Finding critical damage locations by Λ-filtering in finite-element modelling of a girth weld

In structures with uniform material properties, time dependent creep damage will develop in locations where the stress state is aggravated by triaxiality constraints. However, for non-uniform structures, such as welds, where several material zones (differing in microstructure and material properties) are present, the pin-pointing of the “weakest link” is even more challenging. For girth-welded pipes, the unfavourable conditions are known to be found in the vicinity of the heat-affected zone on the outer diameter. To find out how the triaxiality constraints influence accumulated “creep exhaustion”, three girth welds simulated for different steels have been studied using the Λ-filtering technique. The Λ-filtering has been developed for finite element modelling for visualising the combined effect of creep ductility, creep rate and triaxiality constraint evolution assuming rigid plastic deformation for the stress triaxiality-dependent part. In this work, the creep response of three-zone girth welds was studied. In the case of the P23 steel, data for a real (experimental) consumable was used and for the others, assumption on creep strength and creep curve shapes was made according to material property ratios. The creep strain rate formulation used for this work is the logistic creep strain prediction model and its multiaxial implementation has been run in the Comsol multiphysics software package. The results of the weld simulations predict similar behaviour for the P22 and P91 welds, with assumed weld metal behaviour closer to the base material, and differing behaviour for the P23 with an overmatching (experimental) weld metal.