Experimental Study On Mechanical Properties And Constitutive Model Of Fractured Sandstone In Freezing Environment.

Rheology is an important mechanical property of rock.In order to explore the rheological mechanical property and failure mechanism of fractured rock mass in cold region under long-term external load,triaxial mechanical tests were carried out on saturated sandstone with different fracture dip angles in freezing environment at-10℃,and the influence law of fracture geometry characteristics on physical and mechanical parameters and failure modes of rock samples was obtained.Then,triaxial graded loading creep test was carried out,and the deformation and damage evolution law of different rock samples in three stages of creep was analyzed.Finally,the nonlinear viscoelastic-plastic constitutive model of fractured sandstone is established,and the creep characteristic parameters are fitted,and the following conclusions are drawn:(1)The fracture dip angle has a great influence on the development of stress-strain curves of sandstone during loading.The initial segment of the stress-strain curve is "convex",which is due to the fact that the rock mass is mainly loaded by ice bodies in pores and fissures.The peak strength,elastic modulus and shear strength parameters of sandstone are linearly positively correlated with fracture dip angle,while Poisson’s ratio and volume strain are negatively correlated with fracture dip angle,but the variation range is not obvious.The damage evolution equation considering the strength and deformation of fractured sandstone is established by volume strain method combined with Hook’s law.It is found that with the increase of fracture dip angle,the crack initiation and dilatation stresses of the sandstone increase gradually,while the drop state of post-peak stress-strain curve is more obvious,indicating that the elastic stage of stress-strain curve becomes longer,the plastic stage becomes shorter,the brittleness of sandstone increases,and the ability to resist plastic deformation decreases.In addition,the post-peak mesoscopic scan showed that saturated freezing enhanced the degree of cementation between the particles in the sandstone,making the sandstones with different fracture dip angles present a shear failure mode,and the failure surface was more uniform.With the increase of fracture dip angle,the fluctuation range of crack surface area decreases at different scanning heights,but the crack volume on fracture surface increases.The research results can provide theoretical reference for the freezing construction of fractured rock strata and for the follow-up multi-stage loading creep test under low temperature freezing environment.(2)The deformation and damage evolution of different rock samples in the three stages of creep are analyzed.Under different stress levels,the steady creep duration of rock increases gradually with the increase of crack angle,while the creep rate decreases nonlinearly.On the other hand,with the increase of stress grade,the instantaneous deformation and creep deformation also increase.The long-term creep strength of sandstone is obtained by isochronal curve method and steady-state creep rate method.It is found that the long-term strength of sandstone increases significantly with the increase of fracture inclination.Compared with the rock failure mode,it is found that the angle between the main failure surface and the horizontal direction first decreases and then increases.Among them,the rock with 0°and 15° dip angle shows the overall shear through failure mode,and the rock with 75° and 90° dip angle shows tension-shear through failure,and there are no micro-cracks and secondary cracks in the rock sample.(3)according to the creep test results of graded loading,the software element with fractional calculus and the improved nonlinear viscous cylinder are introduced to establish the nonlinear viscoelastic-plastic constitutive model.At the same time,the parameters are identified and verified by 1stOpt software.It is found that the proposed creep constitutive model can accurately describe the graded loading creep curve of fractured sandstone under low temperature freezing environment,especially in the unstable creep stage.At the same time,the effects of loading stress level and crack inclination angle on the parameters of the creep model are analyzed.finally,the creep characteristic parameters are fitted to verify the accuracy and rationality of the creep model.