Material Models for Structural Mechanics
The Structural Mechanics Module without any add-on modules provides the Linear Elastic material with Viscoelasticity and Thermal Expansion modeling capabilities. It also provides access to piezoelectric and magnetostrictive materials.
If you have the optional products Nonlinear Structural Materials Module or Geomechanics Module, many other classes of nonlinear materials are also available. These models can be modified and extended, and custom material models can be defined.
You can also add a material model which you have coded yourself and made available as a binary library file using an External Stress-Strain Relation.
In the COMSOL Multiphysics Reference Manual:
In Table 2-6 and Table 2-7 you can find an overview of the families of materials, and their applicability in the various structural mechanics interfaces.
 
Many of the material models can be augmented by effects like thermal expansion, hygroscopic swelling, initial stresses and strains, external stress, and activation.