Test Material
Under large strains, several material models have regions where the material is unstable. These instabilities might be triggered by a uniaxial deformation limit, or by a combination of multiaxial deformations. These instabilities indicate that the material is used outside the validity range where the material parameters were obtained. Unstable materials generate convergence problems and unphysical solutions, and therefore it is beneficial to test a given material model before running big models.
Some options described below are only available with certain COMSOL products (see https://www.comsol.com/products/specifications/).
Domain Selection
You can only select one domain, so that one material model can be tested at the time. If you want to test different material models, you can sequentially change the domain selection, or add several Test Material nodes with different selections.
Material Tests
Select the Specimen size — Automatic or User defined. For Automatic, a single mesh element with a length of 1 m is used. Select User defined to specify a different Characteristic length L in case the mesh size has an influence in the material law.
Select the Study setup — Stationary or Time dependent, and then select the Test setup — Monotonic, Cyclic, or User defined.
For the Time dependent test, enter the Test time tf. The default is 1 second.
For either the Stationary or the Time dependent test, enter the Number of measurement points in the study Np. The default is 50 points.
For the Monotonic test, the number of measurement points sets up the resolution for the given load or deformation interval.
For the Cyclic test, enter the Number of cycles Nc. In a Stationary test, each cycle has Np/Nc measurement points per cycle. In a Time dependent test, each cycle has a period of tc = tf/Nc, and a time step given by Δt = tf/Np.
For the User defined test, select the Test control — Stretch driven or Force driven. Depending on the choice, you can specify how the prescribed displacement or load varies as a function of time or parameter range. Enter a Parameter name (default is para) to be used in the user defined inputs for the stretch functions or force functions. A parameter with the given name will be created in the Parameters node when clicking Set Up Tests () as shown below.
Tests
Select one or more of the available check boxes to test the material under different load conditions:
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Uniaxial test — Enter the stretch range for the test. The minimum stretch λmin gives the compressive limit, and the maximum stretch λmax gives the tensile limit. A uniaxial compression test is achieved by setting λmax = 1, and a uniaxial extension test by setting λmin = 1. The inputs should be such λmin < λmax.
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Biaxial test — Enter the minimum stretch λmin, the maximum stretch λmax, and the strain biaxiality ratio β for the test.
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Shear test — Enter the maximum shear angle θ for the test.
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Isotropic test — Enter the minimum stretch λmin and the maximum stretch λmax for the test.
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Oedometer test — Enter the axial stretch λmin for the test.
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Triaxial test — Enter the in situ stress σins and the axial stretch λmin for the test.
Isotropic Compression with Modified Cam-Clay Material Model: Application Library path Geomechanics_Module/Verification_Examples/isotropic_compression.
Triaxial and Oedometer Test with Modified Cam-Clay Material Model: Application Library path Geomechanics_Module/Verification_Examples/isotropic_compression_mscc.
Load Group, Material, and Study Generation
The material parameters of a given material model are tested by creating dedicated tests on a single element. In order to do this, a number of fundamental load cases are analyzed. These steps are described below.
On the Material Tests section toolbar, there is an icon Automated Model Setup (). Select:
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Set Up Tests (). This option generates a new 3D Component, which contains a single element to test the material model defined under Domain Selection. When you select it, the following changes will be made to the model:
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A new 3D Component is created.
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A new Solid Mechanics interface is added to this component.
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A number of boundary conditions will be available to the new Solid Mechanics interface. The boundary conditions correspond to the selected tests.
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A Mesh node containing a single element.
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A new Study node will be added based on the Study setup selection. A stationary or time dependent study will contain the Number of measurement points for either a Monotonic or a Cyclic test. This study runs automatically after selecting Set Up Tests.
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A Stop Condition node will be added under the stationary or time dependent study. This condition stops the loading when the element is fully collapsed. See Stop Condition in the COMSOL Multiphysics Reference Manual.
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A new Material Tests node will be added to the Results node. Different plots will show the Tests and the results from the Study node.
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Remove Tests (). This option deletes the 3D Component, the Solid Mechanics interface, and the Mesh, Study, and Results nodes added after selecting Set Up Tests.