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Weld Design andWeld Design andSpecificationSpecification
Jim Glancey, PEJim Glancey, PE
Depts. of Depts. of BioresourcesBioresources EngineeringEngineering& Mechanical Engineering& Mechanical Engineering
University of DelawareUniversity of Delaware [email protected] [email protected]
http:// http:// udel.edu/~jglanceyudel.edu/~jglancey
I cant explain everything . . .I cant explain everything . . .
4 in
6 in
1000 ft
Inside weld entire length
1/4
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Factors in Weld DesignFactors in Weld Design
II Strength (static and/or fatigue)Strength (static and/or fatigue)II Material and the effects of heatingMaterial and the effects of heatingII CostCostII DistortionDistortionII Residual StressesResidual Stresses
II Easy to WeldEasy to Weld
Static StrengthStatic Strength
I Stress - strain diagram
Strain ( ) =
Stress ( )
=F
A
LL
yield ultimate(tensile)
F
F
A
L
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Shear StrengthShear Strength
I In general, material fails in shear due todistortion (at a molecular level)
I Criteria for failure: Ductile: Shear Strength ~ 0.5 Tensile Strength Brittle: Shear Strength ~ 0.75 Tensile Strength
I Weld strength analysis is generally based on
Shear Strength
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Static Strength of WeldsStatic Strength of Welds
F F Normal =
Shear =
Fw * h
Fw * h
2FF
F
1/8
75 o3/8
1/4 Max Normal =
Max Shear =
F0.618w * h
F0.707w * h
Butt
Fillet
h = throat size!
Weld Size vs. Throat SizeWeld Size vs. Throat Size
1/8
75 o3/8
h = plate thickness = weld size
Butt
h = 0.707 * plate thickness0.707 * weld size
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Fillet
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Fatigue StrengthFatigue Strength
Cycles of Loading
FatigueStrength
1000 1,000,0001
EnduranceLimit
Static Tensile Strength
LowCycle
HighCycle Infinite
Life
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Endurance LimitEndurance Limit
II For Steel:For Steel: Endurance Limit = 0.5 * Tensile StrengthEndurance Limit = 0.5 * Tensile Strength
or 100or 100 kpsikpsi , which ever is lower., which ever is lower.
II For Aluminum:For Aluminum: No endurance limit (cannot have an infinite No endurance limit (cannot have an infinite
life)life)
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Factors for Fatigue Stress AnalysisFactors for Fatigue Stress Analysis
Type of Weld Stress Increase
Butt Weld 1.2Transverse Fillet 1.5
Parallel Fillet 2.7
T-butt with corners 2.0
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Strength ConsiderationsStrength Considerations
I Try to minimize the stresses in welds; makethe parent materials carry highest stresses.
I Butt welds are the most efficientI Avoid stress concentrationsI Intermittent weld length should be at least 4
times the fillet sizeI Minimize weld size to reduce potential for
fatigue failure
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Effects of Welding on MetallurgyEffects of Welding on Metallurgy
II Depends on the alloy and welding processDepends on the alloy and welding processII In general, cracking is promoted by:In general, cracking is promoted by:
stress concentrationsstress concentrations
brittle parent material after welding (lowbrittle parent material after welding (lowcarbon steels)carbon steels)
hydrogen in the weld metalhydrogen in the weld metal
impurities in the weld metalimpurities in the weld metal
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Reducing DistortionReducing Distortion
I Prevent overweldingI Intermittent weldingI Minimize number of passesI Place welds near the neutral axis of the partI Balance welds around the neutral axis
I Anticipate shrinkage forcesI Residual stress relief
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Neutral AxisNeutral Axis
II The line (plane) where bending stresses areThe line (plane) where bending stresses arezero.zero.
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