Role of contact couples and couple stress in the behaviour of granular media

Abstract
This paper analyses the interest of taking into account contact couples in the granular material description as well as considering the inadequacy of the micropolar description. The study is made on two types of samples : One which takes into account contact couples, and the other which does not. The response of these two media, which are submitted to a biaxial test, is analysed from both the micromechanic and macroscopic viewpoints. A numerical study which is performed on these two samples shows the influence of the presence of couples on the local static variables as well as on the macroscopic behaviour. A statistical homogenization approach is analysed to simulate the effects of couples. Due to the presence of an internal variable, a numerical study proves that this approach is relevant. This internal variable allows the taking into consideration of the influence of contact couples. A first step in the description of couples versus contact orientation is made by the introduction of the standard deviation. Finally, the inadequacy of the micropolar description which takes into account micropolar stresses is pointed out.