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Weld Design and Weld Design and SpecificationSpecification

Jim Glancey, PEJim Glancey, PE

Depts. of Depts. of BioresourcesBioresources Engineering Engineering & Mechanical Engineering& Mechanical Engineering

University of DelawareUniversity of Delaware

jglancey@udel.edujglancey@udel.edu

http://http://udel.edu/~jglanceyudel.edu/~jglancey

I can’t explain everything . . .I can’t 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

Strength (static and/or fatigue)Strength (static and/or fatigue)

Material and the effects of heatingMaterial and the effects of heating

CostCost

DistortionDistortion

Residual StressesResidual Stresses

Easy to WeldEasy to Weld

Static StrengthStatic Strength

Stress - strain diagram

Strain (ε) =

Stress (σ)

= FA

∆ LL

yield ultimate(tensile)

F

F

A

L

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Shear StrengthShear Strength

In general, material fails in shear due to distortion (at a molecular level)

Criteria for failure:• Ductile: Shear Strength ~ 0.5 Tensile Strength

• Brittle: Shear Strength ~ 0.75 Tensile Strength

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 * hF

w * h

2FF

F

1/875o 3/8

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

1/875o 3/8

h = plate thickness = weld size

Butt

h = 0.707 * plate thickness0.707 * weld size

1/4

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

For Steel:For Steel:•• Endurance Limit = 0.5 * Tensile StrengthEndurance Limit = 0.5 * Tensile Strength

or 100 or 100 kpsikpsi, which ever is lower., which ever is lower.

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 1.2

Transverse Fillet 1.5

Parallel Fillet 2.7

T-butt with corners 2.0

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Strength ConsiderationsStrength Considerations

Try to minimize the stresses in welds; make the parent materials carry highest stresses.

Butt welds are the most efficient

Avoid stress concentrations

Intermittent weld length should be at least 4 times the fillet size

Minimize weld size to reduce potential for fatigue failure

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Effects of Welding on MetallurgyEffects of Welding on Metallurgy

Depends on the alloy and welding processDepends on the alloy and welding process

In general, cracking is promoted by:In general, cracking is promoted by:•• stress concentrationsstress concentrations

•• brittle parent material after welding (low brittle parent material after welding (low carbon 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

Prevent overweldingIntermittent weldingMinimize number of passesPlace welds near the neutral axis of the partBalance welds around the neutral axisAnticipate shrinkage forcesResidual stress relief

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Neutral AxisNeutral Axis

The line (plane) where bending stresses are The line (plane) where bending stresses are zero.zero.

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