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+# This file is a part of JuliaFEM.
+# License is MIT: see https://github.com/JuliaFEM/JuliaFEM.jl/blob/master/LICENSE.md
+
+"""
+Concrete types for continuum mechanics formulations and theories.
+
+Abstract types are in abstract.jl, implementations are in formulations.jl.
+
+Must be included after abstract.jl.
+"""
+
+# ============================================================================
+# CONCRETE THEORY TYPES
+# ============================================================================
+
+"""
+    FullThreeD <: AbstractContinuumTheory
+
+Full 3D analysis with no simplifications.
+
+**DOMAIN-AGNOSTIC**: Can be used by ANY physics domain!
+
+# Usage Across Domains
+
+```julia
+# Continuum mechanics (solid mechanics)
+physics_solid = Physics(
+    formulation = FullThreeD(),
+    field = Displacement{3}(),
+    ...
+)
+
+# Heat transfer (SAME formulation!)
+physics_heat = Physics(
+    formulation = FullThreeD(),
+    field = Temperature(),
+    ...
+)
+
+# Poisson equation (SAME formulation!)
+physics_poisson = Physics(
+    formulation = FullThreeD(),
+    field = Potential(),
+    ...
+)
+```
+
+# Details
+- All six stress/flux components in continuum context
+- No geometric simplifications
+- Most accurate but most expensive
+"""
+struct FullThreeD <: AbstractContinuumTheory end
+
+"""
+    PlaneStress <: AbstractContinuumTheory
+
+2D plane stress assumption (out-of-plane stress = 0).
+
+Applicable to thin plates and membranes where thickness << in-plane dimensions.
+"""
+struct PlaneStress <: AbstractContinuumTheory end
+
+"""
+    PlaneStrain <: AbstractContinuumTheory
+
+2D plane strain assumption (out-of-plane strain = 0).
+
+Applicable to thick sections with no variation in z-direction.
+"""
+struct PlaneStrain <: AbstractContinuumTheory end
+
+"""
+    Axisymmetric <: AbstractContinuumTheory
+
+Axisymmetric analysis (rotation around z-axis).
+
+**DOMAIN-AGNOSTIC**: Can be used by ANY physics domain with axial symmetry!
+
+# Usage Across Domains
+
+```julia
+# Continuum mechanics (pressure vessel)
+physics_vessel = Physics(
+    formulation = Axisymmetric(),
+    field = Displacement{2}(),  # (r, z) displacements
+    ...
+)
+
+# Heat transfer in cylinder (SAME formulation!)
+physics_heat = Physics(
+    formulation = Axisymmetric(),
+    field = Temperature(),  # T(r, z)
+    ...
+)
+```
+
+# Details
+- Geometry and loading symmetric about z-axis
+- No circumferential variations (∂/∂θ = 0)
+- 2D mesh in (r, z) plane represents 3D geometry
+- Examples: Pressure vessels, pipes, rotating disks, cylinders
+"""
+struct Axisymmetric <: AbstractContinuumTheory end
+
+# ============================================================================
+# CONCRETE FORMULATION TYPES
+# ============================================================================
+
+"""
+    ContinuumFormulation{Theory} <: AbstractFormulation
+
+Standard continuum mechanics formulation with theory variant.
+
+This is the fundamental FEM formulation for solid mechanics, heat transfer,
+and other continuum physics problems.
+
+# Type Parameter
+- `Theory <: AbstractContinuumTheory` - Dimensionality/simplification theory
+
+# Examples
+
+```julia
+# 3D elasticity
+physics = Physics(
+    formulation = ContinuumFormulation{FullThreeD}(),
+    field = Displacement{3}(),
+    mesh = mesh,
+    material = steel
+)
+
+# 2D plane stress (thin plate)
+physics_2d = Physics(
+    formulation = ContinuumFormulation{PlaneStress}(),
+    field = Displacement{2}(),
+    mesh = mesh_2d,
+    material = aluminum
+)
+
+# 2D plane strain (thick section)
+physics_2d = Physics(
+    formulation = ContinuumFormulation{PlaneStrain}(),
+    field = Displacement{2}(),
+    mesh = mesh_2d,
+    material = concrete
+)
+
+# Axisymmetric (cylinder)
+physics_axisym = Physics(
+    formulation = ContinuumFormulation{Axisymmetric}(),
+    field = Displacement{2}(),  # (r, z) displacements
+    mesh = mesh_2d,
+    material = steel
+)
+```
+
+# Assembly Dispatch
+
+Assembly methods specialize on theory × field combinations:
+
+```julia
+# 3D solid mechanics
+function assemble!(physics::Physics{ContinuumFormulation{FullThreeD}, Displacement{3}, M, Mat})
+    # Standard 3D displacement-based assembly
+    # Full 6×6 strain-displacement matrix (Bε)
+    # 6×6 constitutive matrix (Dε)
+end
+
+# 2D plane stress
+function assemble!(physics::Physics{ContinuumFormulation{PlaneStress}, Displacement{2}, M, Mat})
+    # 2D assembly with plane stress assumptions
+    # 3×3 reduced strain-displacement matrix
+    # 3×3 plane stress constitutive matrix
+end
+
+# Heat transfer (same formulation, different field!)
+function assemble!(physics::Physics{ContinuumFormulation{FullThreeD}, Temperature, M, Mat})
+    # Thermal assembly (scalar field)
+    # Thermal conductivity matrix
+end
+```
+
+# Implementation Location
+
+Concrete assembly implementations are in:
+- `src/assembly/continuum_3d.jl` - 3D continuum mechanics
+- `src/assembly/continuum_2d.jl` - 2D plane stress/strain
+- `src/assembly/axisymmetric.jl` - Axisymmetric problems
+
+# See Also
+- [`AbstractContinuumTheory`](@ref) - Theory variants
+- Field types: src/fields/api.jl (Displacement, Temperature)
+- Physics coupling: src/physics/api.jl (AbstractPhysics)
+- Assembly: src/assembly/continuum_*.jl
+"""
+struct ContinuumFormulation{Theory<:AbstractContinuumTheory} <: AbstractFormulation end