> For the complete documentation index, see [llms.txt](https://alpaca4d.gitbook.io/docs/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://alpaca4d.gitbook.io/docs/basics/materials.md).

# Materials

Everything in the model needs a material. Alpaca4d has two families, and they are not interchangeable — a component that wants one will refuse the other.

| Family       | For                                                                            | Component                                      |
| ------------ | ------------------------------------------------------------------------------ | ---------------------------------------------- |
| **Uniaxial** | 1D stress–strain. Beam sections, fibres, hinges.                               | [Uniaxial](/docs/basics/materials/uniaxial.md) |
| **nD**       | The full 2D/3D stress–strain law at a Gauss point. Shells, bricks, tetrahedra. | [nD](/docs/basics/materials/nd.md)             |

Both can also be pulled from the [Material Database](/docs/basics/materials/materialdatabase.md), which fills in standard grades of steel, concrete, timber and plastic so you do not have to type *E*, *G*, *ν* and *ρ* by hand.

For the non-linear materials used by fibre sections — `Concrete01`, `Steel01`, `ReinforcingSteel` — see [Moment Curvature](/docs/basics/moment-curvature.md).

## Which goes where

| You are building                                                                                                | Give it                 |
| --------------------------------------------------------------------------------------------------------------- | ----------------------- |
| A beam cross-section (rectangle, circle, I, 2L, hollow, generic)                                                | a **Uniaxial** material |
| A [Plate Fiber Section](/docs/basics/sections/plate-fiber.md) for shells                                        | an **nD** material      |
| An [SSP Brick](/docs/basics/elements/brick.md) or [Tetrahedron](/docs/basics/elements/four-node-tetrahedron.md) | an **nD** material      |
| A fibre in a [Fiber Section](/docs/basics/moment-curvature/fiber-section.md)                                    | a **Uniaxial** material |

## Units

`E`, `G` and stresses are in **kN/m²**; density `Rho` is in **kg/m³**; strains and Poisson's ratios are dimensionless.