Weld Design and Specification
Jim Glancey, PE Depts. of Bioresources Engineering & Mechanical Engineering University of Delaware
[email protected] http://udel.edu/~jglancey
I can’t explain everything . . .
1000 ft 4 in
6 in
1/4” Inside weld entire length
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Factors in Weld Design I Strength (static and/or fatigue) I Material and the effects of heating I Cost I Distortion I Residual Stresses I Easy to Weld
Static Strength I Stress
- strain diagram F A
Stress (σ) =
F A
Strain (ε) =
L
ultimate (tensile)
yield
∆L L
F
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Shear Strength I In
general, material fails in shear due to distortion (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 Welds Butt F
Normal =
F 1/8 75o
Fillet
Shear = 3/8
1/4
2F
F w*h F w*h
F
F Max Normal = 0.618w * h
F
Max Shear =
F 0.707w * h
h = throat size!
Weld Size vs. Throat Size Butt h = plate thickness = weld size 1/8 75o
Fillet
3/8
1/4
h = 0.707 * plate thickness 0.707 * weld size
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Fatigue Strength Static Tensile Strength
Fatigue Strength
Endurance Limit Low Cycle
1
High Cycle
1000
Infinite Life
1,000,000
Cycles of Loading
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Endurance Limit I For Steel:
• Endurance Limit = 0.5 * Tensile Strength or 100 kpsi, which ever is lower. I For Aluminum:
• No endurance limit (cannot have an infinite life)
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Factors 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 Considerations I Try
to minimize the stresses in welds; make the parent materials carry highest stresses. I Butt welds are the most efficient I Avoid stress concentrations I Intermittent weld length should be at least 4 times the fillet size I Minimize weld size to reduce potential for fatigue failure
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Effects of Welding on Metallurgy I Depends on the alloy and welding process I In general, cracking is promoted by:
• stress concentrations • brittle parent material after welding (low carbon steels) • hydrogen in the weld metal • impurities in the weld metal
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Reducing Distortion I Prevent
overwelding I Intermittent welding I Minimize number of passes I Place welds near the neutral axis of the part I Balance welds around the neutral axis I Anticipate shrinkage forces I Residual stress relief
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Neutral Axis I The line (plane) where bending stresses are
zero.
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