oracle: 2D cases (Kirsch plane stress, Lamé plane strain and axisymmetric, rotating disc and cylinder, NAFEMS LE1 with quad8) and J2 uniaxial explicit

The pending 2D LE1 case becomes a real one; the plasticity cases stay
pending with what they still need (static J2 for the collapse pressure,
the transcribed measurements for the Taylor bar).

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

tests/oracle/j2-uniaxial-explicit/README.md

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+# Uniaxial tension of a J2 bilinear-hardening cube to large strain (explicit)
+
+A unit cube of one hex8 (one-point, hourglass control), steel-like (E = 200000, ν = 0.3,
+σ_y = 250, H = 2000, ρ = 7.8e-9), held by symmetry on three faces and stretched on the fourth to
+λ = e^0.2 over 0.01 s (quasi-static), lateral faces free: uniaxial stress at a logarithmic
+strain of 0.2.

tests/oracle/j2-uniaxial-explicit/expected.json

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+{
+ "title": "Uniaxial tension of a J2 bilinear-hardening cube to large strain (explicit)",
+ "kind": "theory",
+ "source": "von Mises plasticity with linear isotropic hardening under uniaxial stress (Simo and Hughes, Computational Inelasticity, Springer 1998, §2.3); logarithmic strain for a hypoelastic–plastic law under coaxial stretching",
+ "quantities": [
+  {
+   "id": "sigma",
+   "quantity": "axial Cauchy stress at a logarithmic strain of 0.2",
+   "measure": {
+    "command": "solve.probe",
+    "params": {
+     "step": "Pull",
+     "elements": [
+      1
+     ]
+    },
+    "pointer": "/elements/0/stress/0"
+   },
+   "value": 643.5643564356436,
+   "tolerance": {
+    "relative": 0.01
+   },
+   "reason": "uniaxial J2 with linear hardening: σ = σ_y + E_t (ε − σ_y/E) in logarithmic strain; the stretch takes 10⁴ wave transits, so inertia and the ringing of the start (ρcv ≈ 0.1% of σ) stay well under 1%"
+  },
+  {
+   "id": "plastic_strain",
+   "quantity": "equivalent plastic strain",
+   "measure": {
+    "command": "solve.probe",
+    "params": {
+     "step": "Pull",
+     "elements": [
+      1
+     ]
+    },
+    "pointer": "/elements/0/plastic_strain"
+   },
+   "value": 0.1967821782178218,
+   "tolerance": {
+    "relative": 0.01
+   },
+   "reason": "ε_p = ε − σ/E, as for the stress"
+  }
+ ]
+}

tests/oracle/j2-uniaxial-explicit/fixture.json

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+{
+  "note": "ModelCraft's accepted results for this case, written by `cargo xtask oracle --bless`. A result outside its band fails CI.",
+  "values": {
+    "plastic_strain": {
+      "band": 1.967821782178218e-10,
+      "value": 0.1967821759864614
+    },
+    "sigma": {
+      "band": 6.435654787724778e-7,
+      "value": 643.5654787724778
+    }
+  }
+}

tests/oracle/j2-uniaxial-explicit/recipe.json

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+[
+ {"command": "mesh.block", "params": {"body": "Cube", "kind": "hex8", "corners": [[0, 0, 0], [1, 0, 0], [1, 1, 0], [0, 1, 0], [0, 0, 1], [1, 0, 1], [1, 1, 1], [0, 1, 1]], "divisions": [1, 1, 1], "faces": {"6": "x0", "3": "y0", "1": "z0", "4": "x1"}}},
+ {"command": "model.set.create", "params": {"name": "all", "geometry": [{"body": "Cube"}]}},
+ {"command": "model.set.create", "params": {"name": "x0", "geometry": [{"body": "Cube", "face": "x0"}]}},
+ {"command": "model.set.create", "params": {"name": "y0", "geometry": [{"body": "Cube", "face": "y0"}]}},
+ {"command": "model.set.create", "params": {"name": "z0", "geometry": [{"body": "Cube", "face": "z0"}]}},
+ {"command": "model.set.create", "params": {"name": "x1", "geometry": [{"body": "Cube", "face": "x1"}]}},
+ {"command": "model.material.create", "params": {"name": "Steel", "elastic": {"type": "isotropic", "young": 200000.0, "poisson": 0.3}, "density": 7.8e-09, "plastic": {"type": "bilinear", "yield_stress": 250.0, "hardening_modulus": 2000.0}}},
+ {"command": "model.section.create", "params": {"name": "cube", "set": "all", "material": "Steel", "kind": {"type": "solid"}, "formulation": {"integration": "reduced"}}},
+ {"command": "model.boundary_condition.create", "params": {"name": "sx", "type": "symmetry", "target": {"set": "x0"}, "plane": "x"}},
+ {"command": "model.boundary_condition.create", "params": {"name": "sy", "type": "symmetry", "target": {"set": "y0"}, "plane": "y"}},
+ {"command": "model.boundary_condition.create", "params": {"name": "sz", "type": "symmetry", "target": {"set": "z0"}, "plane": "z"}},
+ {"command": "model.boundary_condition.create", "params": {"name": "pull", "type": "displacement", "target": {"set": "x1"}, "values": [0.22140275816016985, null, null, null, null, null]}},
+ {"command": "model.step.create", "params": {"name": "Pull", "procedure": {"type": "explicit_dynamic", "end_time": 0.01}, "boundary_conditions": ["sx", "sy", "sz", "pull"], "loads": []}},
+ {"command": "solve.explicit", "params": {"step": "Pull"}, "id": "solve"}
+]