PDF Fatigue Failure ofWelded Joints - CANDU Owners Group
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Fatigue Failure of Welded Joints
Lecture 18
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Lt:!cture Scope
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? Fundamentals of fatigue failure of metals
? Effects of walding on fatigue
? Fatigue design approaches
t
e 18
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? Many types of structure experience fluctuating or repetitive loading
- Bridgl3S - Axles or shafts in machinery and vehicles - Pressure vessels and piping in cydic operation
Lecture 18
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Example' Rot~ting axle
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Stress history at point A
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u)
ID f---I-----.,----#------'\_
a:
t;
..
TIME, T
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Fatigue FaHure
? Fatigue failure is the formation and growth of a craGk caused by repeated or fluctuating loading
? Continued crack growth may end in sudden collapse or fracture when the remaining area is ins ufficient to support the load
Lecture 18
p5
FatJ"gue.Cr'ac",m~: .~ n~"-1r1"]a.?o" on
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? If the stress range is sufficiently high, plastic slip occurs in surface grains
? After a number of cycles microscopic cracks initiate at the slip regions and at microscopic defects
? "Stage 1" cracks are slow to initiate and grow
Grain BoundaI) Surface Micro Cracks
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....afngrt..le Crack Growth
? Stage 2 cracks are less influenced by microstructure. They tend to be oriented normal to the maximum tensile stress
? Each load cycle produces a crack growth increment
? The magnitude of the growth increment depends on the stress intensity, material propel1ies and environment
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Lecture 18
p7
Fatigu9Strength: S-N plot
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? Fatigue strength is commonly represented by a plot of stress range against cycles to failure or "S-N plot"
? However, before S-N data can be used the designer has to have a way of accounting for the relevant stresses
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L '"~ "- SI - ._.. ' - -' _. _. _.
, r-...
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Fatigue life at stress
range SI is Nl cycles
Se is "endurance limit"
below which alternating stresses are considered non-damaging
Se
3E+3 lE+4 3E+4 lE+5 3E+5 lE+6 3E+6
No. of Cycles, N
e 18
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