International Institute of Welding (IIW)

The recommendations of the International Institute of Welding (IIW) provide guidelines for the design and analysis of welded structures. Within these guidelines a catalog of fatigue analysis recommendations for a variety of seam welds is given, which can be used in standard details of the construction.

An Example of an IIW Catalog Entry for Transverse Loaded Butt Welds

The IIW does not give general recommendations on a partial safety factor ym, since such a factor depends largely on circumstances, like the fatigue design strategy consequences when failure occurs. Similar to the yMf safety factor of Eurocode 3, the safety factor ym can be applied in Specialist Durability by dividing the surface influence factor in the fatigue parameters by the value of ym.

To account for insignificant residual stresses in the base material, the IIW recommends using SW/NS/IIW low RS (cat I) and SW/NS/IIW med RS (cat II) for geometrically simple thin walled details. When more complex details with high residual stresses or thick walls are present in the structure the SW/NS/IIW high RS (cat III) is recommended. If no further information of the structure is known, IIW advises to use SW/NS/IIW high RS (cat III). These recommendations also need to be defined under the fatigue parameters of a fatigue analysis case in Specialist Durability.

Similar to the Eurocode 3, the IIW also recommends using the Haibach-Miner rule to calculate the damage accumulation.

The validation area of these methods is restricted to well-defined loading conditions. However in real life applications the loading conditions are multi-axial and changing in time. Still, the method has proven to be successful for thick-walled structures but problems arise when the method is applied in the transportation industry were the sheets much thinner. Due to the strife for weight reduction, the thickness of the sheets continues to decrease. In thin sheets the stress rises up from the nominal stress level in close proximity of the seam weld. This means that in order to capture the structural stress accurately, shell elements are needed for which the edge lengths would need to be much smaller than their thickness. The latter is not possible with the shell element theory. [1] For thin sheet seam weld applications a new method is needed. Fracture mechanics could be able to describe the very local effect in the seam weld. However, at this time a fracture mechanics based solution is too complicated for a standard analysis. For this reason a local stress-based approach RxMS, which can be used for both thick as thin sheets, has been developed. [1]

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