penetration into a real crack will generally be different from the above estimation

–

Crack width is not a constant ( a crack typically narrows with depth)

–

Portions of the crack may be closed

–

Trapped gas or contaminants within the crack limits fluid penetrationion

A liquid penetrant will continue to fill the void until an opposing force balances the capillary pressure. This force is usually the pressure of trapped gas in a void, as most flaws are open only at the surface of the part. Since the gas originally in a flaw volume cannot escape through the layer of penetrant, the gas is compressed near the closed end of a void.

Penetrability

•

High P



High S (High S is not necessarily a good penetrant

–

e.g. Water has high surface tension, yet poor penetrant High Surface Tension Low Wetting Ability

•

Static Penetration Parameter (SPP) = Scosθ

–

Smaller contact angle, θ



higher P

–

Good penetrant 5 o Low Surface Tension High Wetting Ability

•

Width of discontinuity; D(2r)

–

Narrower



higher P



Longer time to rise

–

Finer defect



longer dwell time

Penetrability

•

Viscosity,

 –

Not significantly affect penetrant ability High Surface Tension Low Wetting Ability

•

into discontinuity

–

Strongly affected with temperature



penetrant inspection

–

Kinetic Penetration Parameter, KPP = Scosθ/

•

Highly viscous penetrant: longer time to enter into defect



longer dwell time

•

Drain more slowly and cause excessive loss of



penetrant due to drag Low Surface Tension High Wetting Ability Density

–

has a slight to negligible effect on the performance of a penetrant.

–

Increasing the specific gravity by decreasing the percent of solvent (by volume) in the solution will increase the penetration speed.

–

The gravitational force acting on the penetrant liquid can be working either with or against the capillary force depending on the orientation of the flaw during the dwell cycle

Flaw Entrapment Efficiency

•

Ability of penetrant to form an indication large enough to be detected

•

Factors influenced efficiency

–

Volume of defect

–

Length of defect

–

Contaminants

–

Penetrant dye

–

Processing

Flaw Entrapment Efficiency

•

Volume of defect

–

Size of indication reflects the volume of defect it entered

–

Larger discontinuity: depth or width



More penetrant it holds



More penetrant is present to form indication

•

Length of Defect

–

Affect volume of penetrant

–

Strongly affect the visibility



Very fine defect has insufficient width



Reduce the ability of human eye to detect indication visually



Can only be located when the defect has sufficient length

Flaw Entrapment Efficiency

•

Contaminants

–

Fine and clean discontinuity vs wide and contaminated dicontinuity



Affect the penetration of penetrant

–

Inservice inspection encounter defects contaminated with oil, water and corrosion products



Reduce the volume available for penetration



Water adversely influence contact angle



Acidic or alkaline contaminants also fading the dye visibility



Heat and prolonged exposure under UV light cause penetrant lose their sensitivity

Flaw Entrapment Efficiency

•

Penetrant dye

–

Type of dye



Affect the sensitivity of penetrant visibility in terms of brilliance and intensity of the dye color



Fluorescent dye is more sensitive than color contrast dye

–

Concentration of dye



Different concentration within a classification affect the sensitivity level



Altering dye concentration affect the ability of penetrant penetration

Flaw Entrapment Efficiency

•

Processing

–

Method of processing



Dip and drain allows more volatile constituents of penetrant to evaporate during dwell time



Increases the concentration of dye within remaining penetrant

–

Degree of penetrant removal

•

No background coloration (cleaned component)



contrast penetrant indication

•

high degree of Should be interpreted carefully: over-emulsification or over washing



removes penetrant from defects

• •

Small degree of background coloration : over-washing not occur Brightness of indication must exceed background brightness

What Can NOT be Inspected Via PT?

•

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.

Defect indications become less distinguishable as the background “noise” level increases.

Choices of Penetrant Materials

Penetrant Developer Type I Fluorescent II Visible Method A Water Washable B Postemulsifiable - Lipophilic C Solvent Removable D Postemulsifiable - Hydrophilic Form Dry Powder Wet, Water Soluble Wet, Water Suspendable Wet, Non-Aqueous

PREPARATION AND PRE-CLEANING WATER WASHABLE POST-EMULSIFIABLE SOLVENT REMOVABLE CHECK PENETRANT REMOVAL POWDER DEVELOPER WATER SOLUBLE DEVELOPER WATER SUSPENDABLE DEVELOPER SOLVENT BASED DEVELOPER INSPECT RECORD CLEAN AND PROTECT

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.

Penetrant Properties

• • • • • • •

Wetting ability Specific gravity Volatility Chemical activity Solubility Solvent ability Health hazard

• • • •

Tolerance to contaminants Flammability / flash point Electrical conductivity Availability and cost

Penetrant Properties

• • Wetting ability: – Affect penetrability and bleed-back characteristics – Contact angle and surface tension of penetrant control wetting ability Specific gravity: – Ratio of density of penetrant with density of distilled water at 40  C – Not a problem with oil base penetrant – Penetrant used in tank system must have specific gravity less than 1   to ensure water will not float on top of penetrant prevent penetrant from covering the test object

Penetrant Properties

• • Flash point: – – Temperature at which enough vapor is given off to form combustible mixture Typical min 93  C – Should not be flammable Volatility: – Characterize by vapor pressure or boiling point – Good penetrant usually evaporate too quickly – Low volatility is desirable surface, leave stained and from any discontinuity, leave precipitated dye  so the penetrant dry from the

Penetrant Properties

• • • Chemically inert: – Must be inert, non-corrosive as possible  with the material being tested – chemically compatible Penetrant is contaminant (contain sulphur, sodium, halogen)  potential reactions must be considered – To avoid possibility of embrittlement or cracking over years Viscosity: – Affect thickness of penetration due to molecular/internal friction – Low viscosity penetrant Solubility: – Penetrant contain dye in liquid solution – – Must hold sufficient dye at ambient or high temperature Must not come out from solution if temperature drop

Penetrant Properties

• • Solvent ability: – Solvent must be able to remove surplus penetrant from test specimen – To ensure clean, clear background – Must not dissolve the penetrant in defect Tolerance to contaminants: – Penetrant will be contaminated after a period of time, even if a great care is taken – Must be periodically check to ensure all is well, no residue left

Penetrant Properties

• •

Health hazard:

– Must comply with or exceed the most stringent HSE requirements • Toxicity, odour, skin contact – To prevent allergies or contaminants

Availability and cost:

– Dye materials are easily obtained – Low cost

Penetrant Properties

• Electrical conductivity: – Electrostatic spraying becomes popular  uniform coverage with complicated shapes  Reduces over spraying  Requires less penetrant over all – Spray gun applies –ve charge to penetrant – Test object ground potential – Electrostatic attraction cause penetrant be strongly attracted to the part  low viscosity and easily attracted to the part  Must readily accept and hold the electrical charge

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

–

Level 1/2 – Ultra-Low Sensitivity

As the sensitivity level increases, so does the number of nonrelevent indications.

Therefore, a penetrant needs to be selected that will find the defects of interest but not produce too many nonrelevent indications.

Why is Visible Penetrant Penetrant

•

Visible penetrant is usually red because red stands out and provides a high level of contrast against a light background Red

and Fluorescent Green ?

•

Fluorescent penetrant is green because the eye is most sensitive to the color green due to the number and arrangement of the cones (the color receptors) in the eye.

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.

Photo Courtesy of Contesco

Type of UV light

• • •

Mercury vapour arc lamp

– Street lamp that has filter to reduce the visible light to minimum but allow UV-A to transmit

GE or Westinghouse lamp

– Has separate filter, Hg arc is drawn between electrodes in quartz tube

400 W Hg vapour flood lamp

– Used for very large component

Quality of fluorescent dye

• •

Depends on how efficient dye absorb UV light and convert into visible light Influenced by:

–

The intensity of UV-A light at the surface

–

The ability of dye to absorb UV-A

–

The concentration of dye

–

The ability of dye to produce visible light

–

Film thickness

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.

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.

Basic mechanism of Developer

•

Developer works due to :

–

Capillarity

–

Light scattering

–

Solvent action

Capillarity

• • •

Capillary attraction of developer overcomes the opposing attraction of the discontinuity

– – –

Increase the surface area of indication Spread the penetrant laterally on surface



indication widening Expands the bulk dye into many thin films



brightness enhance

Too large developer particle size will result low capillary pressure Too small will cause block any orifice

Light scattering

•

Very important when involves fluorescent penetrants

–

Brightness of indication is amplified per unit area

–

Each particle provide scattering multiple reflector



both UV-A and fluorescent radiation

–

This improve contrast in dark condition



improved sensitivity of penetrant system

Solvent action

•

Applies to non-aqueous method

–

Have no capability for drawing penetrant out of discontinuity

–

The developer damp the test surface

–

The remaining solvent will bridge the gap between the developer particles and the penetrant in the discontinuity

–

Important for fine defect

Developer properties

• Good developer: – Material must be absorptive to perform blotting action – Must have fine texture – Must mask out background contours and colors – Must be easily and evenly applicable – Must form light and even coat – Must be no fluorescing of developer when fluorescent penetrant is used – Penetrant bleeding must easily wet the material – Must be high color contrast, white is the best – Must be readily removable after test – Must be in-toxic and non-irritant

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!!!

Why Pre-cleaning important?

• • • Penetrant unable to wet the surface of the test object – due to oils, water/hydrates left after evaporation or polishing and buffing lubricants Penetrant is unable to enter a discontinuity (blockage) – Peening or smearing of discontinuity, carbon, scale, paint/coatings, penetrant residues Penetrant bleed out from discontinuity is restricted – Carbon, scale, rust, anodising

Cleaning methods

• • Mechanical methods: – Brushing – Blasting Chemical methods: – Hot solvent degreasing – Vapor degreasing – Cold solvent degreasing – Alkaline degreasing – Acid pickling – Steam cleaning – Paint strippers

Physical Cleaning

•

Grinding

•

Abrasive Blasting

•

Wire brushing Defect Peened or Closed

Light Acid Etching Light Acid applied Thin layer of the surface dissolved

Light Acid Etching The defect opened again to the surface After Acid Etching

Chemical Methods

Hot Solvent Degreasing Solvent Components Heating Element

Vapour Degreasing

Components Condensor

vapour

Drip Tray Solvent Heating Element The most effective method for degreasing

Steam Cleaning

• For large objects

Chemical Methods

• • • • •

Other methods

Cold solvent Degreasing Solvent materials with Emulsifiers Acid / Alkaline Cleaning Paint Removal Ultrasonic Cleaning

Ultrasonic Cleaning

Solvent/ water Components Ultrasonic Crystal

Caution About Metal Smearing

Some machining, surface finishing and cleaning operations can cause a thin layer of metal to smear on the surface and prevent penetrant from entering any flaws that may be present.

Before Sanding Etching of the surface prior to inspection is sometimes required.

After Sanding After Etching

Penetrant Application – Step 2

Many methods of application are possible such as:

– – –

Brushing Spraying Dipping/ Immersing

– –

Flow-on And more

Dwell Time

•

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 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.)

Solvent Removable

•

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 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.

Developerr Application –Step 4

• • •

Bleed the penetrant back to the surface by the reverse capillary action Spread the penetrant into larger area : easier to be seen Improve background contrast

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

Dry powder developer

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 dusting of the part with the powder.

•

The part can also be placed in a developer dust cloud chamber.

Dry powder developer

• • •

ADVANTAGES Easy to handle No hazardous vapours Easy to remove

• • •

DISADVANTAGES Difficult to ensure if it has been properly applied Fine powders can be hazardous Do not offer a high degree of colour contrast

SOLUTION

Aqueous Developer

TWO TYPES SUSPENSION The particles settled down if not agitated

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.

Aqueous Developer

• •

ADVANTAGES No vapours or dust Cheaper than non aqueous

• • •

DISADVANTAGES Difficult to apply evenly Requires drying after application Proper mixing required Too little developer particle : Very weak indications Too much : Developer layer will crack when dry

Developer Application – Step 4 (cont.)

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.

Non-Aqueous Developer

• • ADVANTAGES Most sensitive Useable with fluorescent or colour contrast • • • DISADVANTAGES Hazardous solvents Higher cost Need to be correctly applied

Inspection/Evaluation – Step 5

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.

Non-relevant weld geometry indications Relevant crack indications from an abusive drilling process

Inspection/Evaluation – Step 5

• •

Should be inspect immediately after developer is applied

–

Development time between 0 to 30 min

–

Inspection should be carried out throughout the development time Inspector is the most critical element

–

Acceptance and rejection based on inspector judgment

–

Must have good near vision acuity & capable of colour discrimination

Inspection/Evaluation – Step 5

•

For colour contrast penetrants:

–

Should be viewed in bright white light colour

–

Min 500 lux is recommended = bright daylight

Inspection/Evaluation – Step 5

•

For fluorescent method inspection

– The room must be darkened below 20 lux visible light – Min level of UV-A is 1.0mW/cm 2 – Inspector must be in dark room for 10 min •

To adapt for low light level

– Wear photochromatic spectacles •

To reduce eyestrain

– Should not view continuously more than 30 mins

Inspection/Evaluation – Step 5

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 sketch drawings or photos of indications, adhesive tapes, video, etc

Indications

PRESS FITTED

•

True or Relevant indications

Indications caused by defects •

Non Relevant indications

Indications caused by assembly or geometry of the component

HOLES RIVETS POROSITY

LAP •

False indications

Indications caused by operator faults

SPLINES

Post Clean – Step 6

The final step in the penetrant inspection process is to thoroughly clean the part that has been tested to remove all penetrant processing materials. Penetrant chemicals residues are required to be removed because They may be harmful to the component Penetrant and emulsifier are alkaline: may cause surface pitting, especially on aluminium.

Developer will entrapped moisture: may cause corrosion or They may impair subsequent processing

Penetrant Inspection Systems

Penetrant systems can be highly portable or stationary.

Portable Penetrant System Image courtesy of Nebraska Army National Guard Stationary Penetrant System

CHOICE OF PENETRANT SYSTEM

•

Factors influenced:

– Size and type of defect – Geometry and intricacy – Surface condition – Other factors such as: component material, size and position of item, equipment and expertise available, cost, number of component to be tested

Size and type of defect

•

Wide shallow defect are most likely to be detected using post-emulsifiable method

•

Fine defect is best located with fluorescent method due to high sensitivity of eye to fluorescent than colour contrast indication

Geometry and intricacy

• • •

High intricate components have large number of section changes

Threaded component causes problem during excess penetrant removal

Results in excessive background colouration

–

Reduce the detection of defect

Surface condition

• •

Rough surface is difficult to fully clean

Fluorescent mehod is less suited to test rough component

– Difficulty in adequately monitoring penetrant removal

Other factors

• • •

Component material:

– Solvent removable methods may lead to surface damage due to incompatibility between penetrant and material under test

Size and position of the item

– On-site welds are unlikely tested by fluorescent method due to test method requirement (dark room)

Equipment and expertise available

– Fluorescent method involves flow lines  factory use than onsite test more suited for

Other factors

• •

Cost

–

Water washable method is much cheaper than solvent removable method due to availability of cleaning fluid Number of components to be tested

–

Fluorescent method is recommended for batch inspection due to high sensitivity of human eye to fluorescent indication compare to visible colour contrast

 reduce fatigues during perform the test  inspector able to perform at higher level for longer period

Quality and Process Control

Since penetrant testing involves multiple processing steps, the performance of the materials and the processes should be routinely checked using performance verification tools, which include:

–

TAM Panels

– –

Crack Sensitivity Panels Run Check Panels

• • •

Equipment checks

Overall system performance Control check:

–

water wash temperature and pressure

– –

Colour intensity Penetrant remover check

–

Developer check

– –

UV lamp output check Water removable penetrant, water tolerance check Maintenance check:

–

Equipment cleanliness

–

Airline cleanliness

–

Processing unit

• • • • • •

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.

Summary

•

Penetrant testing (PT) is one of the most widely used nondestructive testing methods.

•

Its popularity can be attributed to two main factors, which are its relative ease of use and its flexibility.

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However, PT involves a number of processing steps that must be closely control to achieve optimal sensitivity.

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Glossary of Terms

Capillary Action - the tendency of certain liquids to travel or climb when exposed to small openings.

Contrast - the relative amount of light emitted or reflected between and indication and its background.

Developer - a finely divided material applied over the surface of a part to help promote reverse capillary action and thus bring out a penetrant indication.

Dwell Time - the period of time that a penetrant or developer must remain in contact with the surface of a part under test.

Emulsification Time - the time allowed for the emulsifier to render the penetrant water washable and thus allow the part to be washed.

Emulsifier - a material applied over a film of penetrant that renders it water washable.

Evaluation - the process of deciding as to the severity of the condition after an indication has been interpreted.

False Indication - an indication caused by improper processing; not caused by a relevant or non-relevant condition.

Flash Time - the time required for the solvent to evaporate from the surface of a part when used to preclean or remove excess penetrant.

Fluorescent Dye - a dye which becomes fluorescent (gives off light) when exposed to short wave radiation such as ultraviolet light.

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Glossary of Terms

Indication - the visible evidence or penetrant bleed-out on the surface of the specimen Interpretation - the process of evaluating an indication in an attempt to determine the cause and nature of the discontinuity. Non-Aqueous Developer - a developer in which developing powder is applied as a suspension in a quick drying solvent Penetrant - a liquid used in fluorescent or visible dye penetrant inspection to penetrate into the surface openings of parts inspected via these methods Relevant Indication - an indication that has been determined not to be false or non-relevant - and actual discontinuity Seeability - the characteristic of an indication that enables it to be seen against the adverse conditions of background, outside light, etc.

Sensitivity - the ability of a penetrant to detect surface openings. Higher sensitivity indicates smaller discontinuities can be detected Ultraviolet Light (or Black Light) - light energy just below the visible range of violet light (356 nanometers).

Viscosity - the resistance of a fluid to the motion of its particles Washability - the property of a penetrant which permits it to be cleaned from the surface of a part by washing with water