Failure Analysis OverviewDave Van Dyke, P.E., MSAWS CWI, ASME EMCFLv II NDT: VT, PT, MT, UTVGO, Inc. Engineers10220 SW Nimbus, Ste K10Portland, OR 97223www.vgoinc.com

Who we areVGO, Inc. Consulting Engineering firmMechanical & Electrical Forensic Engineers (since 1970)Licensed Professional Engineers (PEs)Certified Welding Inspectors (CWIs)Non-Destructive Testing (NDT) and instrumentation techsWork with number of consultants

Services We ProvideForensics / Failure AnalysisOn-site investigations and inspectionsLaboratory analysisTest and MeasurementDesign of experimentsData AcquisitionMechanical and Structural AssessmentsMachine DesignAnalysisInspections

IndustriesIndustrial / ManufacturingManufacturing equipment (presses, fixtures)Consumer ProductsDesign improvement, validationCivil Structures / InfrastructureStructural stabilityUtilities (inspection, testing, failure analysis)Insurance / Legal casesRoot cause, forensics

Tools of the TradeInstrumentationStrain gage, accelerometer, data acquisitionInspectionNDT, weld inspection, cracking, corrosion AnalysisFailure analysis, Fitness for purpose, Stress analysis, Finite Element Analysis (FEA), MetallurgicalTestingLoad testing, cycle testing, instrumented tests, impact test, tensile test

Failure AnalysisVast topic, many failure modes and mechanisms, overlap, disputesAlways exceptionsFocus on:Failure analysis processSome common failuresHighlight some interesting failuresPrimarily steel, but present in most materials

Definition: Failure AnalysisObjective investigation of material facts associated with a part or system failureDetermine:Timeline, chain of eventsRoot-cause of incident / contributing factorsPost incident fitness for purposeRepair optionsMitigating future failures

What is failure?Part and/or system no longer complies with design intent for part or systemSubjective definition based on operationNot always structuralLeaking hydraulic sealsInappropriate stiffness in componentRate of corrosive decay/breakdownPart/system lifetimeOperating/maintenance costsAestheticsAny design parameter

Contributing Factor AreasOriginal DesignMaterial PropertiesManufacturing and processingService FactorsLoadingEnvironmentRepair ProceduresWeld Repair

Failure Analysis ProcessBroad definition:Collection of background dataFormulate hypothesis (plural)Develop test methodologiesImplement tests / collect dataReview results, revise hypothesisEnd by budget (conflicts in articles)ASM process to follow

Investigator RequirementsOBJECTIVENESSVisual CuesVerbal CuesDocumentationNames, dates, times, quantities, historyQuestioning AttitudeVagueness of EnglishOpposing views of an incidentBroad background in failure mechanisms

Technological ToolsPhotography and lightingOptical Microscopy up to 600XScanning Electron Microscopy (SEM) over 10,000XChemical AnalysisSEM/EDSSpark Emission SpectroscopyFourier Transform Infra Red (FTIR)

Optical Microscopy (7)Use polished and etched specimensLimited depth of fieldShows individual grain structure

SEM Image (7)Individual grainsLarge depth of fieldVacuum chamberCharging

SEM-EDS (3)Elements give distinct peaks, often primary and secondary

FTIR (4)Measurement of frequencies that are absorbed by organic media

Technological ToolsMechanical TestingHardness, micro-hardness, tensile, shear, physical testingStress Analysis / FEAMagnitude, principal direction, sensitivityInstrumentationStrain Gages, accelerometers, thermocouples, ect.Non-Destructive Testing (NDT)

Mechanical TestingUp to 100,000 lbf in the labOver 1 million lbf in the field

Finite Element AnalysisUsed to answer particular questionsStress, strain deflection, principal directions, mode shapes, thermal, impact, etc.3500 ton forging press frame, cracking problems

Strain GagesAttach to surface to measure surface strainsAvailable in hundreds of configurationsUsed to calibrate FEA models, measure loads, dynamicSignal conditioning, error elimination, ground loops etc.

NDT Magnetic Particle (MT)ProQuickly inspect large areasPart must be magneticConLimited depth of detectionInterpretation requiredBridge trunion toolmark example

NDT Liquid Penetrant (PT)ProClearly evident surface breaksCan give limited information about depthConfirm other NDT indicationsGood contrastConsCant be used on porous surfaceStrict cleaning requiredTemperature limitationsTexas Shell and Tube example

MT with PT background5 inch Vertical crack toe of SMAW multi-pass weld3 inch MS plateDetected by MTPT background

NDT Ultrasonic (UT, PAUT)ProGood for detecting subsurface crackingQuickly inspect shafts, plates, jointsConsLimitations on inspect-able areasMany discontinuities are shownDifficult to interpret resultsTrojan Example, grain sizeInterstate Bridge, old steel, unknown geometry

ASM Failure Analysis (8)American Society of Metals (ASM) outlineExperts in each areaRemain OBJECTIVEImportant to be thorough, information will be lostFailure analysis becomes less reliable with less information

ASM Failure Analysis (8)1. BackgroundLocation, name, P/N, description, manufacturer, fabricatorFunction of itemMaintenance / cleaning historyOperational historyOperational documentationNormal stress orientationsExtent of incidentPrecipitating eventsDrawings, photographs, reports, inspectionsService deviationsOpinions of related personnel

ASM Failure Analysis (8)2. Visual ExaminationSurvey the entire regionMacroscopically classify the fractureEstimate manner of loadingDetermine associated equipmentObserve colors, contaminants, corrosion products, grinding marks, weld progression, other structures in regionBase material quality, uniformity, coatingsDocument and record

ASM Failure Analysis (8)3. Fractographic ExaminationOften necessary to ascertain failure modeIdentify microstructureNote fracture progressionNote deformationIsolate contaminantsNote colorsAnomaliesDistinguish post incident damage

ASM Failure Analysis (8)4. Chemical AnalysisBase metal compositionContaminant compositionPresence of coating on fracture surfaceCorrosion product composition

ASM Failure Analysis (8)5. Mechanical PropertiesBulk material propertiesAnomalous material propertiesHardnessDuctilityTensile strengthCorrosion susceptibilitySCC susceptibility

ASM Failure Analysis (8)6. Macroscopic ExaminationOverall homogeneityUniform crackingAny differences

ASM Failure Analysis (8)7. Metallographic ExaminationSection polish and etch8. MicrohardnessTraverse across crack, HAZ. Determine gradientsInclusions, metallographic phases

ASM Failure Analysis (8)9. SEM analysisIdentify microscopic fracture modesDuctile dimpling, inter-granular, cleavagePresence of contamination on fracture surface10. MicroprobeSEM-EDS of individual areasGraphite, carbides, precipitates

ASM Failure Analysis (8)11. Residual Stresses and phasesX-Ray diffraction, strain gage methods12. Simulation / TestsFEA, stress analysisTestingConsistent with findings?13. Summarize findings14. Report and Distribute15. Follow-up

ASM Failure Analysis (8)16. Preserve EvidencePackage carefullyDO NOT put fracture faces togetherUse desiccantOther experts, new informationSometimes destructive testing is required

Failure MechanismsParts Fail for many reasons (Deficiency)OperationalPlastic deformation (permanent set, buckling)Excessive deflection (floors, beams, shafts)Excessive vibration (machine mounts)Acoustics issuesFractureDuctile, Brittle, Fatigue, Thin Lip

Failure MechanismsCorrosionThinningStress Concentrators (risers)Corrosion ProductsWearThinningWear ProductsWelding

Synergistic EffectsAll failures have components of multiple failure mechanisms associatedSimultaneous presence, interactingTask is to determine the important ones from the auxiliary modes

Fracture Ductile (1)Plastic deformation present prior to fracture (aluminum, mild steel)Preferred fracture modeStrength increases past yield

Fracture Ductile (10)

Fracture Ductile (2)ANSI 304 SS1000xMicrovoids

Fracture Brittle Typical of glass, cast iron, chalk, HSS, plane strain situations, low temperatureDuctile to brittle transition tempAustenitic SS for cryogenicsNo or limited plastic deformation

Brittle Fracture (10)

Macro- verses Micro-Macroscopic fracture features may differ from microscopic featuresMicroscopically ductile featuresMicro-void coalescenceAssociated energyMicroscopically brittle featuresCleavage of individual grainsIntergranular

Brittle Fracture of Ships (10)

Fracture Inter-granular (4)Fracture proceeds between the individual grain of the materialFailure of grain boundary material

Inter-granular SEM (7)

Fracture Trans-granular (4)Fracture proceeds through the individual grains of materialCleavage

Transgranular

Overload FractureCommonAbuse of deviceInaccurate estimate of service loadingAnalysis errorsInappropriate materialNo account for rare / one-time loadsSometimes Fast Food engineering

Wind Induced VibrationTacoma NarrowsAK Transmission LinesMeasure f1, tensionDynamic onlyOverload? Design?

Ductile OverloadMacroscopicNecking or plastic deformationDull and fibrous fracture surfaceShear lipsOptical MicroscopicPlastic distortion of grainsIrregular trans-granular fractureSEM MicroscopicMicro-voids elongation in direction of loadSingular crack with limited/no branching

Brittle OverloadMacroscopicLittle/no plastic deformationShiny, course, crystalline fracture surfaceChevronsOptical MicroscopicMinimal deformationInter-granular or trans-granularSEM MicroscopicCleavage or inter-granularDiscontinuity or stress riser at origin

Fracture FatigueVery common form of fractureMulti stage cracking phenomenonPh 1 Initiation (ratchet-marks)Ph 2 Propagation (beach marks and striations)Ph 3 Final Fracture (ductile/brittle overload)Cycling component, plastic strains, tensile strains (can be produced by compressive load)

Macroscopic Fatigue (5)Fatigue (Ph 1, 2, 3)Shear lip (plane-stress)What else can we tell (hypothesis)?Material / state?Environment?Significance of stress-riser?Service history?

Fatigue Striations (6)Individual crack propagation stepsBeach marks are made up of varying striation configurations

Chevron Marks (10)Not only fatigue relatedGive crack propagation directionPoint back to initiation site

Preventing FatigueRemain below endurance limit (steels)Be aware of factorsStress risersInclusionsWeld defectsAnisotropic materialsLow occurrence loadsResidual stressesImpart residual / applied compressive stressesTie-rodPeening

Stress Corrosion CrackingTensile stress, susceptible material, appropriate agentChlorides, ammonia, Demo: Austenitic SS in NaCl-H2OBrass SCC on trap primer valves

SCC

SCC

SCC

Incorrect Material PropertiesNot as specified by design, substitution, free machining, cheaper, stainlessMaterial does not meet specChina outsourcing, A36 isnt always

Welding FailuresGTAW (TIG), SMAW (Stick), GMAW/FCAW (MIG)Ductile Fracture, Brittle Fracture, FatigueGeometry: Undercut, overlap, underfillLack of Penn, Lack of fusion, inclusions porosityHAZ microstructure / ductility

Welding FailuresSolidification Cracking (Hot Cracking)Fish EyesHydrogen Embrittlement (Cold Cracking)Lamellar Tearing

Welding FailuresLincoln, Miller, Hobart produce high quality welding wires and electrodesKilling agents (Al, Ti, Si in steel)High Alloy contentDesigned for solidification with good propertiesMost problems occur in HAZ of baseImproper welding procedure / techniqueCleaning and PrepPreheat / Interpass / Post weld heat treatrod selection and joint configuration

Welding Problems CausesHAZ gradients, rate of cooling determinesMicrostructure constituentsHardnessGrain sizeInclusions, lack of fusion, lack of penetration, porosity, produce stress risersLarge residual tensile stressesAmazing that welding and airplanes work

Porosity and Slag (8)Rounded featuresHAZ typically more important, toes

Lack of Penetration (8)Weld is goodStress concentrationHAZ hardness

Undercut (8)Incomplete penetrationUndercutUnderfillJoint design

Lamellar Tearing (9)

Hydrogen cracking AKA cold crackingDelayed, hours to daysSeen in weldingAlso seen in Plated products (Cadium, chromium, black oxide) with improper thermal treatmentMigration of elemental hydrogen to grain boundaries at room temperatureGlycerin demonstration

HydrogenH2 Sources:Acids, electroplating, corrosion byproduct, water, oils, etc.Highly soluble in molten steel, significant drop with declining tempMore of a concern in HSSCan cause brittle fracture at low stressesLoss of ductility

HydrogenHelps drive carbon equivalent requirements for welding alloy steelsXX18 SMAW electrodes, requirements for sealing and rod ovensXX10 SMAW high hydrogen rods for root passPost weld thermal treatment helps remove excess hydrogen, relieve high stresses, lower hardness

Hydrogen cracking FractureInter-granular or trans-granular, changes along lengthInter-granular with higher strengthComplex fractography based on crystalline structure

Cold Cracking Bolts (8)Hydrogen embrittlementSHCS, FHCS, Gr 8 bolts, self-tapping screws, springs, plated parts

Hot CrackingAKA Solidification crackingCenterline cracking in weldsSimilar to shrinkage voids in castingsJoint restraintCan be simulated by external stressDevletian et al.Aluminum welding, 4043 rod

Corrosion RelatedGeneralized/Uniform CorrosionGalvanic CorrosionStress Corrosion Cracking (SCC)Inter-granular CorrosionPitting and Crevice CorrosionSelective LeachingHydrogen Damage/EmbrittlementLiquid Metal Embrittlement (Hg, Cd, Pb, Zn)Biological Corrosion

High Temp FailuresThin lipThick lipCreepGrain growth

Wear FailuresAbrasive WearFrettingRolling Contact FatigueBearingsRoads, potholesSpalling, weldingCorrosive WearErosive WearCavitation Erosion

Examples:Bronze SCC on Columbia DamRe-occurring cracking 3500 ton pressWeld repair of 4140 shaftingWelding of 12L14, sulfide inclusionsWeld quality inspection of water tankSCC criminal case Switzerland/Netherlands

SummaryRemain OBJECTIVEBe aware of various failure mechanismsDesign to ACTUAL loads, not estimatesAnalyze stress concentrationsKnow your materialDesign for ductile fracture

Questions?

Any Questions?

Please feel free to contact me to ask.

References(1) http://www.benbest.com/cryonics/sscurve.gif(2) http://mot.vuse.vanderbilt.edu/mse150/Fracture/Ductile/ductile.htm(3) http://www.science.smith.edu/departments/SEM/Manual99.pdf(4) http://www.corrosion-doctors.org/Forms/scc.htm (5) http://materials.open.ac.uk/mem/mem_mf8.htm (6) http://www.asminternational.org/pdf/spotlights/jfap0502p011.pdf (7) http://corrosion.ksc.nasa.gov/stresscor.htm(8) ASM Handbook volume 11, Failure Analysis and Prevention(9) http://www.twi.co.uk/j32k/protected/band_3/jk47.html(10) http://www.ae.utexas.edu/courses/ase324_huang/Lecture15.pdf