Study On The Mechanism Of Dynamic Size Effect On The Strength Of Rock

Rock strengths exhibit significant differences at different sizes,that is,the size effect of rock strength.However,the direct use of the data obtained in the laboratory for evaluating the stability of the actual rock mass that is unsafe.Therefore,it has great significance to clarify the failure mechanism of rock under static and dynamic loads for disaster prevention and mitigation of underground works.Experiments study showed that the dynamic size effect is opposite to the static size effect of rock strength,but the intrinsic micro-dynamic mechanism of the dynamic size effect of rocks has not been clarified yet.This study used the wing-crack model of sandstone to investigate the dynamic size effect of rock strength by using numerical calculations,and the crack motion equation and loading equation of samples are jointly solved,the mechanism of rock specimen size on strength under dynamic loading,namely the dynamic size effect of rock strength,is analyzed.The critical strain-rate range and its influencing factors in a certain size range were studied,as well as the critical size and its influencing factors under certain strain-rate conditions.The thesis also researched the influence of confining pressure on the size effect of rock strength.The main conclusions are listed as follows:(1)The physical mechanism of dynamic size effect of rock strength is clarified.The physical mechanism is explained as follows: due to the constancy of crack propagation speed at high strain rates,the larger the specimen size is,the longer the crack propagation and penetration time is,the greater the applied stress at the moment of fracture of the specimen is,and the greater the dynamic strength is.That is,the enhancement of specimen is explained as the overload caused by the fracture delay caused by the finiteness of the crack propagation speed.(2)For a certain size range of specimen,the critical strain-rate range of rock was obtained.The larger the specimen size is,the lower the critical strain-rate of dynamic size effect is.The higher the initial crack density is,the lower the critical strain-rate of dynamic size effect is.(3)There exists a critical size of the specimen for a certain strain-rate,and the static size effect dominates when the size is below the critical size,while the dynamic size effect dominates when the size is above the critical size.With the increase of strain-rate,the critical size of the specimen decreases.Increasing the initial crack density of the specimen reduces the critical size of the specimen.(4)The physical mechanism of the size effect of rock strength under confining pressure is clarified.Under low confining pressure,the material of the rock specimen does not tend to be homogenized and its heterogeneity is significant,resulting in a significant size effect on rock strength.However,under high confining pressure,the material of the rock specimen gradually fails from low to high,tending to homogenize the material and to demonstrate better homogeneity,resulting in a less significant size effect on rock strength.The size effect of rock strength is influenced by confining pressure.Under the same size and strain rate conditions,the strength of all specimens increases with increasing confining pressure,and the confining pressure effect on rock strength is significant.The larger the confining pressure,the less significant the size effect of rock strength,that is,the confining pressure can reduce the sensitivity of rock strength to size.Increasing confining pressure increase the critical strainrate range at certain size of specimen and also increase the critical size of a specimen under a given strain rate.