Rocks are mainly composed of minerals in crystalline form and a small amount of amorphous cements. The genesis and environment have important influence on the mineral composition, mineral morphology and interface cement of the rock. Characterization on the process of rock failure and estimation of failure strength based on fracture mechanism and material heterogeneity span a wide range of engineering applications including deep underground excavation mining, geological disposal, core extracting from deep boreholes and rock breakage subject to dynamic loads.
In recent study, researchers from the Institute of Rock and Soil Mechanics (IRSM), Chinese Academy of Sciences (CAS) proposed a novel method which has the advantages of modelling intergranular and transgranular fracturing behaviour of rocks, as well as the grain-scale heterogeneity in rock strengths. They extend the multiscale particle-based discontinuum method to block-based grains, and a hybrid continuum-discontinuum scheme is developed to decrease difficulties in calibrating the redundant contact parameters encountered in grain-based method (see Fig. 1).
Research results indicate that crack initiation and damage stresses are intrinsic properties of rocks determined by grain-scale heterogeneity. Intergranular tensile cracks are primarily initiated as a result of local stress heterogeneity along grain boundaries. The subsequent generation of transgranular shear cracks implies a rapid proliferation of grain-crossing fractures, which will cause large-scale crack interaction and coalescence (see Fig 2).
The effects of grain size, grain morphology and mineralogy on macro mechanical properties, including crack initiation stress, crack damage stress, uniaxial compression strength and elastic modulus, are discussed. The larger grain size contributes to stronger local stress heterogeneity, which results in a lower failure strength of rocks. The crack initiation stress is determined by local heterogeneity and is less affected by the change in average grain size. Grain morphology plays an important role in grain interlocking while a reduction in grain size variance leads to a more homogeneous stress field. The mineralogy is evaluated with the aid of a quartz-mica-feldspar diagram, and the quantitative relationships between mineralogy and the macro-scale mechanical properties of rocks are discussed (see Fig 3).
The newly developed mGbCDM has the ability to characterize the grain-scale fracturing initiation, propagation, interaction and coalescence inter or intra mineral grains in granitic rocks at the laboratory scale. This work was funded by the National Natural Science Foundation of China (NSFC) under grant Nos. 51439008 and 51679231. Funding from the China Scholarship Council is acknowledged for providing supports to the ？rst author (grant No. 201604910678).
Fig 1 The image-based processing of realistic grain morphology in GbCDM
Fig 2 Changes in crack initiation stress, damage stress and peak strength under different confining pressures
Fig 3 Influence of mineral composition on rock strength