failure analysis 06
DESCRIPTION
ANALISIS DE FALLASTRANSCRIPT
Failure Analysis
Prof. Dr. Alber Sadek
Head of Welding Technology and Inspection Dept.
CMRDI - Egypt
Objectives of Failure Investigation:Failure investigation and subsequent analysis should determine the primary cause of failure, based on the determination, corrective action should be initiated that will prevent similar failure.Important contributory causes of the failure must be assessed, new experimental techniques may have to be developed or an unfamiliar field of engineering or science explored.
DESIGN
PROCESS PARAMETERSMATERIAL BEHAVIOR
(SYNERGISM)
LEAKAGE
FRACTURE
FAILURE(INADEQUATE
PERFORMANCE)
DESIGNER
PLANT MANGER
BREAKDOWN
UNACCEPTABLEDIMENSIONAL CHANGE
IMPORTANCE OF FAILURE INVESTIGATIONFailure analysis reveals one or more the following:Deficiencies in designMaterial imperfectionFabrication defectsImproper processingErrors in assemblyService abnormalitiesInadequate or improper maintenanceUnintended or inadvertent factors
Stages involved in Failure Investigation:1. Collection of background data
2. Preliminary examination of the failed part
3. Non-destructive testing / Examination
4. Mechanical Testing
5. Macroscopic Observation
6. Microscopic studies
7. Determination of Failure Mechanism
8. Chemical analysis of the failed portion
9. Analysis of Fracture Mechanics
10. Testing under simulated conditions
11. Analysis and Synthesis of all the evidences, formulation of conclusions
12. Writing of report
NEW TECHNIQUES FOR FAILURE ANALYSISADVANCED NDEMINIATURE SPECIMEN TESTINGX-RAY DIFFRACTION
- RESIDUAL STRESSESMAGNETIC BARKHAUSEN NOISE
- MICROSTRUCTUREIN-SITU METALLOGRAPHYTRACE ELEMENTAL ANALYSIS
- SECONDARY ION MASS SPECTROSCOPY (SIMS)
- AUGER ELECTRON SPECTROSCOPY (AES)
- ELECTRON PROBE MICRO ANALYSIS (EPMA)
- ENERGY DISPERSIVE X-RAY ANALYSIS (EDAX)MODELINGSIMULATION STUDIESFINITE ELEMENT ANALYSIS
TYPES OF FAILURES
MECHANICAL FAILURES
Ductile and brittle failuresFatigue failures
Distortion failuresWear failures
Creep failures
ENVIRONMENTAL FAILURES
Corrosion failuresCorrosion-erosion failures
MECHANICAL-ENVIRONMENTAL FAILURES
Stress-corrosion crackingHydrogen embrittlement
Liquid metal embrittlementCorrosion fatigueFretting fatigue
DUCTILE AND BRITTLE FRACTURESDuctile and Brittle failures are terms that describe the amount of macroscopic plastic deformation that precede fracture
DUCTILE FRACTURETearing of metal accompanied by appreciable gross
plastic deformationGray or fibrous appearance on fracture surfaceExhibit necking – Cup and cone formationMicrovoid formation and its coalescence – Dimpled structure
Dimples on a ductile fractured surface
BRITTLE FRACTURERapid crack propagation with less expenditure of energyWithout gross plastic deformationBright and granular appearance on fracture surfaceLittle or No necking – Plane strain conditionIntergranular / Transgranular mode
Intergranular mode Transgranular mode
FATIGUE FAILURESFatigue fracture is caused by:
Repeated application of cyclic loads
Fatigue cracking results from:
Repeated application of cyclic stresses that are below the static yield strength of the material – High cycle fatigue [HCF]
Repeated application of plastic strain – Low cycle fatigue [LCF]
High Cycle Fatigue Failure of a transmission shaft
Crack origin
Fatigue crack initiationOn planes oriented 45˚ to the applied stress axisIn persistent slip bands on specimen surfaceAt elevated temperature grain boundary may also
act as initiating sitesAt inclusions in the material Characterized by formation of intrusions &
extrusions
Low cycle fatigue•N = < 104 cycles•Total plastic strain range is controlled and changes in stress response monitored•Total strain = Elastic + Plastic strain•Log-log plot of total strain range Δε Vs N •Log-log plot of plastic strain ranges vs N
Coffin – Manson Relation:Δεp Nf
β = C = Constant Δεp = Plastic strain range ; β=Constant (0.5 to 0.6)Nf = Number of cycles to failureLog-log plot between plastic strain ranges vs N
High Cycle FatigueIn the elastic regime
Basquin’s Law
(Δσ/E) Nfα = C = Constant
α = Constant (between 1/8 to 1/15); Nf = Number of cycles to failure
Crack propagates:In planes perpendicular to the applied stress axis
Characterized by striations (beach marks) on the fracture surface
Clarity of fatigue striations depends on:Ductility of the materialStress levels
(High stress levels – Widely spaced; Low stress level – Small spacing)
Photograph of the failed aircraft wheel axle and its fracture surface
BEACH MARKSSTRIATIONS INDICATING SLOW FATIGUE CRACK
GROWTH
MULTIPLE CRACK ORIGINOVERLOAD FAILURE
Failure of a steam turbine blade from a nuclear power plant due to fatigue
To avoid fatigue failures:
Improvement in Design to:
Eliminate or minimize the stress raisersEliminate surface defects during manufactureRelieve tensile residual stressEnsure good surface finish
Creep FailureThermally assisted plastic deformation which is time dependent at constant load or stressAt temp. > 0.3 Tm to 0.4 Tm; [Tm ] = Melting point in Kelvin
Fracture of polycrystalline solids at elevated temperature occurs by:
Nucleation and growth of voids at grain boundary / inclusion sites Grain boundary sliding and grain boundary diffusionRupture due to dynamic recovery or recrystallizationPrecipitates in ferritic steel
Creep - curve
MECHANISM OF CREEP
Primary creep : Work hardening dominates - Decrease in creep rateSecondary creep : Balance between work hardening and recovery Tertiary creep : Loss of cross section, formation and growth of cavities
Particle coarsening, Recovery in dislocation substructure
Failed Turbine wheel assembly from a combustion turbine
Photo-microstructure of the failed turbine blade showing creep cavities and grain boundary carbide precipitation
Design consideration for high temperature application
- Minimum of these three stresses is taken
Stress to cause 1% strain in 105 hours – Long term application
80% stress to cause onset of tertiary in 105 hours
67% stress to cause rupture in 105 hours
Steady state creep:
= Aσn e –Q/RT
A = Constant, σ = Stress n = Stress exponent, Q = Activation energy
Life Prediction Methods•Damage – summation method
Linear Damage Rule - Miners Law – Pure Fatigue
•Frequency – modified stain-range method
Takes care of environmental effectLower frequency promotes fatigue-oxidation interaction
•Strain-range-partitoning method
Takes care of creep-fatigue interactionStrain hold fatigue testsStress-strain hysteresis loop partitioned to obtain contribution of
creep and fatigue in each cycle
•Ductility-exhaustion method
Fraction of ductility consumed in each strain hold test is calculatedApplied to power plant components
ENVIRONMENTAL FAILURECorrosion failure
Corrosion is the unintended destructive chemical or electrochemical reaction of a
material with its environment
Loss due to corrosion in our country is estimated to be 4% of GNP- is equivalent to Rs.24,000 crores per annum
FACTORS THAT INFLUENCE CORROSION
Temperature and temperature gradients at metal environment interface
Relative motion between the environment and the metal parts
Presence of dissimilar metals in electrically conductive environment
Processing and fabrication operations
Storage condition
TYPES OF CORROSION
Uniform corrosionPitting corrosionSelective leachingIntergranular corrosionConcentration cell corrosionCrevice corrosionGalvanic corrosion
CORROSION RATE EXPRESSION
Corrosion rate in metals are expressed as mils per years (mpy) or mm per year (mmpy) [1mpy = 0.0254mmpy]
Safe = < 5mpy Moderate = 5 to 50 mpySevere = > 50 mpy
CORROSION CONTROL METHODSModification of metals
Modification of environmentsChange of metal/environmental potentialUse of nonmetallic materials
Failure of a Monel-400 Boiler heat exchanger from a nuclear power plant
Trepanned portion
SEM PHOTOGRAPH SHOWING THROUGH - THROUGH OPENING AND INTERGRANULAR CORROSION AT THE DEFECTIVE
LOCATION IN MONEL–400 HEAT EXCHANGER TUBE DUE TO SLUDGE DEPOSIT
Intergranular corrosion on high strength
aluminium alloy due to exfoliation
Cu depleted zone
Cu rich second phase
Grain Boundary
Delamination
Photo microstructure of the Delayed Neutron Monitoring tube showing propagation of TGSCC and presence of a long seam weld with weld defect
Branched TGSCC initiation from corrosion pits due to chloride ion attack from the mineral insulation wool from the outer surface of the stainless steel tube
Pitting corrosion observed on cut cross section of a cupro-nickel tube from a turbine lube-oil cooler
MECHANICAL – ENVIRONMENTAL FAILURES STRESS CORROSION CRACKING HYDROGEN EMBRITTLEMENT
STRESS CORROSION CRACKINGSynergistic action of tensile stress and corrosive environment
General Characteristics:Only specific environment cause failure
- Season CrackingMicrostructure of the alloy influences susceptibility
– SensitizationPure metals are less susceptible
Transgranular mode Intergranular mode
Stress corrosion cracking in Stainless steel
In-situ metallography examination on the failed stainless steel dished end
TGSCC on the dished end due to presence of residual stress and improper storage
Hydrogen EmbrittlementCauses a reduction in ductility of the metal due to absorption of hydrogen
Pickup of hydrogen from:
Processing - MeltingFabrication - Welding/ElectropolishingService in a hydrogen environment - Sour gas / refineries
General characteristics of Hydrogen embrittlement:
More susceptible in high strength steels > 1240
MPa
Failure does not occur below a critical stress
Sensitive to strain rate and temperature
Delayed failure – Static fatigue
Hydrogen embrittlement is reversible
MAIN CHARACTERISTIC OF HYDROGEN EMBRITTLEMENT
PHOTOMICROGRAPHS SHOWING BLISTERS AROUND INCLUSION AND DECOHESION OF THE INCLUSION FROM THE MATRIX IN A FAILED AISI 106 Gr. B PIPE
Fracture MechanicsFracture mechanics has developed into a useful tool in the design of
Crack tolerant structuresFracture controlFailure Analysis
Fracture mechanics provide quantative information onCircumstances that lead to the failureTake preventive measures to avoid recurrence of failures
Fracture mechanics is the mathematical analysis:
Mechanical process that lead to fracture failure.Analysis based on established procedure used in solid mechanicsAnalysis concepts employ
Stress and strain field in a cracked bodyStrain energy change during cracking and fracture.
Fracture Mechanics analysis based on:Theory of elasticity -- Linear Elastic Fracture Mechanics (LEFM)Plastic deformation is excessive-- Elastic Plastic Fracture Mechanics (EPFM)
-- Non Linear Elastic Fracture Mechanics (NLEFM)Cracking -- Subcritical Fracture Mechanics (SCFM)
Condition for the onset of Fast Fracture
σ√πa = √EGc
Gc = Energy absorbed by unit area of the crack; Toughness (J/m2) σ = Stress ; a = critical size of the crack
σ√πa = K = Stress intensity factor (MNm-3/2)
Fast fracture occurs when K = Kc ; Kc = EGc
Kc = Critical stress intensity factor or Fracture Toughness
NEW TECHNIQUES & PROCEDURES ACOUSTIC EMISSION TECHNIQUE LEAK TESTING IN PHWR EDDY CURRENT TESTING IN PHWR
ROBOTICS REACTOR VESSEL INSPECTION STEAM GENERATOR INSPECTION (ISI)
IN-SITU REPAIR TECHNOLOGY END SHIELD PRESSURE TUBE REPLACEMENT
DIAGNOSIS AT AN EARLY STAGE IS AN ESTABLISHED WAY TO AVOID FAILURE
AIMING FOR ZERO FAILUREEXPERT SYSTEM ON FAILURE ANALYSISSIGNAL ANALYSIS AND ARTIFICIAL INTELLIGENCE
-- DEFECT CHARACTERISTIONINTELLIGENT PROCESSING OF MATERIALSINTELLIGENT CORROSION MONITORING AND SMART SENSORS
* OPEN * LOGIC
* ANALYTICAL
* REALISTIC
* INTEGRITY
* FIELD SENSE
* CONVICTION & HUMILITY
ENGINEERING
GURU
CONCLUSIONEngineering failure investigation is a detective process of determining why and how things went wrong.
Failure investigation helps us to improve the reliability and safety of machinery / plant and also contributes to the enhanced productivity in addition to preventing many industrial accidents.
Systematic investigations carried out on many failed components has also generated a wealth of useful information.