Browsing by Subject "Regularization method"

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    On the Finite Element Method for Mixed Variational Inequalities Arising in Elastoplasticity
    (1995) Han, Weimin; Reddy, B Daya
    We analyze the finite-element method for a class of mixed variational inequalities of the second kind, which arises in elastoplastic problems. An abstract variational inequality, of which the elastoplastic problems are special cases, has been previously introduced and analyzed [B. D. Reddy, Nonlinear Anal., 19 (1992), pp. 1071-1089], and existence and uniqueness results for this problem have been given there. In this contribution the same approach is taken ; that is, finite-element approximations of the abstract variational inequality are analyzed, and the results are then discussed in further detail in the context of the concrete problems. Results on convergence are presented, as are error estimates. Regularization methods are commonly employed in variational inequalities of this kind, in both theoretical and computational investigations. We derive a posteriori error estimates which enable us to determine whether the solution of a regularized problem can be taken as a sufficiently accurate approximation of the solution of the original problem.
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    Qualitative and Numerical Analysis of Quasi-Static Problems in Elastoplasticity
    (1997) Han, Weimin; Reddy, B Daya; Schroeder, Gregory C
    The quasi-static problem of elastoplasticity with combined kinematic-isotropic hardening is formulated as a time-dependent variational inequality (VI) of the mixed kind; that is, it is an inequality involving a nondifferentiable functional and is imposed on a subset of a space. This VI differs from the standard parabolic VI in that time derivatives of the unknown variable occur in all of its terms. The problem is shown to possess a unique solution. We consider two types of approximations to the VI corresponding to the quasi-static problem of elastoplasticity: semidiscrete approximations, in which only the spatial domain is discretized, by finite elements; and fully discrete approximations, in which the spatial domain is again discretized by finite elements, and the temporal domain is discretized and the time-derivative appearing in the VI is approximated in an appropriate way. Estimates of the errors inherent in the above two types of approximations, in suitable Sobolev norms, are obtained for the quasi-static problem of elastoplasticity; in particular, these estimates express rates of convergence of successive finite element approximations to the solution of the variational inequality in terms of element size h and, where appropriate, of the time step size k. A major difficulty in solving the problems is caused by the presence of the nondifferentiable terms. We consider some regularization techniques for overcoming the difficulty. Besides the usual convergence estimates, we also provide a posteriori error estimates which enable us to estimate the error by using only the solution of a regularized problem.