| Excavation of high steep rock slopes and underground caverns under high ground stress condotions will inevitably lead to redistribution of stress field,which often presents one-direction unloading-tensile deformation damage,resulting in strong tensile fracture characteristics of rock(or rock mass).In order to explore the mechanical characteristics of rock during unloading-tensile under confining pressure,this article,relied on the National Natural Science Foundation of China:Research on the rheological characteristic and time-effective propagation mechanism of hard rock induced by unloading failure under high residual stress(41172243),studied the unloading-tensile deformation and failure of rock induced by unidirectional excavation under three-dimensional stress state.In view of this,the confining pressure effect and the strain rate effect of unloading-tensile mechanical properties of red sandstone induced by axial unloading-tensile under constant confining pressure condition,mesoscopic damage evolution mechanism,strength criterion and constitutive model were investigated based on the theories of rock mechanics,unloading rock mechanics,damage mechanics and elastoplastic mechanics,using laboratory tests,numerical simulations and theoretical methods.The main work and conclusions of this study are as follows:? An axial unloading-tensile device was invented,which could be used to test the axial unloading-tensile mechanical properties of rock under constant confining pressure condition.By means of this device,the confining pressure effect and the strain rate effect of unloading-tensile mechanical properties of red sandstone were studied.The results showed that the axial unloading-tensile mechanical properties of red sandstone were mainly related to the confining pressure.Under the condition of no confining pressure(Pc=0 MPa),the deformation had no unloading stage but only tensile stage,the peak stress was the uniaxial tensile strength,and the mode was tensile failure.Under the condition of low confining pressure(0 MPa<Pc ? 20 MPa),the deformation had both unloading stage and tensile stage,the peak stress was approximately the uniaxial tensile strength and did not change with the confining pressure,and the mode was tensile failure.Under the condition of medium confining pressure(20 MPa<Pc ? 40 MPa),the deformation had both unloading stage and tensile stage,the peak stress would gradually change from the uniaxial tensile strength to 0 MPa with the confining pressure,and the mode was tensile-shear hybrid failure.Under the condition of high confining pressure(Pc>40 MPa),the deformation had no tensile stage but only unloading stage,the peak stress would gradually increase from 0 MPa with the confining pressure,and the mode was shear failure.Under the condition of static or quasi-static condition,the elastic modulus,the peak axial strain and the peak stress of red sandstone during axial unloading-tensile all had good linear relationships with the logarithm lg? of strain rate.? The deformation and fracture process of red sandstone during axial unloading-tensile under different original stress conditions,such as different original hydrostatic stresses,different original axial stresses with constant original confining pressure(30 MPa)and different original confining pressures with constant original axial stress(30 MPa),was simulated.It showed that under different original stress conditions,the deformation included elastic,non-linear and post-peak stages,meanwhile the process of crack propagation included no crack,accelerated propagation and steady propagation stages.With the increase of original hydrostatic stress,the characteristic of non-linear deformation would be more obvious and the post-peak deformation would be disappeared gradually,meanwhile the algebraic value of peak stress first kept constant and then increased,and the ratio of the number of shear cracks to that of total cracks would be larger gradually.When the original confining pressure was 30 MPa,with the increase of the original axial stress,the elastic deformation stage would be longer,and the linear and post-peak deformation stages would be constant,meanwhile the peak stress was approximately-0.33 MPa and constant,and it had few shear cracks.When the original axial stress was 30 MPa,with the increase of the confining pressure,the characteristic of non-linear deformation would be more obvious and the post-peak deformation would be disappeared gradually,meanwhile the shear crack did not appear obviously until the confining pressure reached to 45 MPa.? The mesoscopic damage evolution of red sandstone during axial unloading-tensile under different original stress conditions was analyzed,the types of mesoscopic damage fracture were concluded and the corresponding relationship with the macroscopic failure modes were established,the quantitative evaluation standard of the failure modes was proposed,and the quantitative expression of the mesoscopic damage evolution was derived.The results showed that there were six forms of mesoscopic damage fracture of red sandstone during axial unloading-tensile:the tearing failure induced by the horizontal propagation of the main fracture,the tearing failure induced by the horizontal propagation of the main fracture accompanied with branch fractures,the corrosion failure induced by the horizontal gather of tensile cracks,the corrosion failure induced by the slanting gather of tensile cracks,the corrosion failure induced by one group of S-shaped gather of cracks and the corrosion failure induced by several groups of S-shaped gather of cracks,of which the first two forms were the tensile failure mode,the third form was the tensile-shear hybrid failure mode and the later three forms were the shear failure mode.The failure modes of red sandstone during axial unloading-tensile could be quantitatively evaluated using two parameters,i.e.the peak stress(?ut)and the crack initiation stress(?*).When?ut<?*<0 MPa,the mode was tensile failure.When ?ut<0 MPa<?*,the mode was tensile-shear failure.When 0 MPa<?ut<?*,the mode was shear failure.The mesoscopic damage evolution of red sandstone during axial unloading-tensile could be quantitatively evaluated through a three-parametric non-linear equation,including the shape parameter,the percentile of crack initiation stress and the percentile of peak crack number.? The applicability of the classical Mohr-Coulomb(MC)strength criterion and Hoek-Brown(HB)strength criterion to the unloading-tensile strength of red sandstone was evaluated.The improved Mohr-Coulomb(UT-MC)criterion and Hoek-Brown(UT-HB)criterion were proposed and derived and then the influences of the strain rate on the parameters of two improved criterions were studied.The results showed that two classical criterions could not be directly used to estimate the unloading-tensile strength of red sandstone,nevertheless two improved criterions were of good applicability.In the expressions of two improved criterions,the ratios of tensile-failure critical confining pressure and tensile strength to shear-failure critical confining pressure had hyperbolic relationships with the logarithm lg? of strain rate,the tensile strength and shear-failure critical confining pressure had linear relationships with lg? and the tensile-failure critical confining pressure had a parabolic relationships with lg?.? The elastic-plastic constitutive model and the damage constitutive model of red sandstone during unloading-tensile were established,and their reliabilities were also verified.It was shown that the elastic-plastic constitutive model of red sandstone during unloading-tensile could be divided into three categories according to the original confining pressure level.Under the condition of low confining pressure(0 MPa ? Pc ?20 MPa),the elastic-plastic constitutive model was composed of elastic constitutive model and strain softening constitutive model.Under the condition of medium confining pressure(20 MPa ? Pc ? 40 MPa),the elastic-plastic constitutive model was composed of non-linear elastic constitutive model,elastic constitutive model and strain softening constitutive model.Under the condition of medium confining pressure(Pc ? 40 MPa),the elastic-plastic constitutive model was composed of non-linear elastic constitutive model,elastic constitutive model,plastic constitutive model and strain softening constitutive model.The damage constitutive model of red sandstone during unloading-tensile could be established using the damage variable defined based on the effective stress or the effective area. |
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