Fatigue assessment of welded structures: practical aspects for stress analysis and fatigue assessment

Abstract

A B S T R A C T Analysis of welded structures still remains a challenge for the analyst and in fact cannot be considered as fully solved for practical applications. For many years, a large international aggregation of researchers has developed methods to assess fatigue behaviour of welded structures. Nowadays many suggestions and methods exist to estimate fatigue life of welded structures with respect to nominal, structural, notch stress or fracture mechanics approaches. All of them are still under improvement. The high motivation and many activities of experts in the International Institute of Welding (IIW) group of researchers is a good demonstration of the complexity and need for analysis methods in that field. The purpose of this paper is to provide some discussion on selected methods available. Both authors are giving lectures to transfer methods to industrial applications. It is their experience that a large amount of knowledge has been developed although proper applications require some grading and comments on the use of those methods. This paper should give some comments and recommendations for the practical application of a selection of methods already available. A hierarchical two-step procedure for the assessment of large welded structures will be described and recommended. Also benchmark results are presented on a sample structure for sake of comparison of a few selected methods. Finally a presentation of results obtained by application of selected methods on real structures in comparison with fatigue lives from experiments will be presented. The methods selected within the paper cover the approaches for modelling, structural analysis and assessment of welded structures using finite element analysis (FEA) and stress based concepts for fatigue life estimation. a = weld throat thickness e = element size e 1 = element size of 1st row of elements adjacent to hot spot e 2 = element size of 2nd row of elements adjacent to hot spot k = slope of component fatigue curve l 1 , l 2 , l 3 = distances for structural stress assessment using extrapolation schemes r CAB = weld radius according to CAB concept t = wall thickness A = area F x = force in x-direction F y = force in y-direction K t,r=1 = notch factor for weld toe radius r = 1 mm Correspondence: K. Rother.

An extrapolation method to determine the effective notch stress in welded joints

“…. The presence of a finite radius at the otherwise sharp notch removes the stress singularity at sharp re‐entrant corners . The pioneering studies by Radaj have demonstrated that the fictitious radius at the notch of welded steel joints has a value of 1 mm, based on the microstructural support theory proposed by Neuber .…”

Local Ratcheting by Elastic-Plastic FEA: Criteria and Code Based Approaches

“…It is similar to a quadratic surface stress extrapolation used in the context of structural stress approaches to the fatigue assessment of welds (e.g. 31 ). Another option -not explicitly addressed in EN 13445-3 would be a path linearization with respect of strains across the wall thickness (volumetric FE model presumed).…”

“…The peak strain values die out rapidly and thus the surface strains at the hot spot locations deliver lower resulting strain values. This is a general problem of surface extrapolation methods 31 . The courses of stresses/strains between the extrapolation points and the hot spot have to be verified with respect of plausibility and applicability of the extrapolation procedure 31 .…”

“…This is a general problem of surface extrapolation methods 31 . The courses of stresses/strains between the extrapolation points and the hot spot have to be verified with respect of plausibility and applicability of the extrapolation procedure 31 . The locations of the positions y 1 to y 3 do not only depend on the wall thickness but also on the discretization.…”

Analysis of Optimum Butt Welded Joint for Mild Steel Components Using FEM (ANSYS)

“…Haftirman, et al [4], and Rother and Rudolph [5] were of the opinion that Fatigue is a failure mechanism where the component fails after a period of time in service and also sees a repetitive cyclic stress.…”