The influence of weld geometry on plastic strain history in cyclic loading – a parametric study
The loss of material ductility in the presence of very high amplitude cyclic straining is an observed phenomenon in many extreme conditions like earthquake loadings. This phenomenon is called ultra-low cycle fatigue (ULCF) and failure is often achieved in a few number of cycles – around 5 to 10. Design to ULCF often involves the use of finite element (FE) models to assess the plastic strain history at critical points in a component. In the case of a welded joint, a number of factors contribute to its ultimate cyclic fracture strain, such as the weld and heat affected zone’s (HAZ) material characteristics, and the weld geometry, which cause a local rise in plastic strains at the weld toe. The purpose of the paper is to study the influence of the weld geometry on the plastic strain history of material points near the weld toe, through an FE parametric analysis with nonlinear kinematic hardening. The motivation behind this study stems from a set of experiments conducted on high-strength S770QL steel, whose material properties will be used in this paper. The joint that will be analyzed consists of a welded tube to plate connection. Here 3 geometric parameters will be studied: throat thickness (a – 4, 6, 8 and 10mm, on a 8 mm thick tube wall), weld angle (Θ – 30°, 45° and 60°), and weld toe radius (R – 0, 1 and 2 mm). In addition to these factors, the impact of the type of loading (L – bending, pure torsion and combined proportional bending and torsion during 5 high amplitude cycles) and mesh size (M – 1.0 and 0.5 mm) will also be addressed. As a simplification, the weld material and heat affected zone have the same mechanical properties as the base material. Stress state variables important in ULCF analyses, such as the stress triaxiality and the Lode parameter, are analyzed in addition to the plastic strain history.