Linear kinematics element tests

All element types tested yield the appropriate results for their applicable fundamental modes of deformation. Results for the two-dimensional truss element are illustrated here.

There are two global modes of deformation for a two-dimensional truss: longitudinal and lateral. The longitudinal mode is driven by fixing one end of the truss and prescribing a longitudinal displacement at the other. The axial stresses in the truss element as a result of longitudinal deformation for both small-displacement theory (geometric nonlinearities are neglected) and large-displacement theory (geometric nonlinearities are considered in the step) are shown in Figure 1. As the strains become large, the results diverge because the large-displacement theory accounts for the thinning of the truss as it stretches. The global lateral mode is invoked by prescribing a lateral displacement at one end of the truss element while holding all other degrees of freedom fixed. Results for the lateral case are shown in Figure 2. The nonlinear geometric effect is accounted for only in the large-displacement analysis. The small-displacement analysis ignores the extension of the truss due to its rotation and, therefore, sees no extensional strain due to the prescribed lateral displacements.