Data from: Protocol dependence and state variables in the force-moment ensemble

Stress-based ensembles incorporating temperature-like variables have been
proposed as a route to an equation of state for granular materials. To
test the efficacy of this approach, we perform experiments on a
two-dimensional photoelastic granular system under three loading
conditions: uniaxial compression, biaxial compression, and simple shear.
From the interparticle forces, we find that the distributions of the
normal component of the coarse-grained force-moment tensor are
exponential-tailed, while the deviatoric component is
Gaussian-distributed. This implies that the correct stress-based
statistical mechanics conserves both the force-moment tensor and the
Maxwell-Cremona force-tiling area. As such, two variables of state arise:
the tensorial angoricity and a new temperature-like quantity associated
with the force-tile area which we name keramicity. Each quantity is
observed to be inversely proportional to the global confining pressure;
however only keramicity exhibits the protocol-independence expected of a
state variable, while tensorial angoricity behaves as a variable of
process.