In this paper, the stress-strain curve of material is fitted bg polygonalline composed of three lines. According to the theory of proportional loading in elastoplasticity, we simplify the complete stress-strain relations, which are given by the increment theory of elastoplasticity. Thus, the finite element equation with the solution of displacement is derived. The assemblage elastoplastic stiffness matrix can be obtained by adding something to the elastic matrix, hence it will shorten the computing time. The determination of everg loading increment follows the yon Mises yield criteria. The iterative method is used in computation. It omits the redecomposition of the assemblage stiffness matrix and it will step further to shorten the computing time. Illustrations are givento the high-order element applicatioη departure from proportional loading, the computation of unloading fitting to the curve and the problem of load estimation.
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