# Materials

There are two families of material that you can define in Alpaca4d: **UniaxialMaterial** and **NDMaterial**.

**UniaxialMaterial** is used to construct a material object which represents uniaxial stress-strain relationships. The implemented relationship is Linear Elastic.

**NDMaterial** is used to construct a material object which represents the stress-strain relationship at the gauss-point of a continuum element. The behaviour is an Elastic Isotropic or Elastic Orthotropic.