Explicit X2: large deformation, J2 and Johnson-Cook plasticity, damage and erosion (M10)

- model: materials gain solver-neutral plastic (bilinear isotropic/kinematic,
  hardening table, Johnson-Cook with adiabatic heating), damage (Johnson-Cook
  D1-D5) and erosion (plastic strain); explicit steps gain nonlinear_geometry
  (default on) and stress_rate (jaumann | green_naghdi).
- fem: rotation (Hughes-Winget, Flanagan-Taylor), plasticity (radial return,
  capped bracketed Newton for Johnson-Cook, damage; the update takes the
  mechanical strain increment so thermal strains can be subtracted), closed-form
  hex8 gradients for deformed configurations.
- explicit: updated Lagrangian hex8/tet4, per-element material state, erosion
  with reported energy, plastic work in the energy account.
- solver: warns that the linear solver uses the elastic constants of plastic
  materials (results hash-identical).
- decks: Abaqus *PLASTIC (+ Johnson-Cook keywords), Nastran MATS1/TABLES1,
  LS-DYNA *MAT_PLASTIC_KINEMATIC / *MAT_PIECEWISE_LINEAR_PLASTICITY; what a
  format cannot express is reported as Unsupported.
- engine: model.material.plasticity, model.material.damage (catalogue ready),
  explicit element probe, eroded elements in the solve summary.

Step commits before the rebase onto the DOF framework: branch explicit-x2-steps.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NmAttHhUZTokgPeEJia8sm

crates/model/src/steps.rs

@@ -118,6 +118,30 @@ pub struct ExplicitDynamic {
     /// Artificial bulk viscosity.
     #[serde(default)]
     pub bulk_viscosity: BulkViscosity,
+    /// Large deformation (updated Lagrangian: gradients of the current configuration, stress
+    /// updated in rate form with an objective stress rate); `false` keeps the gradients of the
+    /// undeformed mesh (small displacements and rotations).
+    #[serde(default = "yes")]
+    pub nonlinear_geometry: bool,
+    /// The objective stress rate of large deformation.
+    #[serde(default)]
+    pub stress_rate: StressRate,
+}
+
+fn yes() -> bool {
+    true
+}
+
+/// The objective stress rate a large-deformation stress update uses.
+#[derive(Debug, Clone, Copy, Default, PartialEq, Eq, Serialize, Deserialize)]
+#[serde(rename_all = "snake_case")]
+pub enum StressRate {
+    /// Jaumann (co-rotational with the spin), integrated with the Hughes–Winget rotation.
+    #[default]
+    Jaumann,
+    /// Green–Naghdi (co-rotational with the rotation of the polar decomposition), integrated as
+    /// Flanagan and Taylor do.
+    GreenNaghdi,
 }
 
 fn default_dt_scale() -> f64 {
@@ -142,6 +166,8 @@ impl ExplicitDynamic {
             rigid_walls: Vec::new(),
             hourglass: Hourglass::default(),
             bulk_viscosity: BulkViscosity::default(),
+            nonlinear_geometry: true,
+            stress_rate: StressRate::default(),
         }
     }