Geometrically exact assumed stress-strain four-node element based on the 9-parameter shell model

This paper presents the geometrically exact assumed stress-strain four-node element with nine displacement degrees of freedom per node. The finite element formulation developed is based on the 9-parameter shell model by employing a new concept of interpolation surfaces (I-surfaces) inside the shell body. We introduce three I-surfaces and choose nine displacements of these surfaces as fundamental shell unknowns. Such choice allows us to represent the higher-order shell formulation in a very compact form and to derive in curvilinear reference surface coordinates the strain-displacement relationships, which are objective, i.e. invariant under all rigid-body motions.