PDT 154 MATERIALS TESTING LIQUID PENETRANT TESTING MRS NOORINA HIDAYU JAMIL FACULTY OF ENGINEERING TECHNOLOGY UNIVERSITI MALAYSIA PERLIS

Introduction Also known as Penetrant Testing / Inspection (PT/I), Dye Penetrant Testing/Inspection (DPT/I), Liquid Penetrant Testing (LPT). Penetrant Flaw Detection (PFD).

Intrduction Liquid (or dye) penetrant testing is: surface testing method. used for detecting surface-breaking defects. applicable to all materials - except for high porous (absorbing) materials.

Penetrant Testing: SurfaceTesting Cannot be detected by Penetrant Testing OPEN TO SURFACE/ SURFACE BREAKING SUBSURFACE INTERNAL Penetrant Testing can only detect surface breaking defects Penetrant must be able to enter the defect to form indication

Materials LPT can be used to inspect almost any material provided that its surface is not extremely rough or porous. Materials that are commonly inspected using PT include the following: Metals (aluminum, copper, steel, titanium, etc.) Glass Many ceramic materials Rubber Plastics

Not suitable for porous or very absorbent materials Examples: Wood Cloth/fabric Unglazed ceramic /pottery

LPT Detection Liquid penetrant inspection is used to inspect of flaws that break the surface of the sample. Some of these flaws are listed below: Fatigue cracks Quench cracks Grinding cracks Overload and impact fractures Porosity Laps Seams Surface defect in welds

Fatigue Crack

Quench Crack

A gear with a fractured tooth showing adjacent teeth with incipient cracks. There are also surface cracks due to heavy grinding which line up with the fatigue cracks and might have acted as stress raisers.

Basic Principle of PT Liquid Penetrant & Developer Penetrant testing is a process in which the liquid penetrant is drawn into small openings by capillary action when it is applied to a surface.

After a specified time, excess penetrant is removed from the surface.

Then developer is applied to the surface Then developer is applied to the surface. The developer absorbs residual penetrant drawn from the flaw (reserve capillary action) leaving a bright-colored penetrant “bleeding” through the developer’s white background giving a clear visual indication of cracks, porosity, and other flaws.

Penetrant which pulled out from the defect by the developer and forms indication of the defect Indications

Principle : Capillary Action Interaction of adhesive and cohesive forces

Capillarity The formula = 2S Cos  W = Capillary pressure The ability of a material to enter opening examples: tube or defects The formula = 2S Cos  W = Capillary pressure S = Surface tension  = Contact angle W = Width of opening

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 Less than 90º High 90º Moderate More than 90º Low

Meniscus The contact angle determines the wetting ability The smaller the contact angle the higher the wetting ability

A good penetrant should have concave meniscus Mitre Convex A good penetrant should have concave meniscus

Surface Tension High Surface Tension Low Surface Tension Example: Water Example: Oil

Surface Tension From the formula Capillary pressure = 2S Cos O W The higher the surface tension the higher the capillary pressure

Surface Tension Strongly affected by contamination HIGH LOW

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

Advantages & Disadvantages of LPT

Advantages of LPT The method has high sensitive to small surface discontinuities/defects. The method has few material limitations, i.e. metallic and nonmetallic, magnetic and nonmagnetic, and conductive and nonconductive materials may can be inspected. Large areas and large volumes of parts/materials can be inspected rapidly and at low cost.

Advantages Parts with complex geometric shapes can be routinely inspected. Indications are produced directly on the surface of the part. Aerosol spray cans make penetrant materials very portable. Penetrant materials and associated equipment are relatively inexpensive.

Disadvantages of LPT Only surface breaking defects can be detected. Only materials with a relative nonporous surface can be inspected. Precleaning is critical as contaminants can mask defects.

Disadvantages Metal smearing from machining, grinding, and grit or vapor blasting must be removed before LPT. Defect Peened or Closed

Disadvantages The inspector must have direct access to the surface being inspected. Surface finish and roughness can affect inspection sensitivity. Multiple process operations must be performed and controlled. Post cleaning of acceptable parts or materials is required. Chemical handling and proper disposal is required.

System Classification Type of penetrant Method of penetrant removal Type of developer

PENETRANT REMOVAL DEVELOPERS Colour contrast Solvent Dry powder Systems PENETRANT Colour contrast Fluorescent Dual REMOVAL Solvent Water washable Post emulsifiable DEVELOPERS Dry powder Aqueous Non-Aqueous

PENETRANT Colour Contrast Dual sensitivity Fluorescent

Colour Contrast Also known as Visible Dye Penetrant Uses white light : Daylight or artificial white light Bright coloured dye : usually RED

Fluorescent Penetrant Fluorescent :Material with the ability to convert one wavelength to another. 570nm 365nm UV Light Visible Light Fluorescent Dye

Ultraviolet light

UV-A Source : Mercury vapour arc lamp 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

Fluorescent vs Colour Contrast Fluorescent more sensitive More difficulty in monitoring fluorescent penetrant removal

Basic Processing Steps of a Liquid Penetrant Testing

Procedure Standards specify test methods, eg ASTM E165, ASME Section V, ISO 3879, AS 2062, BS EN571, Mil-L-25135, Mil-L-6866 etc. All materials (penetrant, remover and developer) should be from one supplier as a kit Follow the supplier's directions Parts must be clean and dry and at the correct temperature (~50oC) Precleaning requires removal of all oil, grease, dirt, paint, slag, spatter.

6 Basic Steps Pre-cleaning Penetrant application Removal of excess penetrant Application of Developer Inspection Post-cleaning

Preclean/Surface Preparation Penetrant Application Dwell Penetrant Removal Dry Developer Dry Inspect Post Cleaning/Surface clean

1. Surface Preparation One of the most critical steps of a liquid penetrant inspection is the surface preparation. The surface must be free of oil, grease, water, or other contaminants that may prevent penetrant from entering flaws/defects. The sample may also require etching if mechanical operations such as machining, sanding, grinding or grit blasting have been performed. These and other mechanical operations can smear the surface of the sample, thus closing the defects.

Pre-Cleaning: The most important step Contaminants: Restrict penetrant from entering defects Produce false indications

Surface Preparation Scale Oils and Greases Physical Methods Chemical Methods Capable of removing contaminants on the surface but not in the defects Chemical reactions to remove Scale Slag Corrosion products Carbon Oils and Greases

Methods of Cleaning Physical methods Brushing Blasting Chemical methods Hot solving degreasing Vapour degreasing Cold solvent degreasing Solvent material with emulsifiers Alkaline cleaning Acid pickling Steam cleaning Paint removal

After abrasive cleaning Physical Cleaning Grinding Abrasive Blasting Wire brushing Defect Peened or Closed Before Cleaning After abrasive cleaning

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 Solvent Components Heating Element Hot Solvent Degreasing Heating Element Solvent Components

Vapour Degreasing The most effective method for degreasing Condensor Components vapour vapour Solvent Drip Tray 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 Back

2. Penetrant Application Once the surface has been thoroughly cleaned and dried, the penetrant material is applied by spraying, brushing, or immersing the parts in a penetrant bath. Type 1: Fluorescent Penetrants Type 2: Visible Penetrants Type 3: Dual

Penetrant Application Spraying Methods Brushing Immersion

Penetrant application Electrostatic spraying Special Methods Thixotropic penetrants

Electrostatic Spray Extrator Fan Component +ve charged Penetrant -ve charged

Thixotrophic Penetrant Two stages of viscosity penetrant Penetrant stirred Shear action convert it into liquid Penetrant in Gel Form Simillar to NO DRIP Paint Ideal for overhead works Higher viscosity means longer contact time required Applicable to higher temperature component upto 200OC

Penetrant Application Critical factors Penetration / Dwell time/ Contact time The time allowed for the penetrant to be on the surface of component Component Temperature Low temperature: Penetrant more viscous and sluggish, precipitation occurs. Requires longer Contact time. High Temperature: Penetrant may evaporate, dries up, or flash. Back

3. Penetrant Dwell The penetrant is left on the surface for a sufficient time to allow as much penetrant as possible to be drawn from or to seep into a defect. Dwell times are usually recommended by the penetrant producers or required by the specification being followed. The times vary depending on the application, penetrant materials used, the material, the form of the material being inspected, and the type of defect being inspected. Minimum dwell times typically range from 5 to 60 minutes. Generally, there is no harm in using a longer penetrant dwell time as long as the penetrant is not allowed to dry. Back

4. Excess Penetrant Removal This is a most delicate part of the inspection procedure because the excess penetrant must be removed from the surface of the sample while removing as little penetrant as possible from defects. Depending on the penetrant system used, this step may involve cleaning with a solvent, direct rinsing with water, or first treated with an emulsifier and then rinsing with water .

Removing excess penetrant Penetrant are then classified by the method used to remove the excess penetrant form the part. Method A : Water washable. Method C : Solvent removable. Post Emulsifiable Method B : Lipophilic (Oil-based emulsifier) Method D : Hydrophilic(water soluble detergent emulsifier)

Removal of Excess Penetrant Solvent Removable Water washable Post Emulsifiable The method of removal influences the make-up penetrant.

Removal of Excess Penetrant Solvent Removable Clean of the excess with a lint free cloth. Wipe with a solvent dampened rag. Solvent not be sprayed directly onto the item under test.

Solvent Removable DISADVANTAGES ADVANTAGES Not suited to batch testing 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

Water Washable Penetrant Also known as SELF-EMULSIFIED PENETRANT Pre-mixed penetrant and emulsifier Easily washed by water rinse Oily Penetrant Emulsifier Water Washable Penetrant + =

WATER SPRAY PENETRANT

Penetrant been washed off Shallow wide defects Deep or gross defects shows

Removal of Excess Penetrant Water Washable Penetrant Water shall free from contaminants example: oil Pressure usually below 2 bar Water spray Temperature max 50ºC as BS EN 571

Water WashablePenetrant

Water Washable Penetrant ADVANTAGES Ideal for rough surfaces Suitable for batch testing Cheaper than other methods DISADVANTAGES Susceptible to over washing Least sensitive method Requirement for a water source Corrosion problems

Removal of Excess Penetrant Post Emulsifiable Hydrophilic (Method D) 2 Post emulsification systems Lipophilic (Method B) Emulsification time critical: Too short- too much background Too long-penetrant in shallow/ wide defect removed Determined by experimentation

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 mixed with emulsifier Penetrant in defects left unaffected

Post emulsifiable Now the surface penetrant is water washable Post Emulsifiable Penetrant Emulsifier

Post emulsifiable Water Spray Penetrant mixed with emulsifier

Penetrant in defect not mixed with emulsifier : NOT REMOVED

Emulsifier Lipophilic Oil based Difficult to control emulsification time: too quick (diffusion of oil and oil) Rarely used nowadays Hydrophilic Water based Similar to domestic detergent Easier to control emulsification time Offers OPTIONAL FIRST STAGE RINSE

Lipophilic Remover * The emulsification time is critical A. Apply the penetrant B. Apply emulsifier C. Diffusion Begins E. Rinsing D. Diffusion complete * The emulsification time is critical * Usually not more than 3 minutes

Hydrophilic Remover (Detergent) B. Pre-rinse Clear water Detergent molecules C. Detergent Action Begins A. Apply the penetrant E. Rinsing D. Detergent + Agitation F. Clean Surface

Post emulsifiable ADVANTAGES Maximum penetrating ability Greater control over penetrant removal DISADVANTAGES Not suited to rough surfaces More expensive More time consuming

Drying It is an intermediate stage after penetrant removal Purpose of this stage to remove any droplets and puddles of water from the part. Methods of drying are :- a) wiping with clean, dry, lint - free cloth b) evaporation at ambient temperature c) evaporation at elevated temperature d) forced air circulation e) combination of the methods listed under a) to Back * The surface temperature shall not exceed 50° C during drying

5. Developers Application The role of the developer is to pull the trapped penetrant material out of defects and to spread the developer out on the surface of the part so it can be seen by an inspector. Another function that some developers performs is to create a white background so there is a greater degree of contrast between the indication and the surrounding background.

Developer Application A thin layer of developer is then applied to the sample to draw penetrant trapped in flaws/defects back to the surface where it will be visible. Developers come in a variety of forms that may be applied by dusting (dry powdered), dipping, or spraying (wet developers). Developer

Developer Functions: 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 Properties Easily wetted Contrasting colour Easily removed Absorptive Fine texture Able to mask out background colour Evenly and easily applicable Light and even coat Non-fluorescing Easily wetted Contrasting colour Easily removed Non-toxic and Non-irritant

Developers classify into six standard forms: The developer can be applied as a dry powder, or dissolved or suspended in a liquid carrier. Developers classify into six standard forms: Dry Powder Aqueous - Water Soluble - Water Suspendible Nonaqueous Type 1 Fluorescent (Solvent Based) Nonaqueous Type 2 Visible Dye (Solvent Based) Special Applications

Dry powder developer puffer dust cabinet

electrostatic powder spray guns

Dry powder developer DISADVANTAGES ADVANTAGES Difficult to ensure if it has been properly applied Fine powders can be hazardous Do not offer a high degree of colour contrast ADVANTAGES Easy to handle No hazardous vapours Easy to remove

The particles settled down if not agitated Aqueous Developer TWO TYPES WATER SUSPENDIBLE WATER SOLUBLE The particles settled down if not agitated

Form b - Water Soluble The developer can be applied by: Water soluble developers consist of a group of chemicals that are dissolved in water and form a developer layer when the water is evaporated away. The developer can be applied by: Spraying (the best method) Dipping, pouring, or brushing the solution on to the surface is sometimes used but these methods are less desirable. Parts coated with a water soluble developer must be forced dried.

Water Suspendible This developers consist of insoluble developer particles suspended in water. Water suspendible developers are applied to parts in the same manner as water soluble developers. Parts coated with a water suspendible developer must be forced dried.

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

Non-aqueous Developer Nonaqueous developers are generally recognized as the most sensitive when properly applied. Nonaqueous developers suspend the developer particle in a volatile solvent and are typically applied with a spray gun or aerosol spray cans. Since the solvent is highly volatile, forced drying is not required.

Non-Aqueous Developer ADVANTAGES Most sensitive Dry Powder GAP: Penetrant unable to be developed

Non-Aqueous Developer Solvent can bridge the gap, dilute the penetrant Solvent And develop the penetrant to surface

Non-Aqueous Developer ADVANTAGES Most sensitive Useable with fluorescent or colour contrast DISADVANTAGES Hazardous solvents Higher cost Need to be correctly applied

Solvent Based developer Dry powder developer Solvent Based developer Solvent Developer particle Gap The solvent mix with the penetrant resulting -larger volume The solvent is pull in the defect by capillary force The Penetrant unable to be developed due to the gap Non-Aqueous (Solvent-Based) developer is the MOST SENSITIVE developer Reason: The ability to bridge the gap - solvent action

Form f - Special Applications Special developers that are primarily used when a permanent record of the inspection is required. Plastic or lacquer based developers are commonly used as special developer.

Penetrant Testing Penetrant which pulled out from the defect by the developer forms indication of the defect Indications

Developer action Capillarity Light scattering Solvent action

Penetrant Testing Developer applied to surface Penetrant drawn back out of the defect by reverse capillary action

Light Scattering I0 If

Light Scattering The light scattered by the developer : spread the light to wider angle Improve seeability

Light Scattering If =  I0 ( 1 - e-kcx ) If = Intensity of fluorescence I0 = Intensity of UV light k = Ability of penetrant to absorb UV light c = Concentration of dye x = Thickness of penetrant film = Amount visible light dye will produce

Developer action Capillarity Light scattering Solvent action

Non-Aqueous Developer Solvent can bridge the gap, dilute the penetrant And develop the penetrant to surface Solvent

Developers: Considerations Solvent Based (Non-Aqueous) is the most sensitive developer Problem: Tendency to apply too much-will bury the indication Dry powder developer mainly used with fluorescent penetrant Dry powder restrict bleed out- only stick to wet area where most likely the area of defect Aqueous developer mixture Too low – will produce weak indication Too strong- will crack when dry

Development Aqueous liquid Non-aqueous liquid Dry Powder (Suspendible) (Soluble) - Component must be dry - Applied by :- * Dipping * Blowing * Dust storm cabinet -Dry after application -Applied by *Aerosol -Applied by :- *Immersion *Spraying *Brushing BACK

6. Dry/Indication Development The developer is allowed to stand on the part surface for a period of time sufficient to permit the extraction of the trapped penetrant out of any surface flaws/defects. This development time is usually a minimum of 10 minutes and significantly longer times may be necessary for tight cracks.

Inspection Inspection is then performed under appropriate lighting to detect indications from any flaws which may be present. Indications

Inspection Colour Contrast White light above 500 Lux Fluorescent Indications viewed during and after the development time Lighting levels critical Colour Contrast White light above 500 Lux Fluorescent White light below 10 Lux or 20 Lux UV-A above 1mW / cm2 or 1000µW / cm2

Viewing Conditions COLOUR CONTRAST PENETRANT Minimum 500 LUX Day light or artificial White Light ILLUMINATION: Minimum 500 LUX

Viewing Conditions FLUORESCENT PENETRANT Ambient Background Lighting Maximum 20 lux 5 minutes minimum for operator’s eyes become dark-adapted UV-A IRRADIANCE 1 mW / cm² or 1000µW / cm² Minimum BACK

Clean Surface The final step in the process is to throughly clean the part surface to remove the developer from the parts that were found to be acceptable.