Effect of residual stresses on fracture behavior of weldments

Welding-induced residual stresses are formidable results in weldments due to the nature of highly localized temperature distributions and nonlinear material properties. In the presence of the tensile residual stresses, premature crackings are more likely to occur in services. Existence of high residual stresses has been a concern in many welded structures because the tensile residual stresses increase the crack driving force and reduce the resistance to brittle fracture. They may have detrimental effect on the structural integrity of the weldments. Therefore, analysis of welding residual stresses and fracture behavior is an important subject to be studied for structural integrity of weldments containing crack-like discontinuities.; In this dissertation, fracture assessment procedure and modeling techniques were developed for semi-elliptical surface cracks in welded plates using nonlinear finite element analyses. The fracture assessment procedure consisted of coupled welding and fracture analysis (FEA). The crack face was constrained during welding and removed after welding for the fracture analysis. The results of stress, elastic strain energy, displacement and plastic strains were retrieved from the welding and fracture analysis using a commercial FEA code ABAQUS. These retrieved results were used in the finite element formulation shown in Appendices A and B to calculate path-independent J-integral under residual stress fields. To estimate the condition of residual stresses, a ramp heat input function and element rebirth techniques were used for the deposition of weld beads in multi-pass welding. Generalized plane strain assumption was used in the analysis of residual stresses. Residual stresses were determined without a cracking condition. The physical crack surfaces were restrained from opening using a series of stiff spring elements, which removed after welding for fracture analysis.; The Fitness-For-Service (FFS) concept coupled with Failure Assessment Diagram (FAD) was used to study the effect of residual stresses on the fracture behavior of semi-elliptical surface cracks in the weldments. The fracture behavior of cracks under residual stresses is different from the one without residual stresses when the primary loading is applied because of the residual stress effect on crack driving forces. Different amount of crack driving forces may produce low plasticity or high plasticity at the crack tip. This difference in plasticity at the crack tip is expressed as the plasticity interaction (ρ factor) between the residual stresses and primary load. Assessment of toughness ratios, K_r, in FAD is conducted to study the crack behavior for different geometries of semi-elliptical surface cracks under residual stresses using both API 579 and FEA. When combined loadings are applied, crack depth is influential factor of K_r for shallow cracks (a/t = 0.1 and 0.2) because of small difference of high tensile residual stresses in shallow (top surface) region; however, level of tensile residual stresses is more influential factor than crack depth for the magnitude of K_r in the case of deep cracks (a/t = 0.3 and 0.4). Parametric study was conducted with different aspect ratios of semi-elliptical surface cracks to evaluate the ρ factors for welded plates using nonlinear finite element analyses. The present work also investigates the effect of residual stresses on the FADs with comparison of a general FAD represented in API 579 industry code practice. The effect of residual stresses on FADs shows either more conservatism or less conservatism based on the magnitude of ρ factors.