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Add 2D membrane elasticity for interpolated flex shell mode

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When elastic2d="stretch" is set on an interpolated flexcomp, treat the bounding box boundary as membrane elements rather than volumetric cells. This computes plane-stress stiffness over the boundary faces and updates the runtime force/derivative kernels accordingly.

Interior vertex tracking (moving vertices that follow the deforming shell) is not yet implemented so all mesh vertices need to be on the bounding box surface or the background grid should have no interior nodes (i.e. cellcount should be 1 on at least one axis).

PiperOrigin-RevId: 907654080
Change-Id: I51b90e2f6a1d1b036f9604e42de20e377dc5d3f9
This commit is contained in:
Alessio Quaglino
2026-04-29 10:16:38 -07:00
committed by Copybara-Service
parent 517c113656
commit 9c6a4f76eb
15 changed files with 1378 additions and 349 deletions
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src/user/user_flexcomp.cc
+36 -16
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@@ -884,27 +884,47 @@ bool mjCFlexcomp::Make(mjsBody* body, char* error, int error_sz, const mjVFS* vf
pe->active = true;
mjs_setString(pe->name1, name.c_str());
} else if (equality == 3) {
// create one strain constraint per cell, storing cell index in eq_data
// create one strain constraint per finite element, storing element index
flex->has_strain_eq = true;
int cell_cx = flex->spec.cellcount[0];
int cell_cy = flex->spec.cellcount[1];
int cell_cz = flex->spec.cellcount[2];
for (int ci = 0; ci < cell_cx; ci++) {
for (int cj = 0; cj < cell_cy; cj++) {
for (int ck = 0; ck < cell_cz; ck++) {
// skip empty cells
if (!flex->cell_empty.empty() &&
flex->cell_empty[ci * cell_cy * cell_cz + cj * cell_cz + ck]) {
continue;
bool shell = (doftype == mjFCOMPDOF_TRILINEAR ||
doftype == mjFCOMPDOF_QUADRATIC) &&
flex->spec.elastic2d;
if (shell) {
// shell mode: one constraint per boundary face element
int nelem_fe = 2*(cell_cy*cell_cz + cell_cx*cell_cz + cell_cx*cell_cy);
for (int fe = 0; fe < nelem_fe; fe++) {
mjsEquality* pe = mjs_addEquality(&model->spec, &def.spec);
mjs_setDefault(pe->element, &model->Default()->spec);
pe->type = mjEQ_FLEXSTRAIN;
pe->active = true;
mjs_setString(pe->name1, name.c_str());
pe->data[0] = fe;
pe->data[1] = -1; // sentinel: shell mode
pe->data[2] = -1;
}
} else {
// volume mode: one constraint per 3D cell
for (int ci = 0; ci < cell_cx; ci++) {
for (int cj = 0; cj < cell_cy; cj++) {
for (int ck = 0; ck < cell_cz; ck++) {
// skip empty cells
if (!flex->cell_empty.empty() &&
flex->cell_empty[ci * cell_cy * cell_cz + cj * cell_cz + ck]) {
continue;
}
mjsEquality* pe = mjs_addEquality(&model->spec, &def.spec);
mjs_setDefault(pe->element, &model->Default()->spec);
pe->type = mjEQ_FLEXSTRAIN;
pe->active = true;
mjs_setString(pe->name1, name.c_str());
pe->data[0] = ci;
pe->data[1] = cj;
pe->data[2] = ck;
}
mjsEquality* pe = mjs_addEquality(&model->spec, &def.spec);
mjs_setDefault(pe->element, &model->Default()->spec);
pe->type = mjEQ_FLEXSTRAIN;
pe->active = true;
mjs_setString(pe->name1, name.c_str());
pe->data[0] = ci;
pe->data[1] = cj;
pe->data[2] = ck;
}
}
}