Study On Triaxial Loading And Rheological Mechanism Of CFRP Toroidal Constrained Sandstone

With the rapid development of national economic construction,the demand for energy is increasing year by year,and the difficulty of coal mining is gradually increasing.Consequently,the failure of support system and large deformation and failure of chamber surrounding rock occur from time to time,which brings great difficulties to the mining of coal and other underground resources.In the process of excavation,support and stability or failure of surrounding rock in underground engineering,its stress state is constantly self-adapting and self-adjusting until the complex of surrounding rock and support structure tends to be stable.In this process,the bearing capacity of the surrounding rock mass under the effective constraints provided by the bolt-shotcrete support structure is significantly improved,and the composite structure formed can provide effective constraints and binding force on the surrounding rock of the roadway,thus ensuring the long-term stability of the surrounding rock of the roadway.Therefore,it is necessary to study the mechanical properties and failure mechanism of surrounding rock of underground engineering under effective constraints.Taking the surrounding rock of roadway(sandstone)as the research object,the circular elastic-plastic constraints of different limit stress states on the surrounding rock were realized by applying CFRP with different constraint areas to the standard cylindrical specimen.Specimens of different constraint area of triaxial loading test and rheological test,according to the test results of the damage evolution equation considering the effect of hoop constraint,consider ring to triaxial damage constitutive model of the role of constraints,and sandstone creep model is established based on the theory of minimum energy dissipation,the rheological characteristics of sandstone,before and after the contrast analysis of constraints The particle discrete element method(PFC2D)was used to simulate the failure mechanism of sandstone before and after the constraint.The main results are as follows:(1)With the increase of loading strain rate,the critical point of axial strain from compression to dilatation gradually increases,indicating that loading strain rate can inhibit rock deformation to a certain extent.(2)With the increasing of CFRP constrained area,the proportion of elastic strain energy decreases and the proportion of dissipated energy increases.The damage variable corresponding to the same axial strain decreases gradually,indicating that CFRP circumferential constraint can effectively suppress the damage degree before rock failure.(3)The damage evolution equation and triaxial damage constitutive model of sandstone were established considering CFRP circumvolution constraint effect.Among them,the damage evolution equation is in good agreement with the test data,and can basically describe the pre-peak damage process of circumferential constrained sandstone.The triaxial damage constitutive model curve basically overlapped with the test curve in the pre-peak stage and deviated slightly in the post-peak stage,but it could describe the whole process of triaxial loading failure of CFRP circumferentially constrained sandstone.(4)Based on the numerical calculation method of particle flow,a numerical model of sandstone particle flow is established.The ratio of cracks before peak is,tensile crack : shear pressure crack : shear tension crack ≈1:1:1.Post-peak ratio of cracks is,tensile crack : shear pressure crack : shear tension crack ≈1.6:1:1.The results show that the post-peak fractures of sandstone are mainly tensile.The numerical model of CFRP circumferentially constrained sandstone particle flow is established.With the increasing of CFRP constrained area,the number of fractures corresponding to each characteristic stress point increases gradually,the amplitude of fractures increases gradually every 10 calculation time steps,and the fracture hot spots transition from regular distribution of shear zone to irregular distribution.(5)A fractional creep model based on the principle of minimum energy dissipation is established.Before the accelerated creep,the model curve basically coincides with the test curve,but there is a slight deviation between the test curve and the model curve in the accelerated creep stage,which can describe the whole process of sandstone creep on the whole.(6)The rheological properties of sandstone before and after CFRP confinement are compared and analyzed.The rheological isochronic stress-strain curves of sandstone with different CFRP constrained area all show inflection point under the horizontal action of penultimate second order load,and the larger CFRP constrained area is,the greater the axial and radial long-term strength is.The axial and radial long-term strength are equal after constraint,indicating that CFRP circumferential constraint can significantly improve the radial long-term strength of sandstone.