Improvements in the Fatigue Assessment of Large Welded Structures Using the Nominal Stress Approach

By the fatigue assessment of large welded steel structures such as construction machines structures, the calculation engineer is confronted with a difficulty: the local stress approaches with fictitious notch radius that are very accurate cannot be used on the global structure because of the current computer limitations. Only a nominal stress can be estimated on the whole structure. The accuracy of the current commercial code methods that are using the nominal stress approach is not satisfying for most of the cases. The major problems are the following:

• only one SN-Curve (FAT-class) can be chosen for a weld
• the stress used for the calculation is based on the critical plane concept, not taking into account the direction of the weld (anisotropy of notch effects) and the geometrical weld parameters (e.g. weld throat thickness and penetration)
• choice of the FAT-class when the structural detail is not available in the IIW guideline.

The investigations that have been made in collaboration with LMS Durability development team in the last two years have permitted to elaborate a new nominal stress approach that improves the three issues above. The validation on some weld joints and geometries has shown that this new method allows making a big step forward in the accuracy without affecting the time required for the analysis.