TET10 — prescribed uniform strain on a 10-node tet

Single TET10 (10-node quadratic tet) driven by a fully-prescribed displacement field that matches a uniform strain state. Under all-Dirichlet BCs the solve has a unique solution equal to the prescribed field, so the recovered strain tensor matches the analytical one to machine precision. Useful as a self-contained smoke test that TET10 + femorph_solver.Model.eel() are in lockstep.

from __future__ import annotations

import numpy as np
import pyvista as pv
from vtkmodules.util.vtkConstants import VTK_QUADRATIC_TETRA

import femorph_solver
from femorph_solver import ELEMENTS

Reference 10-node unit tet

Corners at (0,0,0), (1,0,0), (0,1,0), (0,0,1); mid-edge nodes in VTK_QUADRATIC_TETRA order halfway along each edge.

E = 2.1e11  # Pa
NU = 0.30
EPS_XX = 5.0e-4
EPS_YY = -NU * EPS_XX  # Poisson contraction

I = np.array([0.0, 0.0, 0.0])
J = np.array([1.0, 0.0, 0.0])
K = np.array([0.0, 1.0, 0.0])
L = np.array([0.0, 0.0, 1.0])
coords = np.array(
    [
        I,
        J,
        K,
        L,
        0.5 * (I + J),  # 5
        0.5 * (J + K),  # 6
        0.5 * (K + I),  # 7
        0.5 * (I + L),  # 8
        0.5 * (J + L),  # 9
        0.5 * (K + L),  # 10
    ],
    dtype=np.float64,
)

Build the model + prescribed displacement

Apply u = (εxx·x, εyy·y, εyy·z) at every node.

cells = np.concatenate([[10], np.arange(10, dtype=np.int64)])
cell_types = np.array([VTK_QUADRATIC_TETRA], dtype=np.uint8)
grid = pv.UnstructuredGrid(cells, cell_types, coords)

m = femorph_solver.Model.from_grid(grid)
m.assign(ELEMENTS.TET10, material={"EX": E, "PRXY": NU})

for i, (x, y, z) in enumerate(coords, start=1):
    m.fix(nodes=[i], dof="UX", value=EPS_XX * x)
    m.fix(nodes=[i], dof="UY", value=EPS_YY * y)
    m.fix(nodes=[i], dof="UZ", value=EPS_YY * z)

Solve + verify strain recovery

res = m.solve()
eps = m.eel(res.displacement)

print(f"εxx prescribed = {EPS_XX:.3e}  |  recovered (mean) = {eps[:, 0].mean():.3e}")
print(f"εyy prescribed = {EPS_YY:.3e}  |  recovered (mean) = {eps[:, 1].mean():.3e}")
np.testing.assert_allclose(eps[:, 0], EPS_XX, rtol=1e-10)
np.testing.assert_allclose(eps[:, 1], EPS_YY, rtol=1e-10)
np.testing.assert_allclose(eps[:, 2], EPS_YY, rtol=1e-10)

εxx prescribed = 5.000e-04  |  recovered (mean) = 5.000e-04
εyy prescribed = -1.500e-04  |  recovered (mean) = -1.500e-04

Render the deformed tet coloured by εxx

grid = femorph_solver.io.static_result_to_grid(m, res)
node_nums = m.node_numbers
node_to_idx = {int(nn): i for i, nn in enumerate(node_nums)}
grid.point_data["eps_xx"] = np.array(
    [eps[node_to_idx[int(nn)], 0] for nn in grid.point_data["ansys_node_num"]]
)

plotter = pv.Plotter(off_screen=True)
plotter.add_mesh(grid, style="wireframe", color="gray", line_width=1, opacity=0.4)
plotter.add_mesh(
    grid.warp_by_vector("displacement", factor=200.0),
    scalars="eps_xx",
    show_edges=True,
    cmap="viridis",
    scalar_bar_args={"title": "εxx"},
)
plotter.add_axes()
plotter.show()