DesignSpark Mechanical allows you to specify the material properties for the solid objects in your assembly. There is a local library of commonly used industrial & production grade materials.
You can also add a new material to your local library. You will need to list certain physical properties to define the material.
These can be entered in the properties tab.
Example 1: For FR-4 (fire retardant composite material for laminating PCBs), the values can be found here: https://en.wikipedia.org/wiki/FR-4
Example 2 (copper): Depending on the grade of copper you are using, there is one pre-populated in the local library (OFE grade - high electrical conductivity). Just drop-down the material name list to find more.