Analysis of Hot Spot Stress and Alternative Structural Stress Methods

There is a growing need for robust finite element based fatigue assessment procedures for welded joints in offshore/marine structures. Although widely accepted in tubular structures, the applications of the conventional hot spot stress (HSS) procedures in plate structures have proven to be problematic in some joint types. There are a series of on-going international efforts that are intended to address some of the issues to improve the consistency of the HSS calculation procedures for plate structures. In this regard, alternative structural stress (SS) procedures that share some similarities to conventional HSS methods have been developed. The SS method has been shown to be effective in calculating structural stresses directly at failure locations such as at weld toes with minimum mesh-sensitivity. In this paper, detailed analysis and assessments of the conventional HSS and the alternative SS procedures are presented. Starting with the fundamental definitions of both HSS and SS parameters, the underlying mechanics associated with the two types of the stress analysis methods will be discussed by considering series of typical joint types, particularly on some of the important stress concentration characteristics that separate tubular joints from plate joints. Then, the necessary and sufficient conditions required for a stress definition relevant to fatigue and its calculation procedures will be demonstrated for both methods. Areas of concerns and improvements for both methods will be demonstrated by using a series of selected joint types typical of those in marine/offshore structures. Finally, the corresponding S-N data will be used to demonstrate the relative effectiveness of HSS and SS methods in consolidating the data from drastically different joint types into a single S-N curve.