Numerical thermo-structural analysis for residual stresses fields in welded joints
SEPE, RAFFAELE (2008) Numerical thermo-structural analysis for residual stresses fields in welded joints. [Tesi di dottorato] (Inedito)
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There are many welding processes used in industry today, because of its versatility and cheap cost. Fusion welding processes, in particular arc welding processes, are the most important categories that constitute the joining process. These processes induce localized coalescence of metals produced by heating of the materials with an arc, with or without the ap-plication of pressure and with or without the use of a filler metal. However when structures are manufactured by welding a non-uniform temperature distribution is produced. This distribution initially causes a rapid thermal expansion followed by a thermal contraction in the weld and surrounding areas, thus generating inhomogeneous plastic strains accompanied by plastic upsetting so producing residual stresses that remain in the weldment when it is cooled. Distortion and shrinkage are also produced. Distortion and residual stresses cause many problems in the welded structures. They can reduce their performance; in fact the residual stress distribution is the initial stress of a welded structure and it should be combined with the effect of the applied service loading. Several experimental destructive and non destructive techniques for directly measuring residual stress have been developed. But a complete picture of the residual stresses distribution is impossible to obtain by ex-perimental method. So computational simulation plays an indispensable role to solve these complex problems. During last twenty years many welding processes have been simulated by numerical techniques, among which finite element method emerged as the most powerful. A computational simulation of a welding is very complex because the thermal and mechanical behaviours in the welding include high temperature, temperature dependence of material properties and large deformations. In this study simulations of the welding process for butt joints using finite element analyses are presented. Two cases were studied: in the first the residual stress state from a bi-dimensional analysis of the butt joint was compared to results by three-dimensional model. The numerical results were also compared to experi-mental results available in literature showing a very good correlation. In this study a parametric model was adopted and “birth and death” elements were used in single-pass butt welded joint to simulate the weld filler variation with time. In the second case 2D finite element model was carried out to evaluate temperature and residual stress distribution in single-pass butt weld joints by using yet the “birth and death element” technique. A coupled thermo-mechanical non-linear transient analysis was performed to determine the influence of thermal properties and preheating treatment both on tem-perature and residual stress distribution in welding. The simulations were performed with the ANSYS® finite element code. All analyses were performed with temperature dependent material properties and one way thermo-mechanical coupling is assumed, which means that the thermal analysis is completed first, followed by a separate mechanical analysis based on the thermal history.
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