Welding simulation for the calculation of the welding stresses on welded I-girders Birgit Ragotzky Brandenburg University of Technology (BTU), Chair of Steel and Timber Structures Benjamin Launert Brandenburg University of Technology (BTU), Chair of Steel and Timber Structures Thomas Krausche Brandenburg University of Technology (BTU), Chair of Steel and Timber Structures 1. Introduction Welding is one of the most important and widely used joining processes in steel construction. It offers the advantage to build cross section dimensions freely based on individual design requirements. On the other side, welding also involves weld residual stresses and deformations that need to be taken into account in the design process. These develop due to restrained expansion and shrinkage of the weld zone as a result of non-uniform heating of the component. Thereby, thin sheets are expected to show higher distortion while thick sheets are more characterized by higher residual stresses. Both phenomena act directly as well as indirectly on the internal forces and hence also affect the load bearing behavior. The residual stresses, that represent the primary focus of the following contribution, are generally depending on welding parameters, the material as well as on geometrical dimensions. A modern numerical welding simulation can provide a rather realistic assessment of these values compared to available simplified assumptions. These can then be incorporated into the design at an early stage or also support an optimization of the welding process. Nevertheless, the welding simulation also contains several restrictions such as complicated modelling and immense calculation times. Even small-scale specimens can easily take several days depending on component size, meshing and number of welds. In the following, results of numerical welding simulations obtained by two different calculation programs are presented and compared against each other. In particular, the specialized software Simufact.Welding and a freely available add-on to the commercial software code Abaqus, the Abaqus Welding Interface (AWI), are used. The basis for a model calibration and evaluation of the results is provided by experiments carried out on four welded I-girders welded by conventional MAG process. 2. Origins of residual stress Residual stresses are often considered as a superposition of so-called shrinkage stresses and transformation stresses [Fahrenwaldt et al. 2014]. Shrinkage stresses are produced by a restraint of shrinkage of the so-called plasticity zone. In this, the plasticity zone is the zone that experiences plastic deformation during welding. Shrinkage stresses are produced in longitudinal and transverse direction of the weld seam and with increasing thickness also through the thickness. In longitudinal direction, tensile residual stresses often reach the yield stress of the material. Self-equilibrating compressive residual stresses are found apart from the weld (Figure 1a). Additionally, transformation stresses result due to phase transformation in the fusion zone (FZ) and the heat affected zone (HAZ) (Figure 1b). Depending on the cooling behavior, different microstructures with different crystalline structures develop that show different characteristics in strength and ductility. The transformation from austenite into different phase fractions also involves thermal expansion that is restricted by non-transforming areas causing more or less significant local compressive residual stresses. These are then subsequently superposed with shrinkage stresses. The resulting positive effect of a local stress reduction is accompanied by specific microstructure properties such as high hardness and increased susceptibility to cracks. The generation of residual stresses cannot be suppressed. “Local” residual stresses are always created by expansion and shrinkage due to material thicknesses and strengths of the material. For typical structures in steel construction, it is neither possible to stress relieve the components due to their given large sizes. Hence, their calculation and adequate consideration in the design is of particular interest.
Welding simulation for the calculation of the welding stresses on welded I-girders
May 22, 2023
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