A two-phase model of rock is proposed to investigate the mechanism of brittle fracture due to unixial compression, in which rock is considered as a composite material consisted of inclusion (harder grains) and matrix (colloid). The stress slate of matrix region near grains is first calculated using finite element method (FEM). The numerical result shows that tensile stresses are induced easily in the neighboring area or hard grains with the maximum value near grain boundaries. The influence of these tensile stresses on the crack initiation and failure process of brittle rock due to unixial compression is investigated by numerical experiments. It can be concluded that the failure mode of brittle rock under unixial compression is still tensile fracture from point view of microstructure.
Crack initiation, propagation and coalescence in brittle rocks due to uniaxial compression have been investigated by many researchers. Horii and Nemati Nasser[1] tackled this problem using complex analysis and numerical method, several semi analytical stress intensity factor (SIF) formula for inclined crack subjected to uniaxial compression have been proposed by Paul[2] and Baul et al[3]. Recently, Chen and Sun[4] proposed a more accurate semi-analytical SIF formula for pre-existing crack subjected to far-field unixial compression, However, almost all these investigations are based on homogeneous hypothesis. It is a well-known from petrology that rock is or grain structure. It contains various mineral components with quite different mechanical properties even for the same rock. Crack initiation and growth in non-cracked rock due to unixial compression are closely related to its microstructure. Experiments showed that grain size and properties play an important role in rock failure process. A better understanding the mechanism or brittle fracture of rock needs the knowledge of stress-state in the microstructure level. In this paper, brittle rock is modeled as a two-phase composite material consisting of aggregates (harder mineral grains) and matrix (colloid), the idealized microstructure model of rock is depicted in Figure 2. Grains usually possess a higher Young's modulus and lower poisson's ratio with respect to matrix.
Experiments show that cracks under compression rarely go through grains but go around them, which denotes that cracks usually initiate and propagate in matrix. Therefore, present finite element analysis is performed in this region. It can be seen from Figure 1 that such a region is not so wide, the average distance between two adjacent grains is estimated to be as small as 5 to 10% of the radius of an equivalent circular grains for some rocks. In order to investigate the influence of hard grains on stress stale of nearby matrix area, three composite models, which contain three and five circular grains respectively, are used in finite element calculation (see Figure 3). specimen is analysed,the corresponding finite element meshes are plotted in Figure 4. Eight-node isoparametric quadrilateral and six-node isoparametric triangular elements are used in FEM calculation. The obtained maximum principal stress distributions in the matrix are shown in Figure 5.
