330 : Hyperelasticity

(source code)

This examples computes the solution of a nonlinear elasticity problem for hyperelastic media via minimisation of the (neo-Hookian) energy functional

\[\begin{aligned} W(u, F(\mathbf{u})) := \int_\Omega \frac{\mu}{2} (F:F - 3 - 2\log(\mathrm{det}(F))) + \frac{\lambda}{2} \log(\mathrm{det}(F))^2 - B \cdot \mathbf{u} \textit{dx} - \int_{\partial \Omega} T \cdot \mathbf{u} \textit{ds} \end{aligned}\]

where $F(\mathbf{u}) := I + \nabla u$ is the deformation gradient and $\mu$ and $\lambda$ are the Lame parameters. The energy is differentiated twice by automatic differentiation to setup a Newton scheme for a Lagrangian finite element approximation of $\mathbf{u}$, once in the code below to define the kernel for the NonlinearOperator and this kernel is differentiated again in the assembly of the Newton scheme for the nonlinear operator.

The deformed unit cube and the displacement for the default parameters and inhomogeneous boundary conditions as defined in the code looks like this:

module Example330_HyperElasticity

using ExtendableFEM
using LinearAlgebra
using DiffResults
using ForwardDiff
using SimplexGridFactory
using TetGen

# inhomogeneous boundary conditions for bregion 1
function bnd_1!(result, qpinfo)
    x, y, z = qpinfo.x[1], qpinfo.x[2], qpinfo.x[3]
    angle = pi / 3
    result[1] = 0.0
    result[2] = (0.5 + (y - 0.5) * cos(angle) - (z - 0.5) * sin(angle) - y) / 2.0
    return result[3] = (0.5 + (y - 0.5) * sin(angle) + (z - 0.5) * cos(angle) - x) / 2.0
end

# kernel for body and traction forces
function apply_force!(result, qpinfo)
    return result .= qpinfo.params[1]
end

# energy functional (only nonlinear part, without exterior forces)
function W!(result, F, qpinfo)
    F[1] += 1
    F[5] += 1
    F[9] += 1
    μ, λ = qpinfo.params[1], qpinfo.params[2]
    detF = -(F[3] * (F[5] * F[7] - F[4] * F[8]) + F[2] * ((-F[6]) * F[7] + F[4] * F[9]) + F[1] * (F[6] * F[8] - F[5] * F[9]))
    result[1] = μ / 2 * (dot(F, F) - 3 - 2 * log(detF)) + λ / 2 * (log(detF))^2
    return nothing
end

# derivative of energy functional (by ForwardDiff)
function nonlinkernel_DW!()
    Dresult = nothing
    cfg = nothing
    result_dummy = nothing
    W(qpinfo) = (a, b) -> W!(a, b, qpinfo)

    return function closure(result, input, qpinfo)
        if Dresult === nothing
            # first initialization of DResult when type of input = F is known
            result_dummy = zeros(eltype(input), 1)
            Dresult = DiffResults.JacobianResult(result_dummy, input)
            cfg = ForwardDiff.JacobianConfig(W(qpinfo), result_dummy, input, ForwardDiff.Chunk{length(input)}())
        end
        Dresult = ForwardDiff.vector_mode_jacobian!(Dresult, W(qpinfo), result_dummy, input, cfg)
        copyto!(result, DiffResults.jacobian(Dresult))
        return nothing
    end
end


function main(;
        maxvolume = 0.001,  # parameter for grid generator
        E = 10,             # Young modulus
        ν = 0.3,            # Poisson ratio
        order = 3,             # finite element order
        B = [0, -0.5, 0],     # body force
        T = [0.1, 0, 0],      # traction force
        Plotter = nothing,
        kwargs...
    )

    # compute Lame parameters
    μ = E / (2 * (1 + ν))
    λ = E * ν / ((1 + ν) * (1 - 2 * ν))

    # define unknowns
    u = Unknown("u"; name = "displacement")

    # define problem
    PD = ProblemDescription("Hyperelasticity problem")
    assign_unknown!(PD, u)
    assign_operator!(PD, NonlinearOperator(nonlinkernel_DW!(), [grad(u)]; sparse_jacobians = false, params = [μ, λ]))
    assign_operator!(PD, LinearOperator(apply_force!, [id(u)]; params = [B]))
    assign_operator!(PD, LinearOperator(apply_force!, [id(u)]; entities = ON_BFACES, store = true, regions = 1:6, params = [T]))
    assign_operator!(PD, HomogeneousBoundaryData(u; regions = [2]))
    assign_operator!(PD, InterpolateBoundaryData(u, bnd_1!; regions = [1], quadorder = 10))

    # grid
    xgrid = tetrahedralization_of_cube(maxvolume = maxvolume)

    # solve
    FES = FESpace{H1P1{3}}(xgrid)
    sol = solve(PD, FES; maxiterations = 20)

    # displace mesh and plot final result
    displace_mesh!(xgrid, sol[u])
    plt = plot([grid(u), id(u)], sol; Plotter = Plotter, do_vector_plots = false)

    return sol, plt
end


function tetrahedralization_of_cube(; maxvolume = 0.1)
    builder = SimplexGridBuilder(; Generator = TetGen)

    p1 = point!(builder, 0, 0, 0)
    p2 = point!(builder, 1, 0, 0)
    p3 = point!(builder, 1, 1, 0)
    p4 = point!(builder, 0, 1, 0)
    p5 = point!(builder, 0, 0, 1)
    p6 = point!(builder, 1, 0, 1)
    p7 = point!(builder, 1, 1, 1)
    p8 = point!(builder, 0, 1, 1)

    facetregion!(builder, 1)
    facet!(builder, p1, p2, p3, p4)
    facetregion!(builder, 2)
    facet!(builder, p5, p6, p7, p8)
    facetregion!(builder, 3)
    facet!(builder, p1, p2, p6, p5)
    facetregion!(builder, 4)
    facet!(builder, p2, p3, p7, p6)
    facetregion!(builder, 5)
    facet!(builder, p3, p4, p8, p7)
    facetregion!(builder, 6)
    facet!(builder, p4, p1, p5, p8)

    return simplexgrid(builder; maxvolume = maxvolume)
end

end # module

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