Failure Analysis 06

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.