Transcript Failure Investigation - Indira Gandhi Centre for Atomic

Failure Investigation
Dr. Baldev Raj
Director, MCRG
IGCAR, Kalpakkam
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 PARAMETERS
MATERIAL BEHAVIOR
(SYNERGISM)
DESIGNER
PLANT MANGER
FAILURE
(INADEQUATE
PERFORMANCE)
LEAKAGE
FRACTURE
BREAKDOWN
UNACCEPTABLE
DIMENSIONAL
CHANGE
IMPORTANCE OF FAILURE INVESTIGATION
Failure analysis reveals one or more the following:
Deficiencies in design
Material imperfection
Fabrication defects
Improper processing
Errors in assembly
Service abnormalities
Inadequate or improper maintenance
Unintended 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 ANALYSIS
ADVANCED NDE
MINIATURE SPECIMEN TESTING
X-RAY DIFFRACTION
- RESIDUAL STRESSES
MAGNETIC BARKHAUSEN NOISE
- MICROSTRUCTURE
IN-SITU METALLOGRAPHY
TRACE ELEMENTAL ANALYSIS
- SECONDARY ION MASS SPECTROSCOPY (SIMS)
- AUGER ELECTRON SPECTROSCOPY (AES)
- ELECTRON PROBE MICRO ANALYSIS (EPMA)
- ENERGY DISPERSIVE X-RAY ANALYSIS (EDAX)
MODELING
SIMULATION STUDIES
FINITE ELEMENT ANALYSIS
TYPES OF FAILURES
MECHANICAL FAILURES
ENVIRONMENTAL
FAILURES
Ductile and brittle failures
Fatigue failures
Distortion failures
Corrosion failures
Wear failures
Corrosion-erosion failures
Creep failures
MECHANICAL-ENVIRONMENTAL
FAILURES
Stress-corrosion cracking
Hydrogen embrittlement
Liquid metal embrittlement
Corrosion fatigue
Fretting fatigue
DUCTILE AND BRITTLE FRACTURES
Ductile and Brittle failures are terms that
describe the amount of macroscopic
plastic deformation that precede fracture
DUCTILE FRACTURE
Tearing of metal accompanied by appreciable gross
plastic deformation
Gray or fibrous appearance on fracture surface
Exhibit necking – Cup and cone formation
Microvoid formation and its coalescence – Dimpled structure
Dimples on a ductile fractured surface
BRITTLE FRACTURE
Rapid crack propagation with less expenditure of energy
Without gross plastic deformation
Bright and granular appearance on fracture surface
Little or No necking – Plane strain condition
Intergranular / Transgranular mode
Intergranular mode
Transgranular mode
FATIGUE FAILURES
Fatigue 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]
Crack origin
High Cycle Fatigue Failure of a transmission shaft
Fatigue crack initiation
On planes oriented 45˚ to the applied stress axis
In persistent slip bands on specimen surface
At elevated temperature grain boundary may also
act as initiating sites
At 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 Nfβ = C = Constant
Δεp = Plastic strain range ; β=Constant (0.5 to 0.6)
Nf = Number of cycles to failure
Log-log plot between plastic strain ranges vs N
High Cycle Fatigue
 In 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 material
Stress levels (High stress levels – Widely spaced;
Low stress level – Small spacing)
Photograph of the failed aircraft wheel axle and its fracture surface
STRIATIONS INDICATING SLOW FATIGUE CRACK
GROWTH
OVERLOAD FAILURE
BEACH MARKS
MULTIPLE CRACK ORIGIN
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 raisers
Eliminate surface defects during manufacture
Relieve tensile residual stress
Ensure good surface finish
Creep Failure
Thermally assisted plastic deformation which is
time dependent at constant load or stress
At 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 diffusion
Rupture due to dynamic recovery or recrystallization
Precipitates in ferritic steel
Creep - curve
MECHANISM OF CREEP
Primary creep
: Work hardening dominates - Decrease in creep rate
Secondary 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
Photomicrostructure 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 effect
Lower frequency promotes fatigue-oxidation interaction
•Strain-range-partitoning method
Takes care of creep-fatigue interaction
Strain hold fatigue tests
Stress-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 calculated
Applied to power plant components
ENVIRONMENTAL FAILURE
Corrosion 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 corrosion
Pitting corrosion
Selective leaching
Intergranular corrosion
Concentration cell
corrosion
Crevice corrosion
Galvanic corrosion
CORROSION RATE EXPRESSION
Corrosion rate in metals are expressed as mils per years (mpy)
or mm per year (mmpy) [1mpy = 0.0254mmpy]
Safe
Moderate
Severe
= < 5mpy
= 5 to 50 mpy
= > 50 mpy
CORROSION CONTROL METHODS
Modification of metals
Modification of environments
Change of metal/environmental potential
Use of nonmetallic materials
Trepanned portion
Failure of a Monel-400 Boiler heatexchanger
from a nuclear power plant
SEM PHOTOGRAPH SHOWING THROUGH - THROUGH OPENING
AND INTERGRANULAR CORROSION AT THE DEFECTIVE
LOCATION IN MONEL–400 HEAT EXCHANGER TUBE DUE TO
SLUDGE DEPOSIT
Grain Boundary
Cu rich second phase
Cu depleted zone
Delamination
Intergranular corrosion on high strength
aluminium alloy due to exfoliation
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 lub-oil cooler
MECHANICAL – ENVIRONMENTAL FAILURES

STRESS CORROSION CRACKING

HYDROGEN EMBRITTLEMENT
STRESS CORROSION CRACKING
Synergistic action of tensile stress and corrosive environment
General Characteristics:
Only specific environment cause failure
- Season Cracking
Microstructure of the alloy influences susceptibility
– Sensitization
Pure 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 Embrittlement
Causes a reduction in ductility of the metal due to absorption of hydrogen
Pickup of hydrogen from:
Processing
- Melting
Fabrication
- Welding/Electropolishing
Service 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 Mechanics
Fracture mechanics has developed into a useful tool in the design of
Crack tolerant structures
Fracture control
Failure Analysis
Fracture mechanics provide quantative information on
Circumstances that lead to the failure
Take 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 mechanics
Analysis concepts employ
Stress and strain field in a cracked body
Strain 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
DIAGNOSIS AT AN EARLY STAGE IS AN
ESTABLISHED WAY TO AVOID FAILURE
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
AIMING FOR ZERO FAILURE
EXPERT SYSTEM ON FAILURE ANALYSIS
SIGNAL ANALYSIS AND ARTIFICIAL INTELLIGENCE
-- DEFECT CHARACTERISTION
INTELLIGENT PROCESSING OF MATERIALS
INTELLIGENT CORROSION MONITORING AND
SMART SENSORS
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OPEN
* LOGIC
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* ANALYTICAL
* REALISTIC
* INTEGRITY
* FIELD SENSE
* CONVICTION & HUMILITY
CONCLUSION
Engineering 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.