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Idea Transcript
Fatigue in Welded Structures
Lars Damkilde Section for Structural Mechanics Department of Civil Engineering, Aalborg University 1
Stress state Nominal stress, geometric stress and notch stress
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Stress state …
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Nominal stresses will normally be based on beam theory, which is valid sufficiently long from the weld. The nominal stresses define the level of stress state.
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The geometric stresses or the stress concentration factor (SCF) is due to the geometrical changes in the structure near the weld. Neighbouring elements influence through the stiffness.
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The notch stress is the actual stress in the weld. Unfortunately this stress state is virtually impossible to calculate due to the weld process which both induce change in material properties and residual stresses due to heating/cooling.
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Basis for fatigue design for welded structures •
Experimentally based S-N curves for a number of different welding details.
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Calculation of the geometric stress level in the actual structural part. This can either be based on handcalculation with some semi-empirical corrections or by FEM.
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Match of the structural details to one of the known welding details. This step involves engineering judgement.
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Calculation of the damage accumulation taking into account the stress variation and the number of cycles. (based on Paris’ law)
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Accumulation of damage from different stress levels (Palmgren-Miner).
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Eurocode – experimental curves
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Some welding details
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Tubular sections
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Tubular joints – typical in many offshore structures.
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Structural analysis • Global analysis based on 3-D Beam elements. Replacing rigid joint connections with joint flexibility is often beneficial. • Detailed analysis of joints can either be based on semi-empirical formulas or Finite Element analysis. • For non-planar joints the semi-empirical formulas are more questionable. • For most offshore structures fatigue is of great importance.