BMFB 4283NDT & FAILURE ANALYSIS

Lectures for Week 2

Prof. Qumrul Ahsan, PhD Department of Engineering MaterialsFaculty of Manufacturing Engineering

2.0 Liquid Penetrant  2.1 Introduction2.2 Fundamentals2.3 Techniques2.4 Applications

Issues to address

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)

oPT 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 topull 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 4) Apply Developer 2) Apply Penetrant

3) Remove Excess5) Visual Inspection

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 tensionThe surface tension for a droplet of liquid is = ½ (po - pi ) r, where (po - pi ) 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

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

• Wetting is the ability of a liquid to maintain contact with a solid surface, resulting from intermolecular 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 surfaceAt 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.

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

• 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 10o

Contact angle

Degree ofwetting

Strength of:

Sol./Liq.interactions

Liq./Liq.interactions

θ = 0 Perfect wetting strong weak

0 < θ < 90° high wettability

strong strong

weak weak

90° ≤ θ < 180°

low wettability weak strong

θ = 180° perfectlynon-wetting weak strong

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

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:

whereh 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 risefiner defect (hairline) -> greater indication

Penetrability• For cylindrical volume capillary pressure P = 2gl 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

• Static Penetration Parameter (SPP) = Scosθ– Smaller contact angle, θ higher P– Good penetrant 5o

• Width of discontinuity; D(2r)– Narrower higher P Longer time to rise– Finer defect longer dwell time

High Surface TensionLow Wetting Ability

Low Surface TensionHigh Wetting Ability

Penetrability• Viscosity, –Not significantly affect penetrant 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

• 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

High Surface TensionLow Wetting Ability

Low Surface TensionHigh Wetting Ability

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 widthMore penetrant it holdsMore penetrant is present to form

indication

• Length of Defect– Affect volume of penetrant– Strongly affect the visibility

Very fine defect has insufficient widthReduce the ability of human eye to detect

indication visuallyCan 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 penetrationWater adversely influence contact angleAcidic or alkaline contaminants also fading the dye

visibilityHeat and prolonged exposure under UV light cause

penetrant lose their sensitivity

Flaw Entrapment Efficiency

•Penetrant dye

–Type of dyeAffect 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 dyeDifferent concentration within a

classification affect the sensitivity levelAltering dye concentration affect the

ability of penetrant penetration

Flaw Entrapment Efficiency•Processing

– Method of processingDip and drain allows more volatile constituents of penetrant

to evaporate during dwell timeIncreases the concentration of dye within remaining penetrant

– Degree of penetrant removal• No background coloration (cleaned component) high degree

of contrast penetrant indication• 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 MaterialsPenetrant Type

I FluorescentII Visible

MethodA Water WashableB Postemulsifiable - LipophilicC Solvent RemovableD Postemulsifiable - Hydrophilic

Developer FormDry PowderWet, Water SolubleWet, Water SuspendableWet, Non-Aqueous

Penetrant MaterialsPenetrants 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

• 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 40C

– 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

Penetrant Properties• Flash point:

– Temperature at which enough vapor is given off to form combustible mixture

– Typical min 93C– Should not be flammable

• Volatility:– Characterize by vapor pressure or boiling point– Good penetrant usually evaporate too quickly– Low volatility is desirable so the penetrant dry from the

surface, leave stained and from any discontinuity, leave precipitated dye

Penetrant Properties• Chemically inert:

– Must be inert, non-corrosive as possible chemically compatible with the material being tested

– 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 shapesReduces over sprayingRequires 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 partMust 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 Red and Fluorescent Penetrant 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 penetrant is usually red because red stands out and provides a high level of contrast against a light background

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 MethodPenetrants 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 widening

indication– Expands the bulk dye into many thin films enhance

brightness• 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-Clean2. Penetrant Application3. Excess Penetrant Removal4. Developer Application5. Inspect/Evaluate6. Post-clean

Pre-cleaning – Step 1

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

• 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.

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

Before CleaningAfter abrasive cleaning

DefectPeened or Closed

Light Acid Etching

Light Acid applied

Thin layer of the surface dissolved

Light Acid Etching

After Acid Etching

The defect opened again to the surface

Chemical Methods

Hot Solvent Degreasing

Heating Element

Solvent

Components

Heating Element

Solvent

Components Condensor

Drip Tray

vapourva

pour

The most effective method for degreasing

Vapour 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

Ultrasonic Crystal

Solvent/ water

Components

Caution About Metal SmearingSome 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.

Etching of the surface prior to inspection is sometimes required.

Before Sanding

After Sanding

After Etching

Penetrant Application – Step 2Many 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.

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.)

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 stepin 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 diffuseinto 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 4The 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

Aqueous Developer

SOLUTION SUSPENSION

The particles settled down if not agitated

TWO TYPES

Developer Application – Step 4 (cont.)

• 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.

Wet Developer (water- suspended and water- soluble)

Aqueous Developer

ADVANTAGES• No vapours or dust• Cheaper than non-aqueous

DISADVANTAGES• Difficult to apply

evenly• Requires drying

after application• Proper mixing

requiredToo little developer particle : Very weak indicationsToo 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 5In 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/cm2

– 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 5A 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

• True or Relevant indicationsIndications caused by defects

• Non Relevant indicationsIndications caused by assembly or geometry of the component

• False indicationsIndications caused by operator faults

CRACKSPOROSITYLAP

SPLINES

ROUGH PROFILETHREAD

HOLESPOROUS NATURERIVETS

PRESS FITTED

IMPROPER WASHING

Post Clean – Step 6The 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 componentPenetrant and emulsifier are alkaline: may cause surface pitting, especially on aluminium.Developer will entrapped moisture: may cause corrosionorThey may impair subsequent processing

Penetrant Inspection SystemsPenetrant systems can be highly portable or stationary.

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

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 more suited for factory use than onsite test

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.• However, PT involves a number of processing steps

that must be closely control to achieve optimal sensitivity.

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.

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