Nonlinear Elastic Material
The Nonlinear Elastic Material feature is used to model stress-strain relationships which are nonlinear even at infinitesimal strains. It is available in the Solid Mechanics and Membrane interfaces. This material model requires either the Nonlinear Structural Materials Module or the Geomechanics Module.
By adding the following subnodes to the Nonlinear Elastic Material node you can incorporate many other effects:
Note: Some options are only available with certain COMSOL products (see http://www.comsol.com/products/specifications/)
Coordinate System Selection
The Global coordinate system is selected by default. The Coordinate system list contains any additional coordinate systems that the model includes (except boundary coordinate systems). The coordinate system is used when stresses or strains are presented in a local system. The coordinate system must have orthonormal coordinate axes, and be defined in the material frame. Many of the possible subnodes inherit the coordinate system settings.
Nonlinear Elastic Material
To use a mixed formulation by adding the pressure as an extra dependent variable to solve for, select the Nearly incompressible material check box. For a material with a very low compressibility, using only displacements as degrees of freedom may lead to a numerically ill-posed problem.
The available material models depend on the COMSOL products you are using.
Nonlinear Structural Materials Module: Select a Material model: Ramberg-Osgood, Power law, Uniaxial data, Bilinear elastic, or User defined.
Geomechanics Module: Select a Material model: Ramberg-Osgood, Hyperbolic law, Hardin-Drnevich, Duncan-Chang, Duncan-Selig, or User defined.
Density
All nonlinear elastic material models have density as an input. The default Density ρ uses values From material. For User defined enter another value or expression.
Ramberg-Osgood, Power law, Hyperbolic law, Hardin-Drnevich, Duncan-Chang, or Duncan-Selig
Select from the applicable list to use the value From material or enter a User defined value or expression.
From the Specify list select a pair of elastic properties for an isotropic material — Young’s modulus and Poisson’s ratio (the default for Ramberg-Osgood, Power law, Duncan-Chang, and Duncan-Selig) or Bulk modulus and shear modulus (the default for Hyperbolic law and Hardin-Drnevich).
Then depending on the selections, define the applicable parameters:
For Ramberg-Osgood:
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For Power law and Hyperbolic law:
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For Hardin-Drnevich, define the Reference shear strain γref.
For Duncan-Chang, define the Ultimate deviatoric stress qult.
For Duncan-Selig:
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Uniaxial Data
For Uniaxial data enter a value or expression for the Uniaxial stress function σax as a function of the uniaxial strain. The default expression is the linear function (210[GPa])*solid.eax N/m2, which corresponds to a linear elastic material with Young’s modulus 210 GPa.
From the Specify list select how to specify the second elastic property for the material — Bulk modulus or Poisson’s ratio. Then depending on the selection, enter a value or select from the applicable list to use the value From material or enter a User defined value or expression:
When you select Bulk modulus, the Young’s modulus is computed from the tensile part of the Uniaxial stress function σax. When you select Poisson’s ratio, you can either use the tensile part (default), or use the full tensile-compressive function by selecting the check box Use nonsymmetric stress-strain data.
Bilinear Elastic
For Bilinear elastic enter a value or select from the applicable list to use the value From material or enter a User defined value or expression.
User Defined
In the User defined material model, you specify the bulk modulus implicitly by entering the relation between pressure and volumetric elastic strain. Enter a value or select from the applicable list to use the value From material (the default) or enter a User defined value or expression.
Pressure p. The default expression is (-160[GPa])*solid.eelvol N/m2, which corresponds to a linear elastic material with bulk modulus 160 GPa.
Geometric Nonlinearity
The settings in this section affect the behavior of the selected domains in a geometrically nonlinear analysis.
If a study step is geometrically nonlinear, the default behavior is to use a large strain formulation in all domains. Select the Force linear strains check box to always use a small strain formulation, irrespective of the setting in the study step.
When a geometrically nonlinear formulation is used, the elastic deformations used for computing the stresses can be obtained in two different ways if inelastic deformations are present: additive decomposition and multiplicative decomposition. The default is to use multiplicative decomposition. Select Additive strain decomposition to change to an assumption of additivity.
 
When a multiplicative decomposition is used, the order of the subnodes to Nonlinear Elastic Material matters. The inelastic deformation are assumed to have occurred in the same order as the subnodes appear in the model tree.
In versions prior to 5.3, only the additive strain decomposition method was available. If you want to revert to the previous behavior, select Additive strain decomposition. If the results then differ significantly, probably the assumption of additivity is questionable, however.
Studies and Solvers in the COMSOL Multiphysics Reference Manual
Energy Dissipation
To display this section, click the Show button () and select Advanced Physics Options.
Select the Calculate dissipated energy check box as needed to compute the energy dissipated by Creep, Plasticity, Viscoplasticity, or Viscoelasticity.
Location in User Interface
Context Menus
Solid Mechanics>Material Models>Nonlinear Elastic Material
Membrane>Material Models>Nonlinear Elastic Material
Ribbon
Physics tab with Solid Mechanics selected:
Domains>Material Models>Nonlinear Elastic Material
Physics tab with Membrane selected:
Boundaries>Material Models>Nonlinear Elastic Material