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 twodimensional 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 Gaussiandistributed. 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 (α) and a new temperature-like quantity associated with the force-tile area which we name keramicity (κ). Each quantity is observed to be inversely proportional to the global confining pressure; however only κ exhibits the protocol-independence expected of a state variable, whileα behaves as a variable of process.
PACS numbers:arXiv:1802.09641v1 [cond-mat.soft]