Assumed-deformation Gradient Finite Elements with Nodal Integration for Nearly Incompressible Large Deformation Analysis

An assumed-strain finite element technique for non-linear finite deformation is presented. The weighted-residual method enforces weakly the balance equation with the natural boundary condition and also the kinematic equation that links the elementwise and the assumed-deformation gradient. Assumed gradient operators are derived via nodal integration from the kinematic-weighted residual. A variety of finite element shapes fits the derived framework: four-node tetrahedra, eight-, 27-, and 64-node hexahedra are presented here. Since the assumed-deformation gradients are expressed entirely in terms of the nodal displacements, the degrees of freedom are only the primitive variables (displacements at the nodes). The formulation allows for general anisotropic materials and no volumetric/deviatoric split is required. The consistent tangent operator is inexpensive and symmetric. Furthermore, the material update and the tangent moduli computation are carried out exactly as for classical displacement-based models; the only deviation is the consistent use of the assumed-deformation gradient in place of the displacement-derived deformation gradient. Examples illustrate the performance with respect to the ability of the present technique to resist volumetric locking. A constraint count can partially explain the insensitivity of the resulting finite element models to locking in the incompressible limit. Copyright © 2008 John Wiley & Sons, Ltd.