| The tensile strength and fracture toughness are important mechanical parameters in the field of rock mechanics.Due to the relatively low tensile strength of rocks,some rock projects need to use tensile strength to identify and predict common disasters,such as spalling,rockburst,etc.There are also some projects that require the application of fracture mechanics to improve rock breaking efficiency or rock conductivity,such as the application of hydraulic fracturing in oil,natural gas,shale gas and geothermal exploitation.Thus,it has important theoretical significance and engineering practical value to carry out the research on the Brazilian test and fracture toughness test.Brazilian tensile strength and fracture toughness are easily affected by many factors.Laboratory tests have found that non-central initiation of Brazilian discs often occurs.There are large differences in fracture toughness values obtained by different fracture toughness test methods.The fracture mechanism is essential to understand the true rock failure behavior.This paper is based on the National Natural Science Foundation of China "Research on macro/mesoscopic characterization of mechanical behavior of hard brittle rock and the mechanism of strong unloading-temperature coupling of surrounding rock damage"(No.51774020).The macro and mesoscopic methods are used to analyze the influencing factors and fracture mechanism of Brazilian test,mode I and mixed mode I/II fracture tests.The main content and results of this paper are as follows:(1)Acoustic emission monitoring technology is used to evaluate the similarities and differences of the three-dimensional damage and focal mechanism of the sandstone Brazilian disc specimens under linear/non-linear loads.Moment tensor inversion obtains the frequency percentage of isotropic,double couple and compensated linear vector dipole components.It reveals that its rupture is a tensionshear mechanism,which partially explains the reason why Brazilian tensile strength tends to be high.(2)A new method is introduced to automatically calibrate the 3D flat-joint model using the Plackett-Burman design,response surface methodology and optimization method.The Plackett-Burman design obtains the three most important meso-parameters.Response surface methodology is used to evaluate the interaction effect between the meso-parameters.Finally,the problem of obtaining the best set of meso-parameters of 3D flat-joint model is solved using the optimization technique.(3)A meso-analysis method of simulating acoustic emission that can quantitatively identify the source type is proposed to obtain the moment tensor inversion parameters during the test.The influence of contact conditions and heterogeneity of rock on tensile strength,crack initiation point,crack initiation mode,and focal mechanism are obtained,and a failure model transition at the ends of the specimen is proposed.(4)Ultra-high-speed camera and digital image correlation technology are used to obtain the fracture process of straight and oblique notched semi-circular bend sandstone specimens under mode ? and mixed mode ?/? loads.Experiments are carried out to investigate the influence of sample size and loading rate on the mode fracture toughness,and mixed mode fracture experiments with different inclination angles are carried out to study the influence of inclination angle on fracture toughness and fracture mode.The results show that there is a fracture process zone in the sandstone SCB specimen,and there are opening and sliding displacements on both sides of the potential section in the mixed mode fracture test.(5)The 3D flat-joint model mesoscopic models of the semi-circular bend and cracked chevron notched semi-circular bend test of Kowloon granite are generated.The effects of meso-parameters on the mode I fracture toughness of semi-circular bend and cracked chevron notched semi-circular bend tests are determined.The results of the simulating acoustic emission meso-analysis pointed out that the chevron notch is more likely to cause pure mode I fracture due to its higher proportion of tensile source and larger average value of k.(6)The 3D flat-joint model mesoscopic model of the mixed mode I/II fracture test is generated and the source type and relative proportion are analyzed using the simulating acoustic emission meso-analysis method.Three typical classification methods are used to comprehensively and quantitatively analyze the source type.The results show that some tensile mechanisms are not purely tensile,and deviatoric components also play an important role.The increase of notch inclination angle increase the proportion of shear source and decreases k value,which increases the tendency of mode II fracture.(7)A large number of literatures are collected and quantitatively analyzed the general empirical relationships between tensile strength and mode I fracture toughness.The internal connection is analyzed,which can the foundation for engineering design and crack propagation stability evaluation of rock. |
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