| The compressive strength and deformation parameters of rock material are important indices of rock engineering.Because the rock is a typical heterogeneous material,its strength and deformation parameters vary with the sample size.At the same time,the strength and deformation characteristics of rock are influenced by confining pressure and loading rate.In this paper,the static and dynamic compression tests with rock specimens of different sizes were simulated by using PFC2D code.The size effects of rock material under different confining pressures and different loading rates were studied.The simulation results indicate that:(1)The rock strength,peak strain and deformation modulus decrease with increasing specimen size.The size effect of the deformation modulus is relatively small,and even can be ignored.Whereas the change of specimen size has little effect on the failure mode of these rock specimens.(2)The rock strength,peak strain and deformation modulus increase with the confining pressure.Among them,the rock strength and peak strain have linear relations with the confining pressure.While the influence of confining pressure on the modulus is relatively small,and even can be ignored.The failure of rock specimens gradually changes from the splitting mode to shear failure mode into X or Y shearing failure mode with the increase of confining pressure.In addition,the change of rock strength is mainly caused by the change of cohesive force,and the internal friction angle is less affected by the size and loading rate.(3)The rock strength,peak strain and deformation modulus increase with the loading rate,and they are approximately linear with the logarithm of the loading rate.Under the same loading rate range,the increment of rock strength under high confining pressure is smaller than that of the rock strength under low confining pressure.The existence of loading rate has little effect on the rock fracture mode.(4)The increments of rock strength and peak strain for different size specimens at the same confining pressure range are approximately the same.Meanwhile,these two parameters increase with confining pressure and therefore,the size effect of the rock strength and peak strain at the high confining pressure becomes weak.In addition,the increasing ratios of deformation modulus for corresponding scenarios are similar,which causes the confining pressure having little effect on the size effect of deformation modulus.(5)The increment of rock strength,peak strain and compression modulus increases with increasing specimen size at the same loading rate range.Meanwhile,these three parameters increase with loading rates and therefore,the size effect of the rock strength,peak strain and compression modulus at the high loading rates becomes weak.(6)The size effect,confining pressure effect,loading rate effect of rock strength are related to the microscopic failure mechanism of rock.? When compared with small samples,the crack density of large samples is small,which indicates that the relative crack number of large sample is less,and the strength of large sample is smaller.?With the increase of confining pressure and loading rate,more cracks are involved in the expansion(the crack density also becomes larger),which leads to the increase of the strength of the rock.? The slope of the fitting curve of the crack density and confining pressure is basically the same for samples with different sizes,which indicates the increment of the crack density of samples with different sizes is approximately the same at the same confining pressure range.So the strength increment of different samples is basically the same.The slope of the fitting curve of the crack density and the loading rate of large sample is significantly greater than that of small sample.The increments of crack density increases with increasing specimen size at the same loading rate range,so the so the strength increment of large sample is is significantly greater than that of small sample. |
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