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A woven fabric card from KES data drapes wrong on the VBD cloth route

What is wrong

  • A cloth card built from a woven fabric's published KES-F data (areal mass, thickness, bending rigidity B) drapes differently from the same fabric on a Cusick drape tester, even when the solve is converged.
  • With the shipped KES cards' convention (cloth_cotton_plain, cloth_polyester_woven: youngs back-calculated from B at the KES thickness, no elastic.cloth, so the router sends it to CLOTH_VBD), a light polyester twill drapes too stiff (+0.08 to +0.12) and a heavy polyester/viscose plain weave hangs far too low (-0.28). A wool plain weave is within 0.05 on four of five meshes and misses on the fifth.
  • cloth_accuracy="super" does not fix it. The DC is converged in iterations, so the gap is in the model, not the solve: membrane locking, the card mapping and the 2 mm contact thickness (Why, below).
  • The opt-in orthotropic membrane (Workaround) brings the twill and the wool within 0.035 on a warp / weft grid mesh. The heavy plain weave still hangs 0.16 too low, and on a mesh that does not follow warp and weft every fabric drapes far too stiff.

Number

D03M (bench/scenarios/fabric_drape*.py, pack bench/scenarios/packs/fabric_drape_petrak2021.json; its L4 result rows are not shipped) and the R6 law on the same data (alloy.scene.twins.drape_route, dataset bench/rigs/R6.cloth_drape/petrak2021-v1). The data is Petrak, Mahnić Naglić, Rogale and Geršak 2021 (Materials 14:6259): 30 cm specimens clamped between 18 cm discs, KES-FB inputs, each fabric's measured DC the mean of 5 specimens. Alloy: the production route, one L4, 8 worlds per fabric, 8 mm mesh, scored over the last 1 s of 4.5 s.

fabric measured DC super (60 it), mesh seed 0 240 it R6 law, super, mesh seed 7
F1 polyester twill 3/1, 135 g/m², 0.41 mm 0.222 0.302 (+0.080) 0.315 (+0.093) 0.342 (+0.120)
F4 wool plain, 162 g/m², 0.44 mm 0.319 0.306 (-0.013) 0.319 (-0.000) 0.381 (+0.062)
F9 polyester/viscose plain, 367 g/m², 2.42 mm 0.647 0.365 (-0.282) 0.368 (-0.279) 0.365 (-0.282)

The mesh moves these by up to about 0.08. Super on a square-grid disc: - 8 mm: F1 0.315, F4 0.367, F9 0.406; - 5 mm: F1 0.300, F4 0.340, F9 0.429. F1 and F9 miss on every mesh. F4's pass is not robust: on the R6 mesher's seed 7 the fold count, and with it DC, changes (2-5x the world spread of seed 0).

The opt-in orthotropic membrane (KES EM warp / weft stretch, KES G shear, KES B per hinge direction; super, 8 worlds, 2026-11-08):

fabric warp / weft grid disc, 8 mm, 60 it same, 160 it same, contact thickness T0 R6 mesh (jittered hex), seed 7, T0
F1 0.254 (+0.032) 0.257 (+0.035) 0.252 (+0.030) 0.500 (+0.278)
F4 0.314 (-0.005) 0.313 (-0.006) 0.294 (-0.025) 0.395 (+0.076)
F9 0.487 (-0.160) 0.494 (-0.153) 0.487 (-0.160) 0.499 (-0.148)

The same isotropic card convention on that grid disc: 0.315 / 0.367 / 0.406. DC is converged in iterations there. Measured at the end of the drape (8 worlds), the yarns stretch little: the largest yarn strain |w| - 1 is 0.6 % at 60 iterations (99th percentile 0.08 %) and 0.03 % at 160. The largest edge strain, 8 % at 60 iterations and 4.3 % at 160, is the grid's cell diagonals as the sheet shears. On F1, the lightest, the local shear is not converged at 60 iterations: its 99th-percentile shear angle is 9.5°, against 2.2° at 160 (F4 and F9 about 2° at both). sim.health() does not flag shear, so raise the iterations before reading a woven sheet's local shear.

The same fabrics with other isotropic membranes (super, 60 iterations; bending and mass unchanged):

fabric E h from KES shear G (E h = 2 (1 + nu) G) E h from KES tensile (EM secant)
F1 0.217 (-0.005) 0.730 (+0.508), 3 nodes
F4 0.251 (-0.068) 0.724 (+0.405), 4 nodes
F9 0.440 (-0.207) 0.590 (-0.057)

Why

Measured (2026-11-08, L4, super, R6 seed-7 mesh, 8 worlds): the drape on CLOTH_VBD is set by the membrane's in-plane stiffness through locking, more than by the bending. - Under the drape's own loads a fabric stretches about 0.02 %, so a converged sheet would sit at its inextensible limit and its DC would hardly move with E h. On the 8 mm mesh it moves a lot. With the bending fixed, scaling the card's E h by 0.3 / 1 / 3 / 10 gives F1 0.225 / 0.347 / 0.564 / 0.714 (F4 0.269 / 0.370 / 0.508 / 0.720). At E h x10 a 6 mm mesh drapes lower than the 8 mm one (F1 0.587 against 0.714). - That is membrane locking: linear triangles cannot fold into a drape's doubly curved shape without stretching, so the stiffer the membrane, the stiffer the drape, and the mesh changes the answer. Scaling the bending by 0.72 moves DC by only 0.04-0.05. - The card mapping decides where on that curve a fabric sits. The convention above (E from B at the KES thickness) makes the membrane's shear stiffness 7x KES G for F1, and its stretch stiffness 17-42x below KES EM (F1). A woven fabric is stiff along its yarns and soft in shear between them, which no single isotropic E h matches. - The sheet's contact thickness matters too. The rig floors it at 2 mm (5x F1's 0.405 mm), and VBD's self-contact then keeps the folds from closing: 9 folds instead of 12. With only the thickness changed to T0 (E h kept, 16 worlds), F1 drapes 0.280 +- 0.006 instead of 0.355 +- 0.014, and F4 0.298 +- 0.003 instead of 0.365 +- 0.011. - The clamp is not the cause (a pinned rim and an unclamped specimen drape within the world spread), nor is the iteration budget (60 and 240 iterations agree), nor settling (9 s drapes as 4.5 s). - The orthotropic membrane does not lock on a grid whose edges run along warp and weft. Each triangle's two yarn conditions are then its two grid-edge lengths, about two per vertex against three degrees of freedom, so the sheet can shear into a drape like a real woven fabric (a Chebyshev net). On an irregular mesh each triangle adds two independent conditions, about four per vertex, and the stiff yarns lock harder than the isotropic membrane does. - F9 (2.42 mm, 367 g/m²) misses with every membrane. Its inputs match the paper's table. The paper's FAST bending (a cantilever, low curvature) is 2.3-2.8x its KES B for every fabric. As a diagnostic (not the pre-registered mapping), FAST bending on the orthotropic membrane drapes F9 within 0.02 (0.630) but F1 and F4 too stiff (+0.105, +0.079). So neither bending source fits all three: F9 bends stiffer at drape curvatures than its KES B, F1 and F4 do not (bending hysteresis, which Petrak does not report, is one candidate).

Workaround

  • For the drape shape of a light, limp woven fabric, set the card's youngs from KES shear: E = 2 (1 + nu) G / h, with h the sheet thickness you pass to add_cloth. Here it was within 0.07 on F1 and F4.
  • For a heavy fabric whose hang matters, use the tensile stiffness (E = F / (EM h) from KES-FB1). It was within 0.06 on F9.
  • Check one Cusick drape against a measured DC for your fabric before trusting a drape shape.
  • For a woven fabric with KES data, use the opt-in orthotropic membrane on a warp / weft grid:
  • give the material card elastic.cloth (stretch_warp and stretch_weft = KES load / EM strain in N/m, shear = KES G in N/m per radian, bend_warp and bend_weft = KES B in N m);
  • give the object card the route override route: {route: cloth_vbd, reason: ...};
  • use a sheet whose mesh runs along warp (x) and weft (y): add_cloth's grid does. The build warns when fewer than 90 % of the triangles have a warp and a weft edge; the stiff yarns lock on such a mesh. Without the override the card routes to CLOTH_STYLE3D when nothing else moves, and is refused at build beside other moving things (the error names the override). Youngs randomization of such a card is refused.
  • An anisotropic card (elastic.cloth) routes to CLOTH_STYLE3D by default, but it is no alternative for these fabrics today. In one world, clamped or not, the Style3D sheet stays nearly flat (DC 0.98-0.99): its solve locks when the KES stretch stiffness outweighs the inertia per step (cloth-style3d-stretch-lock.md).

Planned fix

WP2 (gap #3, with DC5; WP11 reviews). The law is R6 / D03M: each fabric's DC within 0.05 of its measured value at convergence. Next: - F9: its bending at drape curvatures (FAST fits F9 but not F1 / F4; a KES 2HB for it would test the hysteretic hinge, elastic.bending_hysteresis). - Irregular meshes: map a woven sheet onto a warp / weft grid at build, or a membrane element that does not lock. - R6 at the fabrics' real thickness (the 2 mm floor), then the default route for woven cards (Style3D -> the orthotropic membrane) once the law passes, and a public add_cloth switch for it.