(guide_model_sd_cauchy)=
# Cauchy Continuum Model

The solid material model in SGIO is implemented by
`sgio.model.CauchyContinuumModel`. The legacy dataclass-based
`CauchyContinuumModel` has been removed; the Pydantic implementation that was
previously exposed as `CauchyContinuumModelNew` now owns the canonical name. A
temporary import alias `CauchyContinuumModelNew` is still provided to ease
migration, but it will be deprecated in a future release.

The model captures the standard Cauchy continuum kinematics with three
displacements ({math}`u_1`, {math}`u_2`, {math}`u_3`) and six strain
components ({math}`\varepsilon_{11}`, {math}`\varepsilon_{22}`,
{math}`\varepsilon_{33}`, {math}`\varepsilon_{23}`, {math}`\varepsilon_{13}`,
{math}`\varepsilon_{12}`).

## Key Features

- Pydantic validation on construction and assignment (`validate_assignment=True`)
- Explicit support for isotropic, orthotropic, and fully anisotropic materials
- Typed material queries via `get_matrix_component()` and `get_thermal_expansion()`
- Explicit setters `set_isotropy()`, `set_elastic()`, and `set_strength_constants()`
- First-class JSON serialization via {py:meth}`model_dump` and
  {py:meth}`model_dump_json`

## Creating a Material

The constructor accepts keyword arguments for all engineering constants.
Pydantic validators enforce units and admissible ranges, for example Poisson
ratios.

```python
from sgio.model.solid import CauchyContinuumModel

carbon_ud = CauchyContinuumModel(
    name="UD Carbon/Epoxy",
    isotropy=1,
    density=1570.0,
    e1=138e9,
    e2=9e9,
    e3=9e9,
    g12=5.2e9,
    g13=5.2e9,
    g23=3.5e9,
    nu12=0.32,
    nu13=0.32,
    nu23=0.45,
    strength_constants=[
        1500.0, 1200.0, 800.0,
        900.0, 700.0, 600.0,
        120.0, 110.0, 95.0,
    ],
    cte=[2.5e-6, 2.5e-6, 2.4e-5, 0.0, 0.0, 0.0],
)
```

The preferred mutation path uses explicit typed setters:

```python
from sgio.model import ElasticInputType, MatrixKind, TensorComponent

carbon_ud.set_isotropy('orthotropic')
carbon_ud.set_elastic([210e9, 0.29], input_type=ElasticInputType.ISOTROPIC)

c11 = carbon_ud.get_matrix_component(MatrixKind.STIFFNESS, TensorComponent(1, 1))
```

Legacy `get()` / `set()` helpers remain only as migration shims. New examples
should not depend on them; see {ref}`guide_model_migration_compat` for the
compatibility mapping and removal roadmap.

## Loading from JSON

The example at `examples/load_cauchy_material_from_json/` demonstrates a
complete JSON round-trip. The core logic is straightforward:

```python
import json
from pathlib import Path

from sgio.model.solid import CauchyContinuumModel

payload = json.loads(Path('material.json').read_text())
material = CauchyContinuumModel(**payload)
print(material.model_dump_json(indent=2))
```

This pattern works equally well when the JSON document comes from a database or
an API response, making it easy to integrate SGIO materials into external
pipelines.