Characterisation of LCF performance of X100 weld-joints: Mechanistic yield strength modelling, finite element analyses and DIC testing

Abstract

This paper is concerned with the effect of welding on the fatigue behaviour of X100 material for steel catenary risers. The methodology includes both modelling and experimental characterisation. The modelling combines (i) a physically-based yield strength model to capture the thermally-induced microstructural heterogeneity and associated spatial variations in relative contributions of the key strengthening mechanisms due to welding, and (ii) a five-material cyclic plasticity model with a Coffin-Manson strain-life fatigue model for prediction of cross-weld heterogeneity in cyclic plasticity and fatigue response. The combined non-linear isotropic-kinematic cyclic plasticity behaviour of the five weld joint constituent materials (PM, weld metal (WM) and heat-affected zone (HAZ) subregions) is implemented via a user material (UMAT) subroutine, including Kocks-Mecking monotonic-cyclic evolution of yield stress. The experimental methodology consists of tensile tests with digital image correlation (DIC) for X100 PM and cross-weld samples. The results indicate that the primary phenomenon driving the detrimental effect of welding on fatigue is the evolution of cyclic strain localisation in the inter-critical heat-affected zone (ICHAZ), leading to predicted ICHAZ failure.