An engineering method for constraint based fracture assessment of welded structural components with surface cracks

Abstract

In this study it has been shown that accurate descriptions of crack-tip stress-fields in surface cracked welded plates can be obtained without large 3D FEA models. When a fracture mechanics FE analysis is required in a large construction, existing shell models can be used in combination with a plane strain submodel. The 2D plane strain model is driven by displacements from the global shell model. This technique has been used to simulate crack-tip stress-fields in a surface cracked plate. The crack-tip stress fields are characterised with the J-integral and the constraint parameter, Q. The crack in the global shell model was simulated with line-spring elements. The global behaviour as well as the crack-tip stress-fields of the plane strain submodel have been compared to a 3D solid model. Initially, the crack-tip stress-fields in the plane strain model and the 3D model with surface crack were compared, using the same in-plane mesh and element type. It was found that when first order elements were used, the constraint was higher in 3D than in plane strain. For second order elements, however, the trend was the opposite. By using a correction factor for the load, the load vs. J behaviour and the crack-tip stress-fields of a surface cracked plate can be predicted from a shell analysis with line-spring elements and a plane strain model. Accurate predictions of J and Q were obtained using the shell + submodel technique for homogeneous material and for a weldment with fusion line crack. The shell + submodelling technique was used to assess brittle fracture in two steel weldments with a surface crack using the RKR failure criterion by Ritchie et al. [16]. For the investigated case, the toughness requirements could be relaxed significantly based on the two parameter analysis compared to conventional fracture mechanics analyses.