Abstract
The characteristic far field response spectrum of welded joints – the governing fatigue sensitive locations in steel marine structures – is predominantly linear elastic, meaning mid- and high-cycle fatigue (MCF and HCF) is most important for design. Using the effective notch stress- and the total stress concept, involving respectively Se and ST as intact- and cracked geometry fatigue strength criterion, one MCF-HCF resistance curve has been obtained for all welded joints. A generalised random fatigue limit model explicitly incorporating the MCF life time and HCF strength limit scatter provides statistically the most accurate fatigue strength and fatigue life time estimates. Similar MCF performance is obtained for Se and ST. Although crack growth dominates the MCF damage process, the results for an initiation related criterion like Se and natural crack growth related criterion like ST are similar. Adopting Se rather than ST as fatigue strength criterion naturally related to the crack initiation dominated HCF region showing the largest data scatter may explain the better effective notch stress concept HCF performance. Since the HCF resistance scatter is relatively large, the MCF-HCF generalised random fatigue limit model design curves show approximately 1-slope behaviour. meaning that for design purposes a linear Basquin model approximation rather than a piecewise continuous bi-linear MCF-HCF formulation according to guidelines, standards and classification notes should be adopted.
Highlights
Renewable energy marine structures like floating offshore wind turbines in deep water (Fig. 1) experience cyclic mechanical loading & response conditions, both environment and service induced, meaning fatigue [1] is a gov erning limit state.Fatigue sensitive locations in plane geometries turn up at material scale in micro- and meso-scopic stress concentrations
Using the effective notch stress- and the total stress concept, involving respectively Se and ST as intact- and cracked geometry fatigue strength criterion, one MCF-HCF resistance curve has been obtained for all welded joints
Establishing a design curve, e.g. the R95C75 quantile, near 1-slope behaviour is observed for the fatigue life time range N = 104...109, meaning for engineering purposes a LB model approximation rather than a piecewise continuous bi-linear MCF-HCF formulation according to guidelines, standards and classification notes [26,27,34] should be adopted
Summary
Renewable energy marine structures like floating offshore wind turbines in deep water (Fig. 1) experience cyclic mechanical loading & response conditions, both environment (wind, waves, current, drifting ice) and service (machinery) induced, meaning fatigue [1] is a gov erning limit state. The MCF performance of welded joints HS type C has already been investigated using the effective notch stress concept and total stress concept, involving respectively an intact and cracked geometry based fatigue strength criterion [11]. Adopting different MCF-HCF fatigue resistance curve formulations (Section 2), the effective notch stress concept and total stress concept performance for welded joint HS’s type C, B and A will be investigated (Section 3), taking advantage of explicit weld notch stress (intensity) distribution formulations. Both complete and right-censored data (i.e. failures and run-outs) will be incorporated
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