docs: element library plan, family by family (#30)

Owner 2026-10-13: cover the element families a full CAE code offers, simple ones first (1D and
2D). docs/elements.md lists point/discrete, 1D, 2D, shell/membrane, 3D and special-purpose
families with status and the task that brings each.


Claude-Session: https://claude.ai/code/session_01NmAttHhUZTokgPeEJia8sm

Co-authored-by: jyviko <2501648+jyviko@users.noreply.github.com>
Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

docs/elements.md

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+# Element library
+
+ModelCraft's finite element library (`crates/fem`), family by family, with status. The scope is
+modelled on what a full CAE code offers (Abaqus's element library is the reference for breadth);
+names, formulations and tests are our own, written from the published literature. Deck writers map
+each of our elements to each solver's name (see `crates/decks`). Simple elements first: 1D and 2D,
+then shells, special-purpose and multiphysics elements.
+
+Status: **done** (on main, verified against analytic solutions), **in progress** (an agent task is
+open), **planned**.
+
+## Point and discrete elements
+
+| Element | Use | Status |
+|---|---|---|
+| Point mass | Lumped mass at a node or reference point (isotropic or per direction) | in progress (1D elements I) |
+| Rotary inertia | Inertia tensor at a 6-DOF node | in progress (1D elements I) |
+| Spring to ground / two-point spring | Linear spring in a fixed direction or along the line of nodes | in progress (1D elements I) |
+| Dashpot to ground / two-point | Linear viscous damper (dynamic steps; ignored in static) | in progress (1D elements I) |
+| Nonstructural mass | Mass spread over a region without stiffness | planned |
+| Connectors | Kinematic joints (hinge, slot, universal…) with elasticity, damping, stops | planned (M7) |
+
+## 1D elements
+
+| Element | Use | Status |
+|---|---|---|
+| Truss, 2-node (2D and 3D) | Axial members, cables (compression allowed) | in progress (1D elements I) |
+| Truss, 3-node (2D and 3D) | Curved axial members | in progress (1D elements I) |
+| Beam, 2-node Timoshenko, 3D | Frames, stiffeners; shear deformable | in progress (shells and beams) |
+| Beam, 2-node Euler–Bernoulli (cubic), 2D and 3D | Slender frames; exact for end-loaded prismatic members | planned (1D elements II) |
+| Beam, 3-node Timoshenko (quadratic), 2D and 3D | Curved and stocky beams | planned (1D elements II) |
+| Beam, 2D (planar frames) | In-plane frames with 3 DOFs per node | planned (1D elements II) |
+| Pipe / elbow | Pipes with internal pressure, ovalisation | planned (1D elements II) |
+| Open-section beams with warping | Thin-walled sections in torsion | planned |
+| Rebar / stringers | Reinforcement along edges or embedded | planned |
+
+## 2D elements
+
+| Element | Use | Status |
+|---|---|---|
+| Plane stress tri3, tri6, quad4, quad8 | Thin plates loaded in their plane | in progress (2D elements) |
+| Plane strain tri3, tri6, quad4, quad8 | Long sections (dams, tunnels, extrusions) | in progress (2D elements) |
+| Axisymmetric tri3, tri6, quad4, quad8 | Revolved solids (shafts, vessels, discs) | in progress (2D elements) |
+| Incompatible-mode and reduced-integration quads | Bending-accurate and cheap variants | in progress (2D elements) |
+| Generalized plane strain | Plane strain with a free out-of-plane strain | planned |
+| Axisymmetric with twist | Revolved parts under torsion | planned |
+| Axisymmetric shells (2-node, 3-node) | Thin revolved shells (tanks, domes) | planned |
+| Hybrid (mixed u–p) variants | Nearly incompressible materials | planned |
+
+## Shells and membranes
+
+| Element | Use | Status |
+|---|---|---|
+| Shell quad4 (MITC4), triangle (locking-free) | Thin and thick shells | in progress (shells and beams) |
+| Shell quad8 / tri6 | Curved thick shells | planned |
+| Continuum shells (8-node, on swept hex) | Thick-to-thin shells, composites through thickness | planned (M8) |
+| Membranes (tri3, quad4, quad8) | Fabrics, inflatables, skins without bending | planned |
+| Surface elements | Massless surfaces for loads and contact | planned |
+| Composite layups on shells | Ply tables, orientations | planned (M8) |
+
+## 3D solids
+
+| Element | Use | Status |
+|---|---|---|
+| tet4, tet10 | Free meshes of any part | done |
+| hex8 (full, incompatible modes), hex20 (full, reduced) | Structured and swept meshes | done |
+| wedge6 | Swept meshes from triangles, transitions | done |
+| hex8 one-point with hourglass control, nodal-pressure tet4 (explicit) | Explicit dynamics | done |
+| wedge15, pyramid5/13 | Quadratic transitions, tet–hex transition | planned (M5) |
+| Hybrid (mixed u–p) solids | Nearly incompressible materials (rubber) | planned (M8) |
+| Modified quadratic tet (tet10 for contact/plasticity) | Robust quadratic tets | planned |
+
+## Special-purpose and multiphysics
+
+| Element | Use | Status |
+|---|---|---|
+| Cohesive (2D, 3D) | Adhesive layers, delamination | planned |
+| Gasket | Through-thickness gasket behaviour | planned |
+| Infinite elements | Unbounded domains | planned |
+| Heat transfer (all shapes), coupled temperature–displacement | Thermal and thermal-stress analyses | planned (M8) |
+| Acoustic | Fluid acoustics, structural–acoustic coupling | planned |
+| Rigid elements | Rigid bodies from meshes | planned (M7) |