Damas, J. (José)

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    Towards a relevant set of state variables to describe static granular packings
    (The American Physical Society, 2010-11-04) Damas, J. (José); Pugnaloni, L.A. (Luis A.); Zuriguel-Ballaz, I. (Iker); Maza-Ozcoidi, D. (Diego); Sánchez, I. (Iván); Gago, P.A. (Paula A.)
    We analyze, experimentally and numerically, the steady states, obtained by tapping, of a two-dimensional granular layer. Contrary to the usual assumption, we show that the reversible (steady state branch) of the density-acceleration curve is nonmonotonous. Accordingly, steady states with the same mean volume can be reached by tapping the system with very different intensities. Simulations of dissipative frictional disks show that equal volume steady states have different values of the force moment tensor. Additionally, we find that steady states of equal stress can be obtained by changing the duration of the taps; however, these states present distinct mean volumes. These results confirm previous speculations that the volume and the force moment tensor are both needed to describe univocally equilibrium states in static granular assemblies.
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    Master curves for the stress tensor invariants in stationary states of static granular beds. Implications for the thermodynamic phase space
    (2011) Damas, J. (José); Pugnaloni, L.A. (Luis A.); Zuriguel-Ballaz, I. (Iker); Maza-Ozcoidi, D. (Diego)
    We prepare static granular beds under gravity in different stationary states by tapping the system with pulsed excitations of controlled amplitude and duration. The macroscopic state|defined by the ensemble of static configurations explored by the system tap after tap|for a given tap intensity and duration is studied in terms of volume, V, and force moment tensor, &Sgr;. In a previous paper [Pugnaloni et al., Phys. Rev. E 82, 050301(R) (2010)], we reported evidence supporting that such macroscopic states cannot be fully described by using only V or &Sgr;, apart from the number of particles N. In this work, we present an analysis of the fluctuations of these variables that indicates that V and &Sgr; may be sufficient to define the macroscopic states. Moreover, we show that only one of the invariants of &Sgr; is necessary, since each component of &Sgr; falls onto a master curve when plotted as a function of Tr(&Sgr;). This implies that these granular assemblies have a common shape for the stress tensor, even though it does not correspond to the hydrostatic type. Although most results are obtained by molecular dynamics simulations, we present supporting experimental results.