Similar to the Pmin EOS, except that the effective minimum pressure
falls as the material distends.
At solid density, pmin = pmin0.
At lower densities, pmin = pmin0 (rho/rhoref)eta.
type name
damage
parameters
pmin minimum pressure (real; usually negative to support some tensile stress)
rhoref reference mass density to define solid
eta exponent in weakening law
eos a polymorphic equation of state