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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 jglancey@udel.edu jglancey@udel.edu

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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