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A material in FEBio defines the constitutive response of the domain to which the material is assigned. See Chapter 4↑ for a detailed list of all the available materials in FEBio. It is important to understand that the module defines which materials you can use. For example, the biphasic material cannot be used in the solid module. Although some cases of invalid material use are caught, in many situations the resulting behavior is undefined. The following table shows a list of some of FEBio's special materials and the modules in which they can be used.
In addition to using the proper materials for a given module, it is also important to understand that most material parameters have a limited range in which they define valid constitutive behavior. For example, in a neo-Hookean material the Young's modulus must be positive and the Poisson's ratio must larger than -1 and less than 0.5. FEBio has most of these limits implemented in the code and will throw an error when a parameter value is defined that falls outside the valid range.
Since material parameters can be defined as functions of time through a loadcurve, FEBio will check all parameters at the beginning of each time step. When a parameter has become invalid the run will be aborted.
Some constitutive models are only valid for a limited amount of deformation. All materials in FEBio are designed for large deformation (in the sense that they are objective under arbitrary deformation), but that does not imply unlimited deformation. When the deformation becomes too large, the material response may become unphysical and although FEBio gives an answer, the result may be meaningless (see Section 8.8↓ for a discussion on interpreting the result). Some materials also become unstable at extremely large deformations. A classical example is the Mooney-Rivlin material. For a certain range of values of the material parameters, and under certain loading conditions, the stress-strain response can have a zero slope at large deformations. In that case, FEBio will most likely not be able to converge since the slope of the stress-strain response is used to progress towards the solution.