# Materials

In the material section you can define multiple materials and assign volumes to them. You can add a material to the model by clicking on the Add material button. Afterwards you have to define specific material parameters depending on the chosen analysis type. Of course you can use the default settings as well if you are only setting up a test run or use the built-in material library (Import from material library button) to use some standard material’s properties. At last, you have to use the assignment box to add the corresponding volumes to the material definition.

## Solid Materials

For a solid mechanics analysis every volume that does belong to the simulation domain has to be assigned to a material in order to set up a valid simulation. For example if you are running a solid mechanics simulation, you have to choose a material law that describes the relationship between the strains and the consequent stresses. Please be aware that the material behaviour can be linear or nonlinear (e.g. plastic material) and therefore may affect the numerical effort of the calculation. The Solid material properties and behavior are defined by Thermal solid model. Solid material models available on SimScale are:

### Linear elastic

Linear elastic material deforms elastically through out the analysis, which means that it will return back to its initial state upon unloading, irrespective of the deformation.

### Plastic

Plastic material model describes the material behaviour after the onset of plasticity, which is defined as irreversible deformation in solid material when subject to loading.

### Hyperelastic

Hyperelastic materials are the special class of materials which tends to respond elastically when they are subjected to very large strains. They shows both a nonlinear material behavior as well as large shape changes.

### Creep

Creep is the inelastic, irreversible deformation of structures during time. It is a life limiting factor and depends on stress, strain, temperature and time.

## Fluid Materials

All incompressible analysis types require the material density (a constant) to be specified by the user. For compressible simulations the density is solved as part of the equation of state and does not need to be provided in the material definition. The Fluid material properties and behavior are defined by Thermal fluid model.

### Viscosity Model

For Analysis types that do not include energy/heat, the fluid materials are categorized based on the viscosity model of the fluid. The models generally relate the behaviour of viscosity to strain rate of the fluid.

The 2 fundamental types of viscosity models that define any fluid material are as follows:

### Newtonian Model

In Newtonian fluid model, the local stresses due to the viscous forces in the fluid change linearly with the local strain rate. Here, viscosity is then the constant of proportionality. The implemented Newtonian model assumes a constant kinematic viscosity $$ν$$:

$$ν=(μ/ρ)$$

which is specified by the user in units of $$m^2/s$$.

Some ‘Liquids’ and ‘Gases’, for example water and air follow a ‘Newtonian model’ under standard conditions.

### Non-Newtonian Model

In a Non-Newtonian fluid model, the local shear stress and the local shear rate of the fluid are not related linearly. Here a constant of proportionality cannot be determined and so viscosity is a variable quatity. For these fluids, several Non-newtonian models exist that define the non-linear relation to determine the kinematic viscosity, ν (see link below for details).

Some examples of Non-Newtonain fluids include common substances like ketchup, custard, toothpaste, corn-starch suspensions, paint, blood, and shampoo.