Effective notch stress approach for welds having low stress concentration

The effective notch stress approach for fatigue assessment of welded joints according to the IIW-recommendations is a widely accepted procedure and has already become part of many technical rules and guidelines. The method provides additional means for evaluation of welded joints besides classical nominal and structural stress concepts. In contrast to the latter concepts, effects of many local weld parameters can only be studied analytically using the effective notch stress approach. The effective notch stress approach requires use of a standardized idealization of the weld detail and use of continuum mechanics theory for stress analysis (i.e. use of solid elements for finite element models). The idealization with respect to geometry includes modeling of all weld toes and weld roots using a reference radius together with an associated S-N curve. For joints facing low stress concentration, like butt welds or welds at thin sheets, the method requires checking the fatigue behavior of the base material as well. This sometimes is not considered in using the effective notch stress method, because it is not so obvious. Explanation of background information and a reanalysis of an example from literature is presented here for explanation and demonstration of this concept and of specific needs for welds with low stress concentration. Also the selection of the size of the reference radius will be discussed in this paper by using different radii for the case investigated here. Finally the application limits of the S-N curves as given in the IIW-recommendations is discussed since the selected example is slightly out of the range of application of the S-N curves. - Highlights: • Effective notch stress concept needs special treatment for low stress concentration. • The currently suggested analytical method for such cases is explained in detail. • An incorrect use in a former published example is discussed and corrections given. • Demand for methods to cover low-cycle fatigue behavior is discussed.