Selecting Discretization
Shape Function Order
In structural mechanics analysis, the focus is often on the stresses and strains rather than on the displacements. Since the strains are derivatives of the displacement field, the accuracy of the strains will be one order less than the accuracy of the displacements. For this reason, second-order shape functions are used as default in most of the structural mechanics interfaces. Often this gives the best tradeoff between model size and accuracy.
It is well known that using first-order shape functions in solid mechanics will give an overly stiff solution, unless a very fine mesh is used. This is especially noticeable for triangular and tetrahedral elements. This can, to some extent, be counteracted by using reduced integration, see Using Reduced Integration.
If the purpose of the analysis is only to compute stiffness, rather than stresses, the use of linear shape functions can still be justified. This is the default choice in the Multibody Dynamics interface, available with the Multibody Dynamics Module.
If the solution contains discontinuities, for example when some type of front is moving through the material, first-order elements and a fine mesh is often a good choice, since the advantage of the higher-order elements lies in their ability to represent smooth gradients.
Truss Elements
In the Truss interface the default is to use first-order shape functions, since the elements are mainly used in a context where the axial force in each element is constant.
When truss elements share an edge with other structural elements, you should choose the same discretization in both interfaces, usually quadratic.
Beam Elements
The beam elements have only one set of shape functions, which cannot be changed. The axial displacement and the twist are represented by first-order shape functions, while the bending is represented by cubic Hermitian shape functions. This element can then represent a constant axial force, a constant twisting torque, a linear bending moment, and a constant shear force. This is the exact solution for a beam having no distributed loads.
A consequence of this formulation is that it may not possible to obtain a perfectly conforming approximation if a beam shares an edge with elements from another physics interface.