Introduction to NDT and Penetrant Testing (PT) G.Jothinathan Scientific Officer Gr.I Department of Metallurgical and Materials Engineering I I T M, Chennai 600 036 Email – email@example.com
Introduction To NDTNon Destructive Testing (NDT) is anoninvasive method and the testing is carriedout without impairing further usefulness ofthe material, component and structureThe component can be put into use after theTesting. The signals that are employed do notalter the properties permanently .Method Signal RemarksUT US waves - elastic waves Elastic deformationRT X-rays and gamma rays No damage during the course of testing Radiation damage!!!MT Magnetic field Remnent field- Demagnetisation- interesting and highly paying field- interdisciplinary of metallurgy, physics and chemistry NDT accounts for 1/3 of the cost of an aircraft (without raw material and fabrication cost !!)
Definition of termsNDTNDI Used interchangeably- detection and reporting The scope is broader –serviceability isNDE ascertained Also includes material characterisationMaterial anomalies-interruption ,break,disruptionDiscontinuity- can meet the service requirementsDefect/flaw - renders unsuitable for serviceAnamoly whether Discontinuity or defect/flaw1.Nature of the imperfection –linear-volumetric2.Material –structural or high strength3.Criticality of use – storage vessel or boiler boiler in thermal power plant boiler in nuclear power plantLinear -crack – length > 3 times widthVolumetric - porosity and inclusion
Applications of NDTThe main application of NDT is flaw detection and evaluation.It is also used for geometric dimension measurement. material characterization, bond integrity testing , condition monitoring etcMaterial characterisation:2. E and μ determination3. Grain size evaluation4. Proportion of microstructural phases5. Extent of deformation6. Nodularity of nodular cast iron etc7. Fracture toughness determination
Purpose of NDTFracture mechanics approach – damagetolerance approach (propagates the idea of“live with discontinuities”)A material property, fracture toughness (KIc)is defined and this characterizes the materialbehaviour in presence of discontinuitieswhich the earlier strength of materials designapproach has not taken into account.KIc can be experimentally determined.The stress intensity factor (KI) the stressdistribution around a discontinuityis given by KI = G σ √πa where G isgeometric constant, σ is the working stressand ’ a’ is flaw size.Failure occurs when KI is equal to or greaterthan KIcTo determine KI,, flaw size informationneeded and is provided by NDT personnel.Life extension and life predication methods
APPLIED YIELD TENSILE STRESS STRENGTH (a) The Strength of materials approach APPLIED STRESS FLAW FRACTURE SIZE TOUGHNESS (b) The Fracture Mechanics approachComparison of classical design philosophy with fracture mechanics approach
Visual Testing (VT)This ancient and original method ofexamination by human eyes is still widelyemployed to find gross discontinuities,surface irregularities, roughness andcorrosion products on the surface.Many gadgets like lenses, cameras are used.For surfaces inaccessible to the human eyesas in the case of inside surfaces of pipes andboilers, boroscopes and flexible fibroscopesare employed. These are called endoscopes(internal vision). They are nothing but lensarrangement to transfer the image or theoptical fibers arrangement for transferring theimage.
VT of welds(contd)• Mainly for noncritical welds• Before, during and after welding• Before -Surface roughness and cleanliness (oil)• During - Electrode size and welders performance• After -Dimensional accuracy of weldments Conformity of welds to size and contour requirements
VT of welds (contd)• Acceptability of weld appearance with regard to:• Presence of surface discontinuities such as:• surface roughness weld spatter• cleanliness underfill pores• Undercuts overlaps• Cracks spatter• Establishing definite procedure to ensure uniformity and accuracy
Liquid Penetrant Testing (PT)-is applicable to discontinuities that areopen to the surface or surface connected.-is extension of visual testing-an indication is obtained whose width is very much larger than the actual width of the crack so as to be seen by the unaided eye. Crack indication
Sensitivity and applications-Sensitivity – equal or better than MT – better than RT for surface discontinuities 1 μm x 10 μm x 50 μm can be detectedApplications – on all materials – metals(ferrous and nonferrous) nonmetals(rubber,plastic etc) – all type of defects (open) – leak testingNormally not applicable to porous materials(unfired ceramics and powder metallurgicalparts)Presently Filtered particle penetrant for porous materialsRough surfaces pose problem- > 125 μm-background poses problem
Principle of Penetrant Testing PTHighly coloured (visible or fluorescent)organic dye liquid which is also surfaceactive in nature (called penetrants) is appliedon to the clean surface of the component andallowed sufficient time for penetration intodiscontinuities. The excess surface penetranton the component is removed. This leaves aclean surface of the component with pentrantresiding in the discontinuities. At this pointOf time, developer, which is highly absorptivein nature, is applied. The developer bringsback or bleeds out the penetrant therebyproviding an indication in a contrastingbackground of white colour of developer.1.Preparation of part2. Application of penetrant3. Removal of excess surface penetrant4. Development5. Inspection and Evaluation6. Post Cleaning
Clean surface of Component Penetrant application After removal of excess surface penetrant andRemoval of excess surface penetrant before application developer Inspection and Interpretation Application of developer Fig.1. Principle of Penetrant Testing
Properties of penetrantThe entire penetrant testing is based on theability of the liquid to penetrate intodiscontinuities and later ability to come out.The required properties are 1. Wettability 2. CapillarityThe ability of the liquid to wet the solid surfaceor spread over the solid surface is determined bythe surface energies of the liquid-gas interface,the solid-liquid interface and the solid-gasinterface.Mathematically expressed SSL = γSG - (γLg + γSL)Where: SSL is the wetting ability of liquid on a clean solid. γLg is the surface energy of the liquid-gas interface. γSL is the surface energy of the solid-liquid interface. γSG is the surface energy of the solid-gas interface.The liquid to spread over the solid surface, should replace the previously existing solid –gas interface. This can happen when the energydifference γSG -γSL) is positive or if γSG> > γSL). Or in other words thesurface energy of the solid gas interface should exceed, the surfaceenergy of the solid liquid interface. The difference in these energies isresponsible for the liquid to spread over the solid surface.
The ability of the liquid to spread or wet the solidsurface is related to the contact angle θ, whichquantifies the resultant adhesive and cohesive forces,The contact angle is defined as the angle between thesolid surface and the tangent drawn to the liquid atthe point of contact. It can be seen that spreadingability and contact angle are inversely related. TheFigs. Show the contact angle and wetting ability. a. low contact angle contact angle 90 deg. High contact angleNormally, the penetrants need to have a very lowcontact angle and the commercial penetrantshave contact angles between 0 –5.. Contact angledepends on the solid surface to be wetted. Water-glass has a contact angle of 0 deg. compared towater-silver which is 90 degOnce the liquid wets the surface, the ability of theliquid to rise in the capillary or enter into theopenings is determined by surface tension (T)Hence the main properties of penetrant are T and θHowever, the speed of penetration is determined byviscosity η
Other penetrant required properties1.Visibility2.Nontoxicity (noncorroding etc)Visibility is next most important property of thepenetrantColour contrast ratio of visible dye is ~1:10(The light reflected by the white background to brightred of the dye)Colour contrast ratio of the fluorescent dye is ~1:100(light emitted by the indication to the light emitted bythe dark background)Because of this colour contrast ratio, the indication isbetter seen in the case of fluorescent indication . Thehuman eye brings an effect called halation effect, theability to magnify the indication
Classification penetrantsThere are mainly three types of penetrants namely1. Visible dye or colour contrast penetrant (Type II )2. Fluorescent or brightness contrast penetrant (Type I )3. Dual mode (visible and fluorescent) (Type III)Other unclassified type is . filtered particle penetrant(Type I, II, or III is based based on the type of dyethat is incorporated -visible or fluorescent or both ) Each of these are further classified as methods 1. Water washable WW ( Method A) 2. Post emulsifiable PE lippophilic (Method B) 3. Solvent removable SR (Method C ) 4. Post emulsifiable Hydrophilic ( Method D) -classification is based on the method by which the excess penetrant is removed in the excess penetrant removal step. Simple water washing – water washable Solvent wiping – solvent removable Emulsifying &removing – post emulsifiable lippo & hydro- philic
Composition of penetrants1. Oil base2. Dye material (visible or fluorescent)3. Solvents and stabilising agents.The composition of solvent removable and postemulsifiable penetrant are essentially same.Solvent removable – by solvent actionPost emulsifiable – by dispersing the penetrant into fine particles by the applied emulsifier, making it water removable Water washable - Simple water washing Simple water washing cannot remove the oil base penetrant. Water washable penetrant has one more constituent namely built in emulsifier The moment water is applied, penetrant is dispersed by the in- built emulsifier making it amenable for water washing
Sensitivity of penetrantsAll fluorescent methods are more sensitivethan visible dye penetrant Penetrant Type Visisble dye Fluorescent Properties dye Indication Bright red Yellowish colour green(on excitation) Background White colour Dark (max. 2 of the ft.candles) developer Colour contrast ratio ~1:10 ~1:100 Visibility(seeability) Medium Very high Halation Normal High effect(ability of the eye to magnify the indication) Sensitivity Medium HighIn weld inspection, lower sensitivity methods,namely water Washable and solvent removable areemployed due to surface roughness of the weld.Hence, use of high sensitivity penetrant namelyfluorescent dye penetrant may be preferred.
Preparation of parts3.1.2 The possible contaminants on the componentsare 1.Oil and grease2. Rust or scales (Oxidation products)3. Paints and conversion coatings4. Carbon, Varnish etc5. water 3.1.3 Sources1. Fabrication processes and subsequent treatment2. Surface protection against corrosion3. Surface treatment for improvement of properties3.1.4 Interference by contaminants1.physical blocking of the discontinuity (eg Rust or scale paint or conversion coatings)2.disturbing the balanced composition of the penetrant (eg. Oil, water etc)3. Entering and occupying the discontinuities. Oil, water etc)
Application of penetrantPenetrant can be applied byimmersion, dipping ,spraying, swabbing and pouring.The only requirement is that a thin layer ofpenetrant should be present for the specifiedtime (dwell time) on the surface to be inspected.Dwell time : The total time the penetrant iscontact with the test surfaceincluding the time required for application andfor drain .Dwell time = application time + drain timeNormally 5-30 mts. Depends on the size andnature of discontinuity and the material andsurface condition of the material etc.
Removal of excess penetrantThe excess surface penetrant on the surface iscarefully removed without affecting the penetrantthat is residing in the discontinuity. This is an important step as the unremoved excess surface penetrant will affect subsequently by affecting the contrast of the indication (excessive background) and if any penetrant in the discontinuity is disturbed the volume of the penetrant indication will get reduced (The volume of penetrant is already very small)In any NDT method the Signal to Noise ratio isimportant and this should be as high as possibleS- Volume of penetrant that has entered into the discontinuityN- Unremoved excess surface penetrant in the discontinuityIt is obvious that in PT to achieve a high S/N ratio,the excess penetrant on the surface should beremoved as completely as possible and the penetrantis the discontinuity should not be lost by overwashing
Penetrant removal methods Dissolve and remove (SR)2. Disperse (emulsify) and remove by water PE3. Simply water wash if penetrant contains built in Emulsifier (WW)In each case, care need be exercised so as tocompletely remove the excess penetrant on thesurface completely and at the same time thepenetrant in the discontinuity is not affected.Wash – don’t over wash .In the case, welds, when need be inspectedwithout flushing the crown, the surfaceroughness poses problems giving backgroundcolouration thereby affecting the visibility of theIndication. Hence Solvent removable and waterwashable variations are normally employed.Here overwashing tendency of these methods, istaken to advantage, meaning, the surface is much free from the background colouration. Of course, the sensitivity suffers. Sensitivity depends on the amount of retained penetrant in the discontinuity
Developing an indication3.5.0 DEVELOPINGAfter the removal of the excess surface penetrant ,in developing step, the penetrant from thediscontinuity is brought to the surface so as toform an visible indication.3.6.1 Developer functionsThe developer functions are1.To assist the natural seepage of the penetrant inthe discontinuity and extract or blot out thepenetrant so as to form an visual indication(mechanism of visual indication)2. To provide a contrasting base which enhancesthe detection an indication. Blue-black toyellowish green: white to red3. To spread the penetrant so as to increase theapparent size of the indication4. To mask some confusing indications
Mechanism of development3.6.3 Mechanism of developing action :Natural seepage is assisted and the absoptivedevelopers blot out more penetrant from thediscontinuity. The thickness of the penetrant layeris increased to the levels above the thresholdvisibility .( Some fluorescent penetrant indications can be seen without developing with highintensity black light -3000 microwatt/sq.cm) Particles Developer Absorbed No Developer Penetrant Width is more
Type of developers and the developer used in weld inspectionDeveloper has two variations: Dry and Wet. Inthe case of wet, aqueous and non aqueous.As the sensitivity of wet developers are betterthan, dry developers and since in weldinspection, less sensitive methods namely waterwashable and solvent removable are employed ,it is better to use wet developers (mostlynonaqueous) in weld inspection in spite surfaceroughness. Normally for high surface roughness,dry powder is better suited .
Inspection, interpretation and evaluation Developing action at the end of developer time reveals an indication. In this step the indications are interpreted as to the origin of the indication namely whether True or false indications and if true indication whether relevant or nonrelevant and if relevant as to the nature of discontinuities linear or volumetric. Inspection environement depends on the type of dye - Visible or fluorescent dye white light for visible and black light for fluoresecent True indication Relevant Non relevant False indication Improper processingStandard test specimen in PT3. Aluminum comparator blocks – penetrant quality2. Chrome plated steel specimen – sensitivity
2.8.1 Inspection environmentVisible dye penetrant indications are inspected under white lightand fluorescent dye penetrant indications are viewed underblach light (ultraviolet light?)Electromagnetic radiation spectrumVisible and ultraviolet spectrumVisible light spectrum - 380 nm to 770 nm or ( V to R ) or3800 Ao to 7000 AoUltraviolet spectrum - 10 nm to 380 nm or 100 -3800 AoBlack light spectrum - 320 nm to 380 nm or (with peak at365 nm)Hg arc vapour lamp produces ultraviolet light in addition tovisible light. The filters are used to remove the harmful portionof UV (below 320 nm) and also visible light which affects thefluorescence of the fluorescent dye. The filters that arecommonly used give black light with maximum peak intensity at365 nm..
Colour and fluorescent coloursColour – part of the white light that is reflected by the surfacethat is flooded with the white lightFluorescent colour – Emission of visible light (any colour) on theimpingement of high energy radiation on the surface.In PT & MT, the impingement of black light (UV) leads toemission of yellowish green light Black light Yellowish green light Fluorescent dye
Weld defects that can be detected by PTThe preponderance of weld joints arefusion welded. The followingdiscontinuities can be detected.4. Porosity5. Lack of root penetration (on single side)6. Lack of fusion (at toe of welds)7. Cracks in heat affected zone8. Crater and other cracksWeldments at the site is inspected withvisible dye penetrant using solvent removablemethod
Some hints on penetrant testing of welds• Due to surface roughness of unflushed welds, it is better to use, water washable and solvent removable methods to avoid undesirable background.• In the case of Solvent removable pentrants, usual precaution of wiping with solvent moistened lint free cloth may not yield the desired results because of excessive background. Sometimes, the solvent is directly applied to the weld to overcome this difficulty. Of course fine cracks may be missed• In the case of TIG and MIG where better surface is obtained, it is better to go solvent wiping method• Inspection with multiple pass welds with penetrants requires that each weld bead is inspected prior to the next pass to provide the next the best reliability. Cleaning after each testing
Hints for weld inspection (contd)5.Water washable fluorescent penetrants can be used on large pressure vessels or other large structures by washing with a hose and air drying. Developent with solvent suspended developer can provide good sensitivity Water washable penetrants can be washed more readily from most weld6. It is important to remove all the of the slag from welds before penetrant testing. Grinding off the ripples on rough welds is desirable. Grinding should not smear the metal7. Brazed joints can discontinuities similar to weld beads that can be detected by PT. A braze that does not wet the surface is indication of a poor joint.
Selection guide for NDTMethod Application Advantages DisadvantageVisual Surface Economical test Limited to visual discontinuities acuity of the inspectorLiquid Surface cracks Relatively Cleaning is neededpenetrant and porosity inexpensive & before and after Reasonably fast inspection .surface films hide discontinuitiesMagnetic Surface and Relatively Applicable onlyparticlei subsurface economical and ferromagnetic discontinuities expedient materialsRadiographic Volume Provides Plane discontinuities permanent record discontinuities Surface & must be favourably subsurface aligned with radiation beanUltrasonic Most Sensitive to Small thickness discontinuities planar plate may be discontinuities. difficult to inspect. High penetration Requires skilled capability operatorEddy current Surface and Painted or coated Many variablestesting subsurface can surface signal affect the test be inspected High speed