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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_model.cc
+44 -16
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@@ -2191,12 +2191,20 @@ void mjCModel::SetSizes() {
nflexevpair += (int)flexes_[i]->evpair.size()/2;
nflextexcoord += (flexes_[i]->HasTexcoord() ? flexes_[i]->get_texcoord().size()/2 : 0);
if (flexes_[i]->spec.order != 0) {
int npc = (int)pow(flexes_[i]->spec.order + 1, 3);
int ndof_cell = 3 * npc;
int ncells = flexes_[i]->spec.cellcount[0] *
flexes_[i]->spec.cellcount[1] *
flexes_[i]->spec.cellcount[2];
extra_stiffness_size += ncells * ndof_cell * ndof_cell;
int cx = flexes_[i]->spec.cellcount[0];
int cy = flexes_[i]->spec.cellcount[1];
int cz = flexes_[i]->spec.cellcount[2];
bool shell = (flexes_[i]->elastic2d != 0);
int npe, nelem;
if (shell) {
npe = (int)pow(flexes_[i]->spec.order + 1, 2);
nelem = 2*(cy*cz + cx*cz + cx*cy);
} else {
npe = (int)pow(flexes_[i]->spec.order + 1, 3);
nelem = cx * cy * cz;
}
int ndof_elem = 3 * npe;
extra_stiffness_size += nelem * ndof_elem * ndof_elem;
}
if (flexes_[i]->interpolated || flexes_[i]->rigid) {
continue;
@@ -3476,10 +3484,20 @@ void mjCModel::CopyObjects(mjModel* m) {
m->flex_stiffnessadr[i] = 21 * elem_adr;
} else {
m->flex_stiffnessadr[i] = current_extra_stiffness_adr;
int npc = (int)pow(pfl->spec.order + 1, 3);
int ndof_cell = 3 * npc;
int ncells = pfl->spec.cellcount[0] * pfl->spec.cellcount[1] * pfl->spec.cellcount[2];
current_extra_stiffness_adr += ncells * ndof_cell * ndof_cell;
int pcx = pfl->spec.cellcount[0];
int pcy = pfl->spec.cellcount[1];
int pcz = pfl->spec.cellcount[2];
bool shell = (pfl->elastic2d != 0);
int npe, nelem;
if (shell) {
npe = (int)pow(pfl->spec.order + 1, 2);
nelem = 2*(pcy*pcz + pcx*pcz + pcx*pcy);
} else {
npe = (int)pow(pfl->spec.order + 1, 3);
nelem = pcx * pcy * pcz;
}
int ndof_elem = 3 * npe;
current_extra_stiffness_adr += nelem * ndof_elem * ndof_elem;
}
if (!pfl->stiffness.empty()) {
@@ -3490,10 +3508,20 @@ void mjCModel::CopyObjects(mjModel* m) {
if (pfl->spec.order == 0) {
stiff_size = 21 * pfl->nelem;
} else {
int npc = (int)pow(pfl->spec.order + 1, 3);
int ndof_cell = 3 * npc;
int ncells = pfl->spec.cellcount[0] * pfl->spec.cellcount[1] * pfl->spec.cellcount[2];
stiff_size = ncells * ndof_cell * ndof_cell;
int scx = pfl->spec.cellcount[0];
int scy = pfl->spec.cellcount[1];
int scz = pfl->spec.cellcount[2];
bool shell = (pfl->elastic2d != 0);
int npe, sncells;
if (shell) {
npe = (int)pow(pfl->spec.order + 1, 2);
sncells = 2*(scy*scz + scx*scz + scx*scy);
} else {
npe = (int)pow(pfl->spec.order + 1, 3);
sncells = scx * scy * scz;
}
int ndof_elem = 3 * npe;
stiff_size = sncells * ndof_elem * ndof_elem;
}
mjuu_zerovec(m->flex_stiffness + m->flex_stiffnessadr[i], stiff_size);
}
@@ -3629,8 +3657,8 @@ void mjCModel::CopyObjects(mjModel* m) {
memcpy(m->flex_nodebodyid + node_adr, pfl->nodebodyid.data(), pfl->nnode*sizeof(int));
}
// set interpolation type, only two types for now
m->flex_interp[i] = pfl->spec.order;
// set interpolation type: positive = volumetric, negative = shell mode
m->flex_interp[i] = pfl->spec.elastic2d ? -pfl->spec.order : pfl->spec.order;
// set cell count for multi-cell finite cell method
m->flex_cellnum[3*i+0] = pfl->spec.cellcount[0];