diff --git a/src/physics/elasticity.jl b/src/physics/elasticity.jl
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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
-
-"""
-    ElasticityPhysics <: AbstractPhysics
-
-Solid mechanics with geometric and material nonlinearity.
-
-# Governing Equation
-
-Strong form:
-```
-ρ₀ ∂²u/∂t² = ∇⋅σ + b₀   in Ω
-u = ū                     on Γᵤ
-σ⋅n = t̄                   on Γₜ
-```
-
-Weak form: Find u ∈ U such that ∀v ∈ V:
-```
-∫ ρ₀ ∂²u/∂t²⋅v dV + ∫ σ:∇ˢv dV = ∫ b₀⋅v dV + ∫ t̄⋅v dA
-```
-
-where:
-- u = displacement field
-- σ = stress tensor (from material model)
-- b₀ = body force (per unit undeformed volume)
-- t̄ = traction on boundary
-
-# Formulations
-
-Three geometric configurations:
-
-- `:plane_stress` - 2D, σ₃₃ = 0 (thin structures)
-- `:plane_strain` - 2D, ε₃₃ = 0 (long structures)
-- `:continuum` - 3D general
-
-# Material Nonlinearity
-
-Material model computes: (ε, state_old, Δt) → (σ, 𝔻, state_new)
-
-See `docs/book/material_modeling.md` for details.
-
-# Geometric Nonlinearity
-
-When `finite_strain = true`:
-- Use Green-Lagrange strain: E = ½(∇u + ∇uᵀ + ∇uᵀ∇u)
-- Update configuration each step
-- Geometric stiffness from stress state
-
-# Field Storage
-
-Two types of field storage:
-
-1. **Converged fields** (`store_fields`) - Always computed after convergence
-   - Used for postprocessing, visualization
-   - Efficient computation on GPU then transfer to host
-   - Examples: `:stress`, `:strain`, `:plastic_strain`
-
-2. **Iteration fields** (`store_iteration_fields`) - For debugging only
-   - Stored DURING Newton iterations (before convergence)
-   - Inefficient (lots of data), use sparingly
-   - Examples: `:residual_norm`, `:trial_stress`
-
-# GPU Design
-
-All operations GPU-compatible:
-- Type-stable (no Dict lookups)
-- Zero allocation in hot paths
-- Kernel-friendly (small element loops)
-- Minimal host-device transfers
-
-Workflow:
-```
-1. Transfer geometry + BCs to GPU
-2. Run Newton iterations ON GPU
-3. After convergence: compute postprocessing fields
-4. Transfer results to host
-```
-
-# Example
-
-```julia
-# Linear elasticity
-physics = ElasticityPhysics(
-    formulation = :continuum,
-    finite_strain = false,
-    geometric_stiffness = false,
-    store_fields = [:stress, :strain]
-)
-
-# Finite strain plasticity with debugging
-physics = ElasticityPhysics(
-    formulation = :continuum,
-    finite_strain = true,
-    geometric_stiffness = true,
-    store_fields = [:stress, :strain, :plastic_strain],
-    store_iteration_fields = [:residual_norm]  # For debugging convergence
-)
-```
-
-# References
-
-- Bathe, "Finite Element Procedures"
-- Belytschko et al., "Nonlinear Finite Elements"
-- Simo & Hughes, "Computational Inelasticity"
-"""
-struct ElasticityPhysics <: AbstractPhysics
-    """Geometric formulation: `:plane_stress`, `:plane_strain`, `:continuum`"""
-    formulation::Symbol
-
-    """Use finite strain kinematics (Green-Lagrange strain)"""
-    finite_strain::Bool
-
-    """Include geometric stiffness (σ-dependent) for buckling analysis"""
-    geometric_stiffness::Bool
-
-    """Fields to store after convergence (postprocessing)"""
-    store_fields::Vector{Symbol}
-
-    """Fields to store during iterations (debugging only, inefficient!)"""
-    store_iteration_fields::Vector{Symbol}
-
-    # Inner constructor with validation
-    function ElasticityPhysics(;
-        formulation::Symbol=:continuum,
-        finite_strain::Bool=false,
-        geometric_stiffness::Bool=false,
-        store_fields::Vector{Symbol}=Symbol[],
-        store_iteration_fields::Vector{Symbol}=Symbol[]
-    )
-        # Validate formulation
-        if !(formulation in [:plane_stress, :plane_strain, :continuum])
-            error("Invalid formulation: $formulation. Must be :plane_stress, :plane_strain, or :continuum")
-        end
-
-        # Geometric stiffness only makes sense with finite strain
-        if geometric_stiffness && !finite_strain
-            @warn "geometric_stiffness=true but finite_strain=false. Geometric stiffness typically used with finite strain."
-        end
-
-        new(formulation, finite_strain, geometric_stiffness, store_fields, store_iteration_fields)
-    end
-end
-
-# Convenience constructor for common case
-function ElasticityPhysics()
-    return ElasticityPhysics(formulation=:continuum, finite_strain=false, geometric_stiffness=false)
-end
-
-# Interface implementations
-function get_unknown_field_name(::ElasticityPhysics)
-    return "displacement"
-end
-
-function get_formulation_type(::ElasticityPhysics)
-    return :incremental
-end
-
-function get_unknown_field_dimension(physics::ElasticityPhysics)
-    if physics.formulation in [:plane_stress, :plane_strain]
-        return 2  # 2D problem
-    else
-        return 3  # 3D problem
-    end
-end
-
-"""
-    should_store_field(physics::ElasticityPhysics, field::Symbol, converged::Bool) -> Bool
-
-Check if a field should be stored at this point.
-
-# Arguments
-- `physics`: The physics object
-- `field`: Field name to check
-- `converged`: Whether Newton iteration has converged
-
-# Returns
-- `true` if field should be stored, `false` otherwise
-
-# Logic
-
-- **After convergence** (`converged=true`): Store if in `store_fields`
-- **During iterations** (`converged=false`): Store if in `store_iteration_fields`
-
-# Example
-
-```julia
-physics = ElasticityPhysics(
-    store_fields = [:stress],
-    store_iteration_fields = [:residual_norm]
-)
-
-should_store_field(physics, :stress, true)          # true (converged)
-should_store_field(physics, :stress, false)         # false (not converged yet)
-should_store_field(physics, :residual_norm, false)  # true (debugging)
-should_store_field(physics, :residual_norm, true)   # false (only during iterations)
-```
-"""
-function should_store_field(physics::ElasticityPhysics, field::Symbol, converged::Bool)
-    if converged
-        return field in physics.store_fields
-    else
-        return field in physics.store_iteration_fields
-    end
-end