| 摘 要： | There are some common difficulties encountered in elastic-plas-tic impact codes such as EPIC,NONSAP etc.Most of thesecodes use the simple linear functions usually taken from static pro-blems to represent the displacement components.In such finite ele-ment formulation,the strain and stress components are constants inevery element.In the equations of motion,the stress components ingeneral appear in the form of their space derivatives.Thus,if weuse such form functions to represent the displacement components,the effect of internal stresses to the equations of motion vanishesidentically.The usual practice to overcome such difficulties isto establish as self-equilibrium system of internal forces actingon various nodal points by means of transforming equations of mo-tion into variational form of energy relation through the applica-tion of virtual displacement principle.The nodal acceleration.isthen calculated from the total force acting on this node from allthe neighbouring elements.The transformation of virt
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