# 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. --- # Agent Instructions: Querying This Documentation If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question. Perform an HTTP GET request on the current page URL with the `ask` query parameter: ``` GET https://alpaca4d.gitbook.io/docs/basics/materials.md?ask= ``` The question should be specific, self-contained, and written in natural language. The response will contain a direct answer to the question and relevant excerpts and sources from the documentation. Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.