| In the foundation engineering of deep resource mining and underground space development,the damage and failure characteristics and fracture mechanism of brittle fractured rock have always been the basic scientific problems of rock engineering instability prevention and control.For the common fracture forms in fractured rock mass,a large number of studies have been made on the single flaw and double flaw forms,and gratifying results have been achieved,but there are few studies on the orthogonal cross flaw forms.Therefore,focusing on the scientific problem of fracture damage of fractured rock,this thesis takes orthogonal type cross through flawed red sandstone as the research object,uses MTS815 rock mechanics test system,acoustic emission（AE）technology,and PFC2D numerical simulation.The mechanical response characteristics,crack propagation mechanism,failure mode and acoustic emission characteristics of orthogonal cross-fractured sandstone under different crack dip angles and different strain rates are studied.The main understanding is as follows:（1）The influences of crack dip Angle and strain rate on mechanical properties of fractured sandstone are analyzed.At low strain rates of 10-5/s and 10-4/s,the peak strength and elastic modulus increase with the increase of dip Angleα,and reach the maximum value whenαis 90°.At high strain rates of 10-3/s and 10-2/s,the peak strength and elastic modulus first increase and then decrease with the increase of dip Angleα.The maximum value is obtained whenαis 60°.The peak strain increases first and then decreases with the increase ofα,and reaches the maximum value whenαis 45°.At the same dip Angle,the peak strength,elastic modulus and peak strain increase with the increase of strain rate,and the increase of peak stress and peak strain at high strain rate is significantly higher than that at low strain rate.（2）The crack initiation,crack propagation and macroscopic failure mode of fractured sandstone are analyzed.It is found that the fracture behavior of the samples is less affected by the strain rate,but is closely related to the crack Angleα.Whenαis 0°,60°,90°,the crack first initiates at the secondary crack tip.Whenαis 30°,the crack initiation position starts to change,randomly occurring at the main crack tip or the secondary crack tip.Whenαis 45°,new cracks are first generated at the main crack tip,and the main crack types are tensile cracks.When α≤45°,the main crack plays a dominant role in the failure of the sample,and the existence of the secondary crack has a certain guiding role in crack propagation.Whenα≥60°,the secondary crack begins to control the sample failure,especially whenαis 90°,the sample failure is completely dominated by the secondary crack.The macroscopic failure mode of the sample is mainly the mixed failure of tension and shear,and the pure shear failure and pure tensile failure rarely occur.At low strain rate,shear failure accounts for a relatively high proportion in tensile shear mixed failure,and with the increase of strain rate,the proportion of tensile failure increases gradually.（3）The acoustic emission characteristics of fractured sandstone during compression failure are analyzed.At the initial compaction stage,the AE count and energy are both at a low level,and a surge occurs for the first time at crack initiation.After entering the crack growth stage,the AE count and energy increase several times.Whenαis 45°,the AE count and AE energy surge times are the most,which indicates that more cracks occur in the failure process of the sample at this inclination Angle.The development trend of AE real time b value is characterized by phases.In the early stage of loading,the real time b value is usually at a high level.After entering the crack growth stage,the real time b value drops sharply,and keeps at a low level until the sample is compressed and fails,and the real time b value reaches the minimum.The overall AE b value is less affected by the crack inclination Angle and more affected by the strain rate,showing a trend of decreasing with the increase of strain rate.The failure characteristics of fractured sandstone samples are analyzed based on acoustic emission b value parameter.Based on RA value（rise time/amplitude）and AF value（AE count/duration）,the fracture modes in the damage and failure process of fractured rock are divided into tensile failure and shear failure,and the proportion of tensile failure is significantly larger than that of shear failure.The results show that under the four strain rates,the proportion of tensile failure is significantly greater than that of shear failure,and the internal failure of specimens is mainly tensile failure.（4）PFC2D numerical simulation software is used to construct fractured sandstone samples,and the reliability of the model is verified by comparing the experimental results with the numerical results.The growth trend of the number of cracks in the process of crack propagation is analyzed.The inflection point of the number of cracks is the earliest whenαis 0°,and the latest whenαis 60°.The total number of peak cracks is minimum whenαis 30°,and maximum whenαis 45°.By studying the variation of the contact force chain field between particles,it is found that the initiation of crack is mainly affected by the concentration of tensile stress,and the initiation crack usually occurs first at the crack tip where the tensile stress concentration is obvious.When the stress is loaded to the peak stress,the compressive stress concentration zone is mainly distributed on the left and right sides of the prefabricated crack,while the tensile stress concentration zone is no longer distributed near the prefabricated crack,but gradually distributed in the whole sample with the evolution and expansion of the crack.Based on the change of particle displacement field,the meso-fracture mechanism of sample initiation failure is analyzed.The initiation failure at the crack tip is all the tensile fracture caused by type I displacement field,which explains the reason why the tensile crack first appears. |
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