Coupled thermal-mechanical fixed-grid finite-element model with application to initial solidification

Computational modeling of mechanical behavior in a solidifying body is of great potential benefit to understand and improve such material processes as foundry shape casting, continuous casting, and welding. Understanding the history of shape and stress during solidification processes is important for process development. This thesis is directed towards realizing this opportunity by developing and evaluating computational procedures for stress modeling of processes with solidification.; To represent deformation phenomena properly over the wide range of temperatures encountered in solidification processes, it is accepted that unified elastic-viscoplastic models are better than classic creep and plasticity theory. However, these models are difficult to implement in finite-element analysis. Three numerical time-integration schemes are evaluated to integrate such constitutive models. They include the explicit forward-Euler scheme, the implicit backward-Euler scheme and the alternating implicit-explicit scheme based on the operator-splitting technique. Several constitutive algorithms are examined. The performances of these methods are compared using a computationally-demanding solidification test problem with known solution. Results indicate that a formulation comprised of the alternating implicit-explicit time integration scheme and the bounded Newton method at the local level calculation is the most robust, accurate and efficient method.; A coupled transient finite-element model based on the fixed-grid implementation of this formulation is developed to simulate temperature, shape, and stress development in a solidifying steel shell during the initial stage of continuous casting. This verified model is applied to predict the distorted shape of a vertical section through the solidifying shell, during a sudden fluctuation in liquid level at the meniscus. The results show that thermal stress causes the exposed portion of the thin shell to bend towards the liquid, when there is a severe, sudden drop in liquid level. The subsequent rise in liquid level increases the bending. These results illustrate an important mechanism contributing to the formation of transverse surface depressions and short longitudinal surface cracks associated with severe liquid level fluctuations.