Issue 37

P.S. van Lieshout et al., Frattura ed Integrità Strutturale, 37 (2016) 173-192; DOI: 10.3221/IGF-ESIS.37.24 173 Focussed on Multiaxial Fatigue and Fracture Comparative study of multiaxial fatigue methods applied to welded joints in marine structures P.S. van Lieshout, J.H. den Besten, M.L. Kaminski Delft University of Technology, Netherlands , , A BSTRACT . Marine structures are particularly prone to action of waves, winds and currents with stochastically varying composition, intensities and directions. Therefore, resultant stresses may cause multiaxial fatigue in specific welded structural details. For the assessment of multiaxial fatigue in welded joints, a wide variety of methods have been suggested. However, there is still no consensus on a method which can correctly account for non-proportional and variable amplitude loading. This paper beholds a comparative study of multiaxial fatigue methods applicable for design of marine structures. For the purpose of comparison several load cases were defined including non-proportional and variable amplitude loadings with different normal and shear stress amplitude ratios. Three types of methods are compared: those described by three different codes (i.e. Eurocode 3, IIW and DNV-GL), those described by three different multiaxial fatigue approaches from literature (i.e. Modified Carpinteri-Spagnoli Criterion, Modified Wohler Curve Method and Effective Equivalent Stress Hypothesis) and an approach based on Path-Dependent-Maximum-Range multiaxial cycle counting. From this study it has been concluded that non-proportional variable amplitude loading has a significant negative impact on the fatigue lifetime estimates, and that further research and experimental testing are essential to come to a consensus. K EYWORDS . Multiaxial fatigue; Welded joints; Marine structures; Non-proportional; Variable amplitude loading; Constant amplitude loading. I NTRODUCTION ost welds in structural details of marine structures are predominantly subjected to uniaxial stresses due to the stiffness distributions in typical structural member assemblies like stiffened panels, frames and trusses. However, there are also welds which could be subjected to multiaxial stresses induced either by geometry [1, 2] or loading. Such stresses may lead to a significant reduction of the fatigue resistance of welded steel [3]. Considering that the majority of marine structures are (relatively thick) plated structures, such fatigue lifetime reductions are generally caused by the combined effect of a dominant normal and shear stress (mixed Mode-I and Mode-III). Currently, fatigue design of marine structures is predominantly based on uniaxial fatigue criteria assuming a governing Mode-I. These criteria are then used in combination with a damage accumulation hypothesis (e.g. Miner’s rule) and cycle counting method (e.g. rainflow counting) to determine the fatigue lifetime. However, such an approach can be non- conservative for structural details where the welds are subjected to multiaxial stresses, especially when these are non- proportional, i.e. out-of-phase (OP). Over the last few decades intensive efforts have been made to develop multiaxial fatigue approaches which are able to deal with difficulties such as (random) variable amplitude (VA) loading and non-proportionality. This has resulted, M