Fluctuations, Correlation and Representative Elementary Volume (REV) in   Granular Materials
  In general, the mechanics of granular matter is described using continuum mechanics approach; this requires to introduce the concepts of stress and strain, which are averaged quantities, so that this needs also to introduce the notion of representative elementary volume (REV) above which averaged quantities have some physical meaning. As local quantities fluctuate spatially in granular matter; a local measure of stress and strain shall exhibit fluctuations too, whose typical amplitude depends on the sampling size L. This paper discusses this problem and the causes for large scale correlation. The mean stress s applied to a plane surface of size L*L is calculated and its fluctuation amplitude Ds is found when local forces are not correlated; it is found that Ds/s scales as 1/L . It is shown also that large scale fluctuations of stress can always be interpreted as an inhomogeneous stress field and that static equilibrium modifies the mean stress applied to a rod (in 2d), even if it does not perturb the contact force distribution. This last result is compared to experiment, which indicates that the number N of contacts per rod (in 2d) is 2<N<3 . Pacs # : 5.40 ; 45.70 ; 62.20 ; 83.70.Fn 
P. Evesque