Fatigue Welded

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Fatigue in Welded Structures

Lars DamkildeSection for Structural MechanicsDepartment of Civil Engineering, Aalborg University


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Stress state

Nominal stress, geometric stress and notch stress

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Stress state

- 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.

- 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.

- The notch stress is the actual stress in the weld. Unfortunately this stressstate 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.

Calculation of the geometric stress level in the actual structural part. This

can either be based on handcalculation with some semi-empiricalcorrections or by FEM.

Match of the structural details to one of the known welding details. This

step involves engineering judgement.

Calculation of the damage accumulation taking into account the stressvariation and the number of cycles. (based on Paris law)

Accumulation of damage from different stress levels (Palmgren-Miner).

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Eurocodeexperimental curves

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Some welding details

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Tubular sections

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Cumultative damage - Palmgren-Miners rule

Principle is to decompose the load history into a number of

cycles at different stress level. The damage for each level of

stress is calculated and the total damage is summed.

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Assumption for Palmgren-Miners rule

The order of loads is of no importants. That means that large load

amplitudes followed by smaller load amplitudes are equal to

smaller load amplitudes followed by larger amplitudes.

Some research results indicate that the so-called non-uniformity

index has importance, and will add to the accumulated index.

In practise the experimental data are hard to get, and they have a

large scatter.

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Load historycalculation

The load history shall be transformed into a number of cycles at

different levels. Time under load is of no importance and only the

amplitude stresses are of interest.

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The stress levels from a load history

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Rain Flow countinga method for dividing the load historyinto cycles.

The principle of dividing the process into closed loops.

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More complicated

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Rain-Flow counting can be programmed and calculated onthe fly (continously).

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Tubular jointstypical 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-empiricalformulas or Finite Element analysis.

For non-planar joints the semi-empirical formulas are more

questionable.

For most offshore structures fatigue is of great importance.

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SCF-factors for tubular joints:

Some definitions

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Different joint types - planar joints

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SCFvalues are generally high.

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Calculation of hot-spot stresses

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SCFvalues depend on the joint type.

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Stress points in a K/Y-joint

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Decomposing of a joint into two loading types.

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Geometrical definitions.

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Formulas for the SCF-factors

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