The influence of fluid filtration on the strength of porous fluid-saturated brittle materials

The paper is devoted to the study on how the strength of fluid-saturated permeable brittle materials depends on strain rate. The study has been carried out by means of a numerical simulation using a hybrid cellular automata method and a coupled model, which takes into account the interplay of deformation of solid skeleton, pore pressure change and fluid filtration. It has been found that the influence of pore fluid on the material strength is determined by the competition of fluid pore pressure change (due to the volume deformation of solid skeleton) with filtration. On the basis of a parametric study we obtained the combinations of physical and mechanical characteristics of solid skeleton and fluid as well as of linear dimensions of the samples, which uniquely defines the dependence between the strength of the deformed fluid-saturated sample and the strain rate. By the examples of uniaxial compression and constrained shear tests we have shown that the character of influence of the fluid filtration on the sample strength is determined by the sign and magnitude of pore volume change during the course of deformation. Under the loading accompanied by a decrease in the pore volume and increase in the pore pressure, fluid redistribution reduces the local maxima of pore pressure and thereby provides an increase in strength of the samples. Under the loading conditions which determine an increase in the pore volume and pore pressure drop, the filtration maintains the fluid pressure and thereby reduces the strength of the samples. Based on the simulation results, we have constructed the generalized logistic dependences between the samples strength of brittle permeable materials and strain rate, mechanical properties of liquid and solid skeleton and sample dimensions. These results show that the non-stationary character of the related deformation and filtration processes determines a significant variation of the strength in the samples of permeable materials even at low strain rates.