Dye Penetrant Inspection - · PDF fileLiquid Penetrant Inspection TWI. ... A.K.A. Dye...

Copyright © 2003, TWI Ltd World Centre for Materials Joining Technology

Liquid Penetrant Inspection

TWI


Liquid Penetrant Inspection

• Surface inspection method

• Applicable to all non-porous, non-absorbing materials

A.K.A. Dye Penetrant Inspection(DPI)

Penetrant Flaw Detection (PFD)

Penetrant Testing (PT)


6 Steps in Penetrant flaw detection

• Surface preparation

• Penetrant application

• Removal of excess penetrant

• Application of developer

• Inspection

• Post cleaning and protection


1. Surface Preparation

• Physical Methods • Chemical Methods

• Scale

• Slag

• Corrosion products

• Carbon

• Oils and Greases

• Paints


Chemical Methods

• Vapour Degreasing

• Hot Solvent Degreasing

• Cold solvent Degreasing

• Solvent materials with Emulsifiers

• Acid / Alkaline Cleaning

• Steam cleaning

• Paint Removal


2. Penetrant Application

• Penetrating fluid applied to component and drawn into defect by capillary action


Penetrant Application

Critical factors

• Penetration / Dwell time

• Component Temperature


Principle : Capillary Action

• Interaction of adhesive and cohesive forces


Penetrant Properties

• Good Wetting Ability

• High Surface Tension

• Viscosity


Wetting Ability

• Liquids having good wetting ability have a low contact angle.

• Liquids having a contact angle of 90º or less will act as penetrants.

• Contact angle is strongly affected by surface cleanliness.


Contact Angle

Contact Angle


LOW HIGH

Contact Angle


Surface Tension

HIGH LOW


Surface Tension

• Strongly affected by contamination


Viscosity

• Viscosity is a measure of internal friction

• It affects the rate at which a liquid flows

• Viscosity has a strong effect on the time taken for capillary action to work

• A high viscosity penetrant will require a longer contact time and a longer development time

• A low viscosity penetrant may drain too quickly from vertical or overhead surfaces


Types of Penetrant

• Colour Contrast

• Fluorescent

• All other factors being equal, fluorescent penetrants provide the best sensitivity

• Each of the above may be water washable, solvent removable or post-emulsifiable (in order of increasing sensitivity)


2. Penetrant Application

Methods

Spraying

Brushing

Immersion

Preceded by a visual inspection


Penetrant application

• Special Methods

Thixotropic penetrants

Electrostatic spraying


3. Removal of Excess Penetrant


• Penetrating fluid removed from component surface (but not from defect)


Removal of Excess Penetrant

• Water washable

• Solvent Removable

• Post Emulsifiable

Penetrants are formulated for removal by one or sometimes more than one of the above.



• Water Washable Penetrant

Spray wash

Minimise mechanical

action

Pressure as low as possible

Temperature less than 50ºC


Water Washable Penetrant

ADVANTAGES

• Usable on rough surfaces

• Suitable for batch testing

• Cheaper than other methods

DISADVANTAGES

• Susceptible to over washing

• Least sensitive method

• Requirement for a water source


Post Emulsifiable Penetrant

Stages

• Immerse component in penetrant

• Immerse component in emulsifier

• Emulsifier diffuses into the penetrant making it water washable

• Water wash removes excess penetrant / emulsifier

• Penetrant in defects left unaffected



2 Post emulsification systems

Hydrophilic

Lipophilic

Contact time critical

Determined by experimentation


Post emulsifiable

ADVANTAGES

• Maximum penetrating ability

• Greater control over penetrant removal

DISADVANTAGES

• Not suited to rough surfaces

• More expensive

• More time consuming



Clean off the excess with a lint free cloth.

Wipe with a solvent dampened rag.

Solvent Removable

Thou shalt not spray the cleaner directly

onto the item under test.


Solvent Removable

ADVANTAGES

• Portability

• No water supply needed

DISADVANTAGES

• Not suited to batch testing

• Requires hand wiping so time consuming

• More expensive than water washable

• Potentially hazardous chemicals


Drying

Hot air recirculating oven (max 80ºC)

Forced warm air

Dry clean compressed air

Component temperature shall not exceed 50ºC


4. Development


• Developer applied to surface


Developer Properties

• Absorptive

• Fine texture

• Able to mask out background colour

• Evenly and easily applicable

• Light and even coat

• Non-fluorescing

Easily wettedContrasting colourEasily removedNon-toxic and Non-irritant


Developer action


• Developer applied to surface

• Penetrant drawn back out of the defect by reverse capillary action


Developer action

Capillary Action - Increases the size of an indication far beyond the actual defect size; increases the “thickness” of an indication, thereby increasing it’s colour brilliance.Light scattering - Aids the conversion of UVA to visible light when using fluorescent systems; reduces background glare when using visible systems.Solvent Action - Solvent combines with penetrant, reducing penetrant viscosity, thereby inducing a more rapid, more efficient bleed-out.


Light Scattering

I0If

IfIfIf


No Developer With Developer


DevelopmentDry powder

Component must be dryApplied by Dipping BlowingDust storm cabinet

Aqueous liquidDry after application

Applied by Immersion SprayingBrushing

Non-Aqueous liquidApplied by

Aerosol


Dry powder developer

ADVANTAGES

• Easy to handle

• No hazardous vapours

• Easy to remove

DISADVANTAGES

• Difficult to see if properly applied

• Fine powders can be hazardous

• Does not offer the best degree of colour contrast


Aqueous Developer

• Solutions


Aqueous Developer

• Solutions • Suspensions


Aqueous Developer

ADVANTAGES

• No vapours or dust

• Cheaper than non-aqueous

DISADVANTAGES

• Difficult to apply evenly

• Requires drying after application


Non-Aqueous Developer

ADVANTAGES

• Most sensitive

• Usable with fluorescent or colour contrast

DISADVANTAGES

• Hazardous solvents

• Higher cost

• Need to be correctly applied


Developer Sensitivity

• Dry powder 100 - 140 %

• Aqueous solution 110 - 150 %

• Aqueous suspension 120 - 200%

• Non-Aqueous 120 - 240%


System Classification

• Type of penetrant

• Method of penetrant removal

• Type of developer


System classification

• PENETRANT

• Colour contrast

• Fluorescent

• Dual

REMOVALWater washableSolventPost emulsifiable

DEVELOPERSDry powderAqueousNon-Aqueous


5. Inspection

• Indications viewed as soon as practicable after developer application with final assessment taking place after a minimum development time has elapsed.


30 Seconds 1 Minute 15 Minutes


5. Inspection

Colour Contrast

• White light above 500 lux

Fluorescent

• White light below 20 lux

• UV-A above 1000µW / cm2

Lighting levels critical


5. Inspection

Fluorescent

• Allow 15 minutes lamp warm-up

•Allow 5 minutes dark adaptation

•Do not wear photo-chromatic spectacles


6. Post Cleaning

DPI residues are required to be removed because

• They may be harmful to the component

or

• They may impair subsequent processing


Advantages of DPI

• Applicable to all non-porous materials

• Able to test large parts with a portable kit

• Batch testing

• Applicable to small parts with complex geometry

• Simple,cheap, easy to interpret

• Good sensitivity

• Training requirements typically less than for other NDT methods.


Disadvantages of DPI

• Will only detect defects open to the surface

• Careful surface preparation required

• Not applicable to porous materials

• Temperature dependent

• Cannot retest indefinitely

• Compatibility of chemicals


Penetrant Systems

PENETRANT

Colour contrast

Fluorescent

Dual

REMOVAL

Solvent

Water washable

Post emulsifiable

DEVELOPERS

Dry powder

Aqueous

Non-Aqueous


Selection of System

• Nature of discontinuities (size and type)

• Geometry and intricacy

• Surface condition

• Component material and application

• Size and position

• Equipment and expertise available

• Cost

• Number of components to be tested


Control Checks

• Tank levels

• Overall system performance


Overall System Performance

• Chromium plated cracked test panel

• Cracked test piece

• Quench cracked aluminium alloy block


Control Checks

Tank levelsOverall system performanceRinse water temperatureOven temperatureEquipment cleanlinessAirline filtersUV-A filters


Control Checks

UV-A / visible light levelsFluorescent / colour intensitySuppliers checkHydrophilic remover dilutionDeveloperMeter / gauge calibrations


Control Checks - Frequency

Tank levels Overall system performanceRinse water temperatureOven temperatureEquipment cleanlinessAirline filtersUV-A filtersUV-A / visible light levels

DailyDailyDailyDailyDailyWeeklyDailyMonthly


Control Checks - Frequency

Fluorescent / colour intensitySuppliers checkHydrophilic remover dilutionDeveloperMeter / gauge calibrations

MonthlyAnnual when freshDailyAnnual


UV(A)


Electromagnetic Spectrum

10-10 10-8 10-6 10-4 10-2 1cm 102 104 106 108

Wavelength

Electric Waves

TV

Microwaves

Infra red

Ultra violet

X-rays & Gamma

Light


10 100 200 300 400 500 600 700

ULTRAVIOLET VISIBLE

LIGHT LIGHT

A Damaged

Black Light EmitsUV-AUV-BUV-C



Fluorescence

UV-A Source : Mercury vapour arc lamp

+

Filter

Precautions

• Avoid looking directly at the lamp

• Do not use if filter is cracked, damaged or incorrectly fitted


Fluorescence and the


10 100 200 300 400 500 600 700

ULTRAVIOLET VISIBLE

LIGHT LIGHT

Emits DualAbsorbs


Fluorescent v Colour Contrast

• Fluorescent more sensitive

• Less operator fatigue with fluorescent

• More difficulty in monitoring fluorescent penetrant removal

• Fluorescence may degrade under UV(A), when exposed to acid and high temperatures


Any Questions Please ?

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