Sufficient stressSufficient stressCharacterised byCharacterised by --Characterised by Characterised by --Very fast crack propagationVery fast crack propagation
Many steels exhibit ductile fracture at elevated temperatures andMany steels exhibit ductile fracture at elevated temperatures andMany steels exhibit ductile fracture at elevated temperatures and Many steels exhibit ductile fracture at elevated temperatures and brittle fracture at low temperatures.brittle fracture at low temperatures.
The temperature above which a material is ductile and below The temperature above which a material is ductile and below which it is brittle is called the which it is brittle is called the Nil Nil -- Ductility TransitionDuctility TransitionTemperatureTemperature..
This temperature is not precise and will vary according This temperature is not precise and will vary according to heat treatment heat input notches etcto heat treatment heat input notches etcto heat treatment, heat input, notches etc.to heat treatment, heat input, notches etc.
TYPES OF FRACTURETYPES OF FRACTURE
Appear when yielding and deformation
Ductile fractureDuctile fracture
Appear when yielding and deformation precedes failure
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Effect of a notch on tensile ductilityEffect of a notch on tensile ductility
TYPES OF FRACTURE.TYPES OF FRACTURE.
d d
A notch also adversely affect fatigue strength!
Courtesy of Douglas E. Williams, P.E., Welding Handbook, Vol.1,Courtesy of Douglas E. Williams, P.E., Welding Handbook, Vol.1,Ninth Edition,reprinted by permission of the American Welding SocietyNinth Edition,reprinted by permission of the American Welding Society
•• it is the result of overloadingit is the result of overloading•• evidence of gross yielding or plastic evidence of gross yielding or plastic
deformationdeformation•• the fracture surface is rough and tornthe fracture surface is rough and torn
the surface shows 45the surface shows 45°° shear lips or haveshear lips or have•• the surface shows 45the surface shows 45°° shear lips or have shear lips or have surfaces inclined at 45surfaces inclined at 45°° to the load to the load direction (because maximum shear direction (because maximum shear plane is at 45plane is at 45°° to the load!)to the load!)
TYPES OF FRACTURE.TYPES OF FRACTURE.Three requirements for brittle fracture:Three requirements for brittle fracture:
•• TEMPERATURETEMPERATURE
•• STRESS CONCENTRATIONSTRESS CONCENTRATION
•• STRAIN RATESTRAIN RATE
TYPES OF FRACTURE.TYPES OF FRACTURE.Factors affecting brittle fracture:Factors affecting brittle fracture:
•• Temperature (transition curve, convergence of YSTemperature (transition curve, convergence of YSTemperature (transition curve, convergence of YS Temperature (transition curve, convergence of YS and UTS as the temperature is reduced)and UTS as the temperature is reduced)
•• Crystalline structure (b.c.c. vs. f.c.c.)Crystalline structure (b.c.c. vs. f.c.c.)•• Material toughnessMaterial toughness•• Residual stressResidual stress•• Strain rate (YS increase but UTS remain constant)Strain rate (YS increase but UTS remain constant)•• Material thickness (restrain due to surrounding Material thickness (restrain due to surrounding
BRITTLE FRACTURE.BRITTLE FRACTURE. BRITTLE FRACTURE - CASE STUDY.BRITTLE FRACTURE - CASE STUDY.Three requirements for brittle fracture:Three requirements for brittle fracture:
•• TEMPERATURETEMPERATURE
•• STRESS CONCENTRATIONSTRESS CONCENTRATION
•• STRAIN RATESTRAIN RATE
Hydraulic proof test at 10Hydraulic proof test at 10°°CC
Small pre existing cracks in the HAZ Small pre existing cracks in the HAZ adjacent to SAW, flange to shelladjacent to SAW, flange to shell
Residual stresses as PWHT had not been Residual stresses as PWHT had not been sufficient for full stress reliefsufficient for full stress relief
BRITTLE FRACTURE - CASE STUDY.BRITTLE FRACTURE - CASE STUDY.Three requirements for brittle fracture:Three requirements for brittle fracture:
•• TEMPERATURETEMPERATURE
•• STRESS CONCENTRATIONSTRESS CONCENTRATION
•• STRAIN RATESTRAIN RATE
November 1954. Irish SeaNovember 1954. Irish Sea
Stress concentration at bulkhead Stress concentration at bulkhead positionspositions
Gale force winds ( 8Gale force winds ( 8--9 on Beaufort 9 on Beaufort Scale). 10m waves.Scale). 10m waves.
BRITTLE FRACTURE - CASE STUDY.BRITTLE FRACTURE - CASE STUDY.
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•• There is little or no plastic deformationThere is little or no plastic deformation
There is little or no plastic deformation There is little or no plastic deformation before failurebefore failure
•• The crack surface will show chevron The crack surface will show chevron marks pointing back to the initiation marks pointing back to the initiation pointpoint
•• In case of impact fracture, the surface isIn case of impact fracture, the surface isIn case of impact fracture, the surface is In case of impact fracture, the surface is rough but not torn and will usually have rough but not torn and will usually have a crystalline appearancea crystalline appearance
•• The surface is 90 Deg to the loadThe surface is 90 Deg to the load
TYPES OF FRACTURE.TYPES OF FRACTURE.
Causes for brittle fracture:Causes for brittle fracture:
P f ld d f t ( lit )P f ld d f t ( lit )•• Presence of weld defects (poor quality)Presence of weld defects (poor quality)
•• Poor toughness in parent material (wrong Poor toughness in parent material (wrong choice)choice)
•• Poor toughness in HAZ (to high heat input)Poor toughness in HAZ (to high heat input)Poor toughness in HAZ (to high heat input)Poor toughness in HAZ (to high heat input)
•• High level of residual stress (no PWHT, High level of residual stress (no PWHT, wrong design)wrong design)
INTEGRANULAR CLEAVAGEINTEGRANULAR CLEAVAGE
CrackCrack
Crack grows by travelling Crack grows by travelling around the grainsaround the grains
Final fracture willFinal fracture will
TRANSGRANULAR CLEAVAGETRANSGRANULAR CLEAVAGE
CrackCrack
Crack grows by splitting each grain Crack grows by splitting each grain along a flat planealong a flat plane
Final fracture will consist of a series of Final fracture will consist of a series of small flat planes, each corresponding small flat planes, each corresponding to one grainto one grain
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FATIGUE FRACTURE.FATIGUE FRACTURE.Fatigue cracking is a service failure occurring Fatigue cracking is a service failure occurring under cyclic stress conditions.under cyclic stress conditions.
It will normally occur at a change of section.It will normally occur at a change of section.
All materials are susceptible to fatigue failure.All materials are susceptible to fatigue failure.
Fatigue failures start at a specific point andFatigue failures start at a specific point andFatigue failures start at a specific point and Fatigue failures start at a specific point and propagate with each stress cycle at a rate that propagate with each stress cycle at a rate that depends on the applied stress. depends on the applied stress.
Identified by Identified by beach marksbeach marks on the fractured face.on the fractured face.
There are two stages in the process of fatigue There are two stages in the process of fatigue cracking;cracking;
1. Time during which the fatigue crack is 1. Time during which the fatigue crack is produced (nucleated). Usually at a change produced (nucleated). Usually at a change in section or notch.in section or notch.
2 C k i t ll l d i li d2 C k i t ll l d i li d2. Crack grows incrementally as load is applied.2. Crack grows incrementally as load is applied.
Concentric rings are produced, Concentric rings are produced, Beach MarksBeach Marks, , that radiate from the the origin. Each cycle of that radiate from the the origin. Each cycle of stress causes a single ripple.stress causes a single ripple.
Fatigue fracture occurs in structures subject to Fatigue fracture occurs in structures subject to repeated application of tensile stress Crack growth isrepeated application of tensile stress Crack growth is
repeated application of tensile stress. Crack growth is repeated application of tensile stress. Crack growth is slow (in same cases, crack may grow into an area of slow (in same cases, crack may grow into an area of low stress and stop without failure).low stress and stop without failure).
Fatigue fracture surface Fatigue fracture surface smooth in appearancesmooth in appearance
Secondary mode of failure Secondary mode of failure ductile fracture rough ductile fracture rough fibrous appearance fibrous appearance
TYPES OF FRACTURE.TYPES OF FRACTURE.Fatigue fracture distinguish features:Fatigue fracture distinguish features:
•• it initiate from stress concentration pointsit initiate from stress concentration points•• load is considerably below the design or yield stress level load is considerably below the design or yield stress level •• the surface is smooththe surface is smooth•• the surface is bounded by a curvethe surface is bounded by a curve•• bands may sometimes be seen on the smooth surface bands may sometimes be seen on the smooth surface --
”beachmarks”. They show the progress of the crack front ”beachmarks”. They show the progress of the crack front from the point of originfrom the point of originp gp g
•• the surface is 90the surface is 90°° to the loadto the load•• final fracture will usually take the form of gross yielding final fracture will usually take the form of gross yielding
(as the maximum stress in the remaining ligament (as the maximum stress in the remaining ligament increase!)increase!)
•• fatigue crack need initiation + propagation periodsfatigue crack need initiation + propagation periods
TYPES OF FRACTURE.TYPES OF FRACTURE.
Fatigue Fracture, Beach Marks & Final Ductile Failure.Fatigue Fracture, Beach Marks & Final Ductile Failure.
TYPES OF FRACTURE.TYPES OF FRACTURE.The result of fatigue & brittle fracture:The result of fatigue & brittle fracture:
INTERIOR OF THE BRACEINTERIOR OF THE BRACEPRIMARY MODE OF FAILURE.PRIMARY MODE OF FAILURE.
BRACE D6BRACE D6 FATIGUE CRACK ORIGINATING FATIGUE CRACK ORIGINATING FROM HYDROPHONE SUPPORT FROM HYDROPHONE SUPPORT FILLET WELD.FILLET WELD.
HYDROPHONE SUPPORT TUBEHYDROPHONE SUPPORT TUBE
SECONDARY MODE OF FAILURE:SECONDARY MODE OF FAILURE:
BRITTLE FRACTURE. ALTHOUGH SOME BRITTLE FRACTURE. ALTHOUGH SOME DUCTILE TEARING HAS OCCURRED DUCTILE TEARING HAS OCCURRED AROUND THE CIRCUMFERENCE OF THE AROUND THE CIRCUMFERENCE OF THE BRACE.BRACE.
•• Check effect of possible weld defects; Check effect of possible weld defects; define weld qualitydefine weld quality
•• Use improvement techniquesUse improvement techniques
••Provide for inspection in service for fatigue Provide for inspection in service for fatigue crackscracks
Creep is defined as a slow deformation under Creep is defined as a slow deformation under
TYPES OF FRACTURE.TYPES OF FRACTURE.Creep fractureCreep fracture
constant load at elevated temperatures.constant load at elevated temperatures.
Can occur in materials which are operated for Can occur in materials which are operated for extensive periods at high temperatures.extensive periods at high temperatures.
The reason for creep fracture is the flow (or The reason for creep fracture is the flow (or plastic deformation) of metals when held for plastic deformation) of metals when held for long periods of time at stresses well below long periods of time at stresses well below their normal yield strength.their normal yield strength.
TYPES OF FRACTURE.TYPES OF FRACTURE.
•• Creep is a timeCreep is a time--temperature dependant temperature dependant
•• Section under stress continue to deform Section under stress continue to deform even if the load is maintained constanteven if the load is maintained constant
•• Creep is most likely when operating near Creep is most likely when operating near the recrystallization temperature of that the recrystallization temperature of that y py pmaterialmaterial
•• Usually appear in case of process plant Usually appear in case of process plant equipment, due to heating and cooling equipment, due to heating and cooling cyclescycles
