TWI Industrial Member Report Summary 855/2006
By L Wei
The fatigue assessment method based on the use of S-N curves for welded joints is well established. However, it is only suitable for a limited range of classified welded joints where the nominal stress can be well defined. In practice, the approach cannot be easily adapted for use on complicated geometries for which nominal stresses are not easily defined, especially if stresses are obtained by finite element analysis (FEA).
Two fatigue design and analysis methods are available for extending the applicability of the nominal stress based S-N curve approach: the structural hot spot stress (SHS) method and the equivalent structural stress (ESS) method which is based on the consideration of fatigue crack growth governed by linear elastic fracture mechanics. In comparison with the SHS approach, the strength of the ESS method is the ability to account for some effects, eg, the thickness effect, but it involves more complicated calculations. In contrast, the SHS method provides a straightforward and practical method for using the results of FEA to account for stress concentration due to geometric effects in the fatigue assessment of welded joints. However, further work is required to validate the approaches for calculating structural hot spot stresses with FEA, to establish structural hot spot stress based design curves, and to improve the effectiveness of the SHS approach.
- To investigate and compare existing FEA procedures of determining structural hot spot stresses.
- To provide recommendations for FEA-based fatigue design of welded structures.
- To identify the possibility for improving the effectiveness of the structural hot spot stress approach in assessing the fatigue lives of different types of welded structure.