Experimental characterisation and computational modelling for cyclic elastic-plastic constitutive behaviour and fatigue damage of X100Q for steel catenary risers

New higher strength steels are required for deep and ultra-deepwater steel catenary risers (SCRs). In this work, the cyclic elastic-plastic-damage behaviour of X100Q, a candidate next-generation SCR material is experimentally characterised and modelled. The material is shown to exhibit early life (primary) fatigue damage followed by the more conventional (secondary) fatigue damage; as a result, it is necessary to demarcate the observed cyclic softening into dynamic recovery and damage-induced softening. An automated constitutive parameter optimisation process in combination with a new two-stage cyclic damage evolution model successfully predicts the effect of strain-range on damage evolution. The model is implemented in a user material (UMAT) subroutine for multiaxial application, within a hierarchical global-local modelling methodology for dynamic fatigue analysis of an SCR girth weld geometry. The interdependency between fatigue damage-induced material degradation and cyclic plasticity at the weld is shown for a range of load cases.