Material Models in Metafold for Grasshopper

This page explains the material models available in the Metafold plugin for Grasshopper: what each one is trying to capture physically, what its inputs mean, and what kinds of real-world materials it's a good fit for. Use it as a reference when you're not sure which material block to wire up for a given part.

💡 Tip: You don't need to understand the underlying math to use these — the goal here is just to help you pick the right model and get its inputs in the right ballpark. Some of these also appear as "Metafold DTB" prefixed versions in the DTB simulation guides (e.g. Metafold DTB Mooney-Rivlin Material) — those are the DTB-ready wrappers around the same underlying material concepts described below.

Setting Input Values

Each input on these material components can be filled in two ways:

• A Panel — a yellow text/number box wired into the input, letting you type or edit the value directly and see it update live (this is what's shown wired into most of the components in the screenshots above).
• Right-click the input — right-clicking directly on an input name lets you type a value in on the spot, without needing a separate Panel component — a faster option for quick, one-off values.

A few inputs aren't numbers at all but simple true/false toggles built into the component itself (shown as a small switch in the corner of the component, rather than a wired input) — these are turned on or off directly on the component rather than fed a value from outside.

Explicit heat conduction is turned off in this setup, so Thermal Conductivity (TC) and Specific Heat (SH) values aren't relevant here — you can leave them at any placeholder value without affecting the simulation.

If a material's model is set to Rigid, it behaves as fully rigid regardless of whatever values are given for G and K — those inputs are effectively ignored once Rigid is selected.

Rubbers, Foams & Hyperelastics

These models describe soft, stretchy materials — the stress they carry depends mainly on how far they've been stretched or compressed, not on any permanent damage.

Neo-Hookean (MF Neo-Hook Mat)

The concept: The simplest "stretchy rubber" model. It says a material gets steadily stiffer the more you stretch or squeeze it, in a smooth, predictable way. It doesn't capture the more complex S-curve stiffening that some real rubbers show, but it's a solid, cheap default when you don't have detailed test data for a part.

Inputs:

• G — Shear Modulus: how stiff the material feels when you try to distort its shape
• K — Bulk Modulus: how strongly it resists being compressed in volume
• D — Density
• TC — Thermal Conductivity
• SH — Specific Heat

Good fit for: Generic foams and rubbers where you don't need to match a very specific stress-strain curve — simple EVA foam, basic silicone, everyday rubber gaskets.

Example values:

• Soft foam: G = 150,000 Pa, K = 300,000 Pa, D = 100 kg/m³
• Medium foam: G = 344,500 Pa, K = 644,500 Pa, D = 150 kg/m³
• Firm rubber: G = 800,000 Pa, K = 1,500,000 Pa, D = 300 kg/m³

Mooney-Rivlin (MF Mooney Mat)

The concept: Also a "stretchy rubber" (hyperelastic) model, but with two stiffness numbers instead of one. That extra flexibility lets it match the way real rubbers and foams actually behave — relatively soft at first, then stiffening more sharply as you compress or stretch further.

Inputs:

• C1, C2 — two stiffness constants that together shape the stress-strain curve
• PR — Poisson Ratio: how much the material bulges outward when you squash it (close to 0.5 means it barely changes volume, like rubber)
• D, TC, SH — Density, Thermal Conductivity, Specific Heat

Good fit for: The most common go-to for rubber-like or foam-like parts — midsole foams, rubber outsoles, silicone components.

Example values:

• Soft foam: C1 = 200,000 Pa, C2 = 100,000 Pa, PR = 0.45
• Medium midsole foam: C1 = 658,000 Pa, C2 = 356,000 Pa, PR = 0.48
• Firm outsole rubber: C1 = 900,000 Pa, C2 = 450,000 Pa, PR = 0.49

Maxwell Weichart (MF Maxwell Mat)

The concept: For materials whose stiffness depends on how fast you load them — squeeze quickly and it feels stiff, squeeze slowly and it feels soft, because the material's internal structure has time to relax under slow loads. This is the classic way to represent damping and energy loss. The DTB version's Modes table is a Prony series: each row adds one more relaxation-time/modulus pair (one more Maxwell element) to the chain, letting you layer multiple time-dependent responses together. The simpler MF Maxwell Mat likely exposes just one mode's worth of this by default.

Inputs:

• K — Bulk Modulus
• Gt — a shear-stiffness term
• VM — a relaxation/viscosity parameter controlling how quickly the material "gives" under sustained load
• D, TC, SH — Density, Thermal Conductivity, Specific Heat

Good fit for: Memory foam, viscoelastic gel inserts, damping pads — anything where the speed of impact changes how firm it feels.

Example values:

• Soft memory foam: K = 300,000 Pa, Gt = 100,000 Pa
• Firm damping pad: K = 800,000 Pa, Gt = 300,000 Pa

Viscoelastic Transversely Isotropic (MF Visco Trans-Iso Mat)

The concept: Combines two behaviors at once — time-dependent stiffness (like the Maxwell model) and direction-dependent stiffness, but for materials with just one special direction (like fibers all running one way) rather than three like the Orthotropic model. This model was originally developed for fiber-reinforced soft tissue.

Inputs:

• K — Bulk Modulus (bulk_modulus)
• c1, c2 — Mooney-Rivlin constants for the background matrix material
• c3–c5 — shape the fiber-direction stiffening curve: c3 scales the exponential fiber stress, c4 controls how the fibers "uncrimp" (straighten) before carrying load, c5 is the stiffness of already-straightened fibers
• FS — fiber stretch (fiber_stretch, λ*): the stretch at which fibers finish straightening and start carrying full load
• DoS / FO — the fiber direction vector (direction_of_symm)

• MFS, MMS — failure strains (max_fiber_strain, max_matrix_strain): the strain at which the fiber or matrix is considered to fail, if failure checking (failure_option — a true/false toggle) is switched on
• VS — a six-mode viscoelastic Prony series (y1–y6 relative moduli, t1–t6 relaxation times) describing how the material relaxes over time
• D, TC, SH — Density, Thermal Conductivity, Specific Heat

Good fit for: Fiber-reinforced composites and tissue-like materials (tendons, some fiber-reinforced 3D prints) with one dominant fiber direction.

Example values: The fiber-direction and viscous-relaxation parameters (c1–c5, FS, VS) are specific to the fiber/matrix combination and are usually derived from test data rather than typical published numbers — as a starting point, set D, TC, SH from the base material (e.g. a nylon-fiber composite: D ≈ 1,400 kg/m³) and treat the rest as calibration inputs tuned against a known stress-strain curve.

Metals & Plastics (Elastic-Plastic Behavior)

These models describe materials that behave elastically up to a point, then deform permanently once pushed past their limit — unlike the rubbers and foams above, which spring back to their original shape.

Hypoelastic (MF Hypo Mat)

The concept: A lighter-weight elastic model that builds up stress incrementally, step by step, based on how the material is currently deforming — rather than from a full stretch-based formula like the hyperelastic models above. It's simpler and cheaper to compute, and works well as long as deformations stay relatively small. It has no plasticity or yield behavior at all.

Inputs:

• G — Shear Modulus
• K — Bulk Modulus
• D, TC, SH — Density, Thermal Conductivity, Specific Heat

Good fit for: Metals or plastics undergoing small, mostly-elastic deformation — thin plastic clips, small metal springs — where you don't need the full rubber-like stretch behavior.

Example values (typical published metal properties — verify against a supplier datasheet for critical work):

• Aluminum (6061): G ≈ 26 GPa, K ≈ 69 GPa, D ≈ 2,700 kg/m³
• Titanium (Ti-6Al-4V): G ≈ 44 GPa, K ≈ 110 GPa, D ≈ 4,430 kg/m³

Elastic-Plastic (MF Elastic-Plastic Mat)

The concept: For materials that spring back up to a point, then permanently deform if pushed past their limit — like bending a paperclip too far. This model captures both the initial springy (elastic) stage and the permanent (plastic) deformation stage once the material yields.

Inputs:

• G — Shear Modulus
• K — Bulk Modulus
• YC — Yield Condition: which yield surface the material uses to decide when plastic deformation begins (e.g. von Mises) — a model choice, not a numeric strength value
• SC — Stability Check: an optional check for material/localization instability (e.g. shear banding) — also a model choice rather than a number

• A, B, C, n, m — the Johnson-Cook flow stress parameters: σ = (A + Bεⁿ)(1 + C·ln ε̇)(1 − Tᵐ), where A is the initial yield stress, B and n control strain hardening, C controls strain-rate sensitivity, and m controls thermal softening. It's part of a modular framework with pluggable yield condition, stability check, flow-stress model, and equation-of-state — Johnson-Cook is one of several selectable flow-stress models.
• D, TC, SH — Density, Thermal Conductivity, Specific Heat

Good fit for: Metals (aluminum, steel brackets), rigid plastics that dent or crack under enough force, and structures deliberately designed to deform permanently to absorb impact.

Example values (yield strength varies a lot by alloy/temper — check a datasheet for your exact grade):

• Aluminum 6061-T6: G ≈ 26 GPa, K ≈ 69 GPa, Yield (YC) ≈ 276 MPa
• Mild steel: G ≈ 79 GPa, K ≈ 160 GPa, Yield (YC) ≈ 250 MPa

A, B, C, n, m are the Johnson-Cook constants above — published values exist for common alloys (e.g. Al 6061-T6: A ≈ 324 MPa, B ≈ 114 MPa, n ≈ 0.42, C ≈ 0.002, m ≈ 1.34), though exact figures vary by source and temper, so treat these as a starting point rather than a certified spec.

UCNH (MF UCNH Mat)

The concept: An extended version of the Neo-Hookean model with isotropic hardening plasticity built in — meaning that once the material yields, its yield surface expands uniformly in every loading direction as it strengthens (rather than shifting off-center, which is what kinematic hardening models do). Its volumetric (pressure) response uses the standard compressible Neo-Hookean relation p = ½·K·(J − 1/J), where J is the relative volume change — with an optional simplified power-law equation of state that kicks in under strong tension to avoid instability.

Inputs:

• G — Shear Modulus
• K — Bulk Modulus

• EOS — a true/false toggle (useModifiedEOS) switching between the standard volumetric law above and a simplified power-law version for extreme tension — built into the component, not a numeric input
• UP — initial pressure (initial_pressure): an optional pre-existing stress state in the material, active only if initial-stress is enabled
• YS — Yield Stress (yield_stress): the initial stress level at which plastic deformation begins
• HM — Hardening Modulus (hardening_modulus): how quickly the yield surface expands as the material strengthens
• A — initial equivalent plastic strain (alpha, default 0): lets you start the material already partway hardened, e.g. if it's been pre-strained
• D, TC, SH — Density, Thermal Conductivity, Specific Heat

Good fit for: High-rate impact scenarios — foams or polymers under very fast or high-pressure loading, such as helmet liners or protective padding under impact — rather than everyday slow compression.

Example values: Like the fiber model above, EOS/UP/YS/HM are impact-specific calibration parameters rather than off-the-shelf numbers — start from your base polymer's Neo-Hookean values (G, K, D) and tune the pressure/yield terms against high-rate test data if available.

Rigid (Non-Deforming)

Not a "material" in the deforming sense at all — this is for tools and fixtures in the simulation that stay perfectly stiff.

Rigid Material (Metafold DTB Rigid Material)

The concept: For parts that don't deform at all during the simulation — typically the tool doing the compressing (a piston or a Last) rather than the part being tested. It still needs physical properties since it interacts with the deformable parts, but it won't stretch, squish, or bend no matter how much force is applied.

Inputs:

• Shear Modulus, Bulk Modulus — set very high, since the part shouldn't deform
• Density
• Thermal Conductivity
• Specific Heat

Good fit for: Any simulation stand-in for an essentially rigid tool — a compression platen, a mold plate, a metal punch.

Example values: Since this part shouldn't deform, set Shear Modulus and Bulk Modulus well above the stiffest deformable part in your assembly (e.g. 1×10¹¹ Pa or higher) so it behaves as effectively rigid — Density, Thermal Conductivity, and Specific Heat can just match the real tool material (e.g. steel: D ≈ 7,850 kg/m³).

GPU-Only Material Models

The two models below require a GPU to run — they can only run with the Experimental (GPU) Solver. To enable this check Use Experimental Solver on the MF Simulation Configuration.

Hyperfoam (MF Hyperfoam Mat)

The concept: Purpose-built for foams — materials with a lot of internal air that can collapse and compress a great deal before they start pushing back hard, unlike solid rubbers which barely change volume. This is a proper N-term Ogden hyperfoam model — the same formulation used in Sandia's LAME material library — where the principal stresses follow τₖ = Σᵢ (2μᵢ/αᵢ)·(λₖ^αᵢ − J^(−αᵢβᵢ)), letting you layer multiple Ogden terms for a more accurate soft-then-firm "cushioning" curve.

Inputs:

• T — the set of Ogden terms: each term is a (μ, α, Poisson ratio) triple — μ sets that term's stiffness contribution, α shapes how sharply it stiffens with stretch, and each term can have its own Poisson ratio. A maximum of 3 terms are allowed.

• Gt — the overall shear modulus
• VS — volumetric/compressibility behavior, derived from each term's Poisson ratio
• D, TC, SH — Density, Thermal Conductivity, Specific Heat

Good fit for: Cushioning and impact foams — open- or closed-cell EVA, polyurethane foam, packaging foam.

Example values:

• Soft packaging foam: T = {100,000, 6, 0.06}, Gt = 150,000 Pa
• Firm impact foam: T = {247,500, 8, 0.085}, Gt = 344,500 Pa
• Metafold's built-in default (confirmed from source): μ = 620,000 Pa, α = 4.0, Poisson = 0.05, Density = 130 kg/m³ (single-term Ogden)

Orthotropic (MF Orthotropic Mat)

The concept: A linear elastic model (small-deformation Hooke's law, not a large-stretch hyperelastic model like the others on this page) for materials that are stiffer in some directions than others — think plywood (strong along the grain, weaker across it), woven fabric, or a 3D-printed part that's stronger along its print direction. Instead of one stiffness value, it needs stiffness defined along three directions.

Inputs:

• E1, E2, E3 — stiffness along each of the three principal directions
• nu12, nu13, nu23 — how the material bulges in one direction when compressed in another
• G12, G13, G23 — shear stiffness between each pair of directions
• dA, dB — vectors defining the material's local direction axes (so the model knows which way is "stiff")

• D, TC, SH — Density, Thermal Conductivity, Specific Heat

Good fit for: Woven textiles, plywood or composite panels, FDM 3D-printed parts with a clear grain/layer direction.

Example values: Directional stiffnesses vary a lot by material — as a rough starting point for an FDM 3D print (stiffer along the print direction): E1 ≈ 2 GPa (along layers), E2 ≈ E3 ≈ 1.2 GPa (across layers), best confirmed with a simple tensile test in each direction.