The Membrane Interface
The Membrane (mbrn) interface (), found under the Structural Mechanics branch () when adding a physics interface, is mainly used to model prestressed membranes, but can also be used to model a thin cladding on a solid. Membranes can be considered as plane stress elements on boundaries in 3D with a possibility to deform both in the in-plane and out-of-plane directions. There is also a version of the membrane interface for 2D axisymmetric problems. The membrane interface is then applicable to lines since that is what represents boundaries.
The difference between a shell and a membrane is that the membrane does not have any bending stiffness. In most applications, the membrane is used by itself and not as a cladding. A tensile prestress is then necessary in order to avoid singularity because a membrane with no stress or compressive stress has no transverse stiffness. To include the prestress effect, you must enable geometric nonlinearity for the study step.
The Linear Elastic Material is the default material, which adds a linear elastic equation for the displacements and has a Settings window to define the elastic material properties. This material model can also be combined with viscoelasticity.
With the Nonlinear Structural Materials Module, you can also model Nonlinear Elastic and Hyperelastic materials, and add options such as Plasticity, Creep, and Viscoplasticity.
When this physics interface is added, these default nodes are also added to the Model Builder: Linear Elastic Material, Free (a condition where edges are free, with no loads or constraints), and Initial Values. In the case if axial symmetry, an Axial Symmetry node is also added. From the Physics toolbar, you can then add other nodes that implement, for example, loads and constraints. You can also right-click Membrane to select physics features from the context menu.
Settings
The Label is the default physics interface name.
The Name is used primarily as a scope prefix for variables defined by the physics interface. Refer to such physics interface variables in expressions using the pattern <name>.<variable_name>. In order to distinguish between variables belonging to different physics interfaces, the name string must be unique. Only letters, numbers, and underscores (_) are permitted in the Name field. The first character must be a letter.
The default Name (for the first physics interface in the model) is mbrn.
Structural Transient Behavior
From the Structural transient behavior list, select Include inertial terms (the default) or Quasistatic. Use Quasistatic to treat the elastic behavior as quasistatic (with no mass effects; that is, no second-order time derivatives). Selecting this option gives a more efficient solution for problems where the variation in time is slow when compared to the natural frequencies of the system.
For problems with creep, and sometimes viscoelasticity, the problem can be considered as quasistatic. This is also the case when the time dependence exists only in some other physics, like a transient heat transfer problem causing thermal strains.
Reference Point for Moment Computation
Enter the default coordinates for the Reference point for moment computation xref. The resulting moments (applied or as reactions) are then computed relative to this reference point. During the results and analysis stage, the coordinates can be changed in the Parameters section in the result nodes.
Discretization
In the Membrane interface you can choose not only the order of the discretization, but also the type of shape functions: Lagrange or serendipity. For highly distorted elements, Lagrange shape functions provide better accuracy than serendipity shape functions of the same order. The serendipity shape functions will however give significant reductions of the model size for a given mesh containing quadrilateral elements.
The default is to use Quadratic serendipity shape functions for the Displacement field.
Dependent Variables
The dependent variable (field variable) is for the Displacement field u which has three components (u, v, and w). The name can be changed but the names of fields and dependent variables must be unique within a model.
Vibrating Membrane: Application Library path Structural_Mechanics_Module/Verification_Examples/vibrating_membrane