When membrane elements are used separately, not supported by other structural elements, a prestress is necessary in order to avoid a singularity. The unstressed membrane has no stiffness in the normal direction. It is the geometrically nonlinear effects (stress stiffening) which supply the out-of-plane stiffness. A prestress can be given either through initial stress and strain or through a tensile boundary load. Prestress is not necessary in cases where inertia effects are included in a dynamic analysis. A small prestress can, however, still be useful to stabilize the analysis in the initial state. In order to obtain the prestress effect, you must select
Include geometric nonlinearity in the settings for the study step.
The dependent variables are the displacements u,
v, and
w in the global
x,
y, and
z directions, and the displacement derivative
unn in the direction normal to the membrane. For anisotropic materials, the tangential displacements derivatives
u1n and
u2n are additionally added as dependent variables.
The dependent variables are the displacements u and
w in the global
r and
z directions, and the displacement derivative
unn in the direction normal to the membrane in the
rz-plane. For anisotropic materials, the tangential displacements derivatives
u1n and
u2n are additionally added as dependent variables.
The fundamental difference between Linear Elastic Material and
Linear Elastic Material, Layered is that in
Linear Elastic Material the material properties are assumed to be constant through the thickness. In the
Linear Elastic Material, Layered model, there is a numerical integration in the thickness direction, and it is also possible to store states, such as inelastic strains, at different through-thickness locations.
When the Composite Materials Module is available, the Linear Elastic Material, Layered model can be used to model multilayered membrane. This is the main use of this material model. It is, however, also used for the
Thermal Expansion, Layered multiphysics coupling, even if there is just a single layer.
For each layer, you have the option to set the resolution in the thickness direction. In a layered material, this is the Mesh elements property in the layer definitions. When working with a single layer material, then it is the
Mesh elements property in the
Shell property group. For membrane analysis, you can set this value to ‘1’ since in-layer variations are not part of the theory.