| After the excavation and unloading of underground rock mass engineering,the stress state of surrounding rock mass changes from the original three-way compression to two-way compression.Due to the effects of rebound tension and bidirectional compression,the tension deformation will occur at the unloading surface,and the rock mass on the unloading surface is in a stress state of two-way compression and one-way tension.To simulate a series of stress adjustment processes during excavation and unloading,the triaxial extension test is extracted.To explore the deformation and failure characteristics,rupture mechanism,and energy evolution characteristics of red sandstone at different unloading rates and different confining pressures,the laboratory test method was adopted,and the MTS815 test platform was used in conjunction with the compression-pull load conversion device.A series of triaxial extension tests were carried out on“I”shaped specimens,and the failure characteristics of the specimens were obtained by 3D laser scanning and SEM scanning.After analyzing the test results,the following conclusions are drawn:（1）The triaxial extension tests at different confining pressures and different strain rates were carried out on the"I"-shaped red sandstone specimens.The results show that the elastic modulus,failure strength and axial ultimate strain show an increasing trend with the increase in confining pressure.The failure strength,axial ultimate strain,and elastic modulus all increase with the increasing strain rate,and the strain rate sensitivity of the three increases with the increasing confining pressure.（2）The failure characteristics of the specimens were obtained by 3D laser scanning and SEM scanning.After analysis,the fracture mechanism of red sandstone at different confining pressures was obtained:at 0MPa confining pressure,cemented substances failure and intergranular failure are the main fracture pattern.At the confining pressures of 10MPa and20MPa,the intergranular failure is the main fracture pattern.At 30MPa and40MPa,intergranular failure and transgranular failure are the main fracture pattern,and the fracture surface shows smooth due to shear friction.At50MPa and 70MPa,shear friction marks caused by shear stress appear.Confining pressure causes the failure mode of specimens gradually change from tension failure to hybrid failure（tension–shear fracture）,and then to shear failure.The confining pressures corresponding to the three failure modes are 0-20MPa,30-40 MPa,and>50 MPa.（3）The fracture mechanism of red sandstone at different strain rates:at lower strain rates,cemented substances failure,and intergranular failure are the main fracture pattern,and the failure surface is undulatory and rougher.With the increase in strain rate,more and more transgranular failures occur,and the failure surface becomes smoother.At a higher strain rate(1×10-2s-1),the fracture surface displays many small cleavages.These phenomena indicate that the internal micro-cracks of red sandstone have sufficient time to develop into one or several macro-main cracks along the weak structural plane at a low strain rate,and the failure plane fluctuates greatly.As the strain rate increases,the micro-cracks have no time to expand along the weak surface,and the mineral grains around the micro-cracks are jointly subjected to stress,resulting in more transgranular failure.Mineral grain failure requires greater stress,which is manifested as an increase in failure strength on a macroscopic scale.In addition,the shortening of the micro-crack propagation time means that the expansion area is also smaller,and adjacent micro-cracks cannot penetrate each other,which is manifested as many small cleavages on the failure surface.（4）The single cyclic loading-unloading triaxial extension tests at confining pressures of 10MPa,30MPa,50MPa,and 70MPa were carried out.The results show that the input energy density（IED）,elastic energy density（EED）,and dissipation energy density（DED）all grow exponentially with the increasing stress level.IED grows the fastest,EED grows second,and DED grows the slowest.The increase of confining pressure can prolong the elasticity enhancement stage,but before the peak stress,the energy storage capacity at different confining pressures converges.In addition,the linear energy storage and dissipation laws under triaxial extension were determined.With the increase in the IED,both the EED and DED grow linearly.There is also a strong linear relationship between the EED and DED.（5）Based on the linear energy storage and energy dissipation laws,the peak strain energy storage index wp and the corresponding theoretical calculation method are proposed.The theoretical calculation method can better describe the experimental data of the strain energy storage index with the input energy.The function curve of the theoretical calculation method and the experimental data both converge to the ideal peak strain energy storage index wT with the increase of the input energy.After verification,the theoretical calculation method calculates wp with high accuracy and stability.（6）As the input energy increases,wp converges to the ideal peak strain energy storage index wT.wT can roughly estimate wp,which reflects the pre-peak elastic energy storage capacity.With the increase of confining pressure,wT showed a trend of increasing first and then decreasing,which explained the mechanism of failure mode transition under the triaxial extension from the perspective of energy.（7）Based on the linear energy dissipation law,the damage variable DT is proposed.At the initial loading stage,the DT is small and grows very slowly.As loading progresses,the growth rate gradually became faster,and the growth curve of DT approximated an oblique line in the middle and late stages.In addition,it was found that DT grows exponentially with the increase in stress level at different confining pressures.At the confining pressure of 10MPa,DT grows fastest,and 30MPa and 50MPa slow down in turn.The damage growth rate of 70MPa is similar to that of 50MPa.59 pictures,11 tables,128 references... |
