Abstract

Non-uniform cooling of steel cross-sections during the manufacturing process generates a state of residual stresses in the cross-section. Design codes describe the distribution of these stresses in different ways. This work aims to numerically investigate the influence of these models on the behavior of bare steel and steel-concrete composite sections by the curves: flexural stiffness-bending moment, moment-curvature and yield curves (initial and full yield). These procedures are important for the study of the simplified curves used in some methodologies of the refined plastic hinge method (RPHM) analysis. The study will use the strain compatibility method (SCM), where, if the axial strain of the cross-section point is known, the section stiffness is obtained using the tangential Young's modulus derived from the materials constitutive relationship. A fiber discretization algorithm is applied and the residual stresses are explicitly inserted into the fibers automatically. The methodology was calibrated using the moment-curvature relationship and the flexural stiffness-bending moment curve. These results were numerically stable and good convergence with literature data was obtained. In general, the residual stress model of the American standard (AISC, 2016) defines a larger elastic region within the interaction diagrams then European model (CEN, 2005). The results obtained showed that the initial yield curves for steel I-sections under minor axis bending require revision for application to RPHM, mainly due to the loss of symmetry in relation to the ''M'' axis in the normal force-bending moment (''NM'') interaction diagram.

Abstract

This work describes the local–global strategy proposed for the computation of residual stresses in friction stir welding (FSW) processes. A coupling strategy between the analysis of the process zone nearby the pin tool (local level analysis) and the simulation carried out for the entire structure to be welded (global level analysis) is implemented to accurately predict the temperature histories and, thereby, the residual stresses in FSW. As a first step, the local problem solves the material stirring as well as the heat generation induced by the pin and shoulder rotation at the heat affected zone. The Arbitrary Lagrangian Eulerian (ALE) formulation is adopted to deal with the rotation of complex pin shapes. A thermo-rigid-viscoplastic constitutive law is employed to characterize the viscous flow of the material, driven by the high-strain rates induced by the FSW process. A mixed temperature–velocity–pressure finite element technology is used to deal with the isochoric nature of the strains. The output of this local analysis is the heat generated either by plastic dissipation or by friction, and it is used as the power input for the welding analysis at structural (global) level. The global problem is tackled within the Lagrangian framework together with a thermo-elasto-viscoplastic constitutive model. In addition, in this case, the mixed temperature–displacement–pressure format is introduced to deal with the deviatoric nature of the plastic strains. The outcomes of this analysis are the distortions and the residual stresses after welding. The material used in this work is stainless steel 304 L; however, the methodology presented is applicable to a wide range of materials. The proposed numerical strategy is validated by the experimental evidence.