In this paper flat shell elements are formed by the assemblage of discrete Mindlin plate elements RDKTM and either the constant strain membrane element CST or the Allman's membrane element with drilling degrees of freedom LST. The element RDKTM is a robust Mindlin plate element, which can perform uniformly thick and thin plate bending analysis. It also passes the patch test for thin plate bending, and its convergence for very thin plates can be ensured theoretically. The singularity of the stiffness matrix and membrane locking are studied for the present elements. Numerical examples are presented to show that the present models indeed possess properties of simple formulations, high accuracy for thin and thick shells, and it is free from shear locking for thin plate/shell analysis.Keywords Mindlin plate theory, Shear locking, Drilling degrees of freedom, Displacement functions of a Timoshenko beam
IntroductionIn the formulations of shell elements there are three distinct choices: (1) The flat shell element, that is obtained by combining a membrane element for plane elasticity and a bending element for flat plates; (2) The curved shell element, that is formulated according to shell theory; and (3) The degenerated shell element, that is formed based on the three dimensional solid theory. Among those formulations, the flat shell element is considered simplest and avoids complex shell equations. Therefore, the application of flat shell elements is widespread in engineering.Triangular flat shell elements may be used to represent an arbitrarily shaped shell as a faceted surface for curved shell structures. In this case, convergence to the exact geometry is obtained when the mesh is refined. Usually, triangular flat shell elements consist of triangular constant strain membrane elements and triangular bending plate elements. To study this type of shell element, the first step is the choice of an applicable plate element and an applicable membrane element. However, shear locking has been a major problem for the Mindlin plate element. It is well known that for Mindlin plates that only C 0 continuity is required and that the difficulties of C 1 continuity for thin plate elements can be avoided. Moreover both thin plate and thick plate analysis can be integrated in the element model. Initially, Mindlin plate elements use strain-displacement relations to obtain bending and transverse shear strain. In this case, bending energy is written in terms of nodal rotations only, whereas shear strain energy is given in terms of both nodal rotations and deflections. When the plate becomes thin, transverse shear effects are reduced and nodal deflections become associated only with the vanishing shear energy. This is a difficult situation to uphold and shear locking is soon observed. In order to avoid this, reduced integration [1, 2] and selective integration [3,4] technique are widely used. In the 3-noded triangular element, a single Gauss point integration is used for calculating the shear strain energy. However, it is found that such el...