The core's test material: a compressible neo-Hookean "jelly", τ = μ(F·Fᵀ − I) + λ·ln(J)·I, plus a
Kelvin–Voigt viscosity τ += η·J·(C + Cᵀ) so it wobbles and settles like gelatine instead of bouncing like
a rubber ball. No plasticity. Sag under its own weight is about g·L/c² for a block of height L, so the
default (c ≈ 11.6 m/s) holds its shape at a few metres.
The core's test material: a compressible neo-Hookean "jelly", τ = μ(F·Fᵀ − I) + λ·ln(J)·I, plus a Kelvin–Voigt viscosity τ += η·J·(C + Cᵀ) so it wobbles and settles like gelatine instead of bouncing like a rubber ball. No plasticity. Sag under its own weight is about g·L/c² for a block of height L, so the default (c ≈ 11.6 m/s) holds its shape at a few metres.