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Bending stiffness for curved shells in flex.

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PiperOrigin-RevId: 796361841
Change-Id: I105ea235fe2a8e1bcbde67276d77fa92eed214a1
This commit is contained in:
Alessio Quaglino
2025-08-18 04:12:34 -07:00
committed by Copybara-Service
parent 972ffd7b90
commit b66175eba6
18 changed files with 488 additions and 250 deletions
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test/plugin/elasticity/elasticity_test.cc
-175
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@@ -28,181 +28,6 @@ namespace {
using ElasticityTest = PluginTest;
// -------------------------------- flex ------------------------------------
TEST_F(ElasticityTest, FlexCompatibility) {
static constexpr char flex_xml[] = R"(
<mujoco>
<worldbody>
<body name="parent">
<flexcomp name="soft" type="grid" count="3 3 3"
radius="0.01" dim="3"mass="1">
<pin id="2"/>
<elasticity young="5e4" poisson="0.2"/>
</flexcomp>
</body>
</worldbody>
</mujoco>
)";
char error[1024] = {0};
mjModel* m = LoadModelFromString(flex_xml, error, sizeof(error));
ASSERT_THAT(m, testing::NotNull()) << error;
mjData* d = mj_makeData(m);
mj_deleteData(d);
mj_deleteModel(m);
}
// -------------------------------- shell -----------------------------------
TEST_F(ElasticityTest, ElasticEnergyShell) {
static constexpr char cantilever_xml[] = R"(
<mujoco>
<worldbody>
<flexcomp type="grid" count="8 8 1" spacing="1 1 1"
radius=".025" name="test" dim="2">
<elasticity young="2" poisson="0" thickness="1"/>
</flexcomp>
</worldbody>
</mujoco>
)";
char error[1024] = {0};
mjModel* m = LoadModelFromString(cantilever_xml, error, sizeof(error));
ASSERT_THAT(m, testing::NotNull()) << error;
mjData* d = mj_makeData(m);
mj_kinematics(m, d);
mj_flex(m, d);
// check that a plane is in the kernel of the energy
for (mjtNum scale = 1; scale < 4; scale++) {
for (int e = 0; e < m->flex_edgenum[0]; e++) {
int* edge = m->flex_edge + 2*(m->flex_edgeadr[0] + e);
int* flap = m->flex_edgeflap + 2*(m->flex_edgeadr[0] + e);
int v[4] = {edge[0], edge[1], flap[0], flap[1]};
if (v[3]== -1) {
continue;
}
mjtNum energy = 0;
mjtNum volume = 1./2.;
for (int i = 0; i < 4; i++) {
for (int j = 0; j < 4; j++) {
for (int x = 0; x < 3; x++) {
mjtNum elongation1 = scale * d->flexvert_xpos[3*v[i]+x];
mjtNum elongation2 = scale * d->flexvert_xpos[3*v[j]+x];
energy += m->flex_bending[16*e+4*i+j] * elongation1 * elongation2;
}
}
}
EXPECT_NEAR(
4*energy/volume, 0, std::numeric_limits<float>::epsilon());
}
}
mj_deleteData(d);
mj_deleteModel(m);
}
// -------------------------------- membrane -----------------------------------
TEST_F(PluginTest, ElasticEnergyMembrane) {
static constexpr char cantilever_xml[] = R"(
<mujoco>
<worldbody>
<flexcomp type="grid" count="8 8 1" spacing="1 1 1"
radius=".025" name="test" dim="2">
<elasticity young="2" poisson="0" thickness="1" elastic2d="stretch"/>
<edge equality="false"/>
</flexcomp>
</worldbody>
</mujoco>
)";
char error[1024] = {0};
mjModel* m = LoadModelFromString(cantilever_xml, error, sizeof(error));
ASSERT_THAT(m, testing::NotNull()) << error;
mjData* d = mj_makeData(m);
mj_kinematics(m, d);
mj_flex(m, d);
mjtNum* metric = m->flex_stiffness + 21 * m->flex_elemadr[0];
// check that if the entire geometry is rescaled by a factor "scale", then
// trace(strain^2) = 2*scale^2
for (mjtNum scale = 1; scale < 4; scale++) {
for (int t = 0; t < m->flex_elemnum[0]; t++) {
mjtNum energy = 0;
mjtNum volume = 1./2.;
int idx = 0;
for (int e1 = 0; e1 < 3; e1++) {
for (int e2 = e1; e2 < 3; e2++) {
int idx1 = m->flex_elemedge[3*t+e1 + m->flex_elemedgeadr[0]];
int idx2 = m->flex_elemedge[3*t+e2 + m->flex_elemedgeadr[0]];
mjtNum elong1 =
scale * m->flexedge_length0[idx1] * m->flexedge_length0[idx1];
mjtNum elong2 =
scale * m->flexedge_length0[idx2] * m->flexedge_length0[idx2];
energy += metric[21*t+idx++] * elong1 * elong2 * (e1 == e2 ? 1. : 2.);
}
}
EXPECT_NEAR(
4*energy/volume, 2*scale*scale, std::numeric_limits<float>::epsilon());
}
}
mj_deleteData(d);
mj_deleteModel(m);
}
// -------------------------------- solid -----------------------------------
TEST_F(ElasticityTest, ElasticEnergySolid) {
static constexpr char cantilever_xml[] = R"(
<mujoco>
<worldbody>
<flexcomp type="grid" count="8 8 8" spacing="1 1 1"
radius=".025" name="test" dim="3">
<elasticity young="2" poisson="0"/>
<edge equality="false"/>
</flexcomp>
</worldbody>
</mujoco>
)";
char error[1024] = {0};
mjModel* m = LoadModelFromString(cantilever_xml, error, sizeof(error));
ASSERT_THAT(m, testing::NotNull()) << error;
mjData* d = mj_makeData(m);
mj_kinematics(m, d);
mj_flex(m, d);
mjtNum* metric = m->flex_stiffness + 21 * m->flex_elemadr[0];
// check that if the entire geometry is rescaled by a factor "scale", then
// trace(strain^2) = 3*scale^2
for (mjtNum scale = 1; scale < 4; scale++) {
for (int t = 0; t < m->flex_elemnum[0]; t++) {
mjtNum energy = 0;
mjtNum volume = 1./6.;
int idx = 0;
for (int e1 = 0; e1 < 6; e1++) {
for (int e2 = e1; e2 < 6; e2++) {
int idx1 = m->flex_elemedge[6*t+e1 + m->flex_elemedgeadr[0]];
int idx2 = m->flex_elemedge[6*t+e2 + m->flex_elemedgeadr[0]];
mjtNum elong1 =
scale * m->flexedge_length0[idx1] * m->flexedge_length0[idx1];
mjtNum elong2 =
scale * m->flexedge_length0[idx2] * m->flexedge_length0[idx2];
energy += metric[21*t+idx++] * elong1 * elong2 * (e1 == e2 ? 1. : 2.);
}
}
EXPECT_NEAR(
energy/volume, 3*scale*scale, std::numeric_limits<float>::epsilon());
}
}
mj_deleteData(d);
mj_deleteModel(m);
}
// -------------------------------- cable -----------------------------------
TEST_F(ElasticityTest, CantileverIntoCircle) {