Development Of Rock Direct Tensile Centring Device And Study On The Micro Fracture Mechanism Of Rock Stretching

In recent years,with the rapid development of underground and slope engineering in fields such as hydropower,transportation,energy,and mining,there have been more and more accidents involving rock mass tensile instability and failure.This is mainly because the tensile strength of rocks is much lower than the compressive strength,making the failure of rock masses in engineering often begin in the tensile stress zone or tensile shear stress zone.Therefore,it is of great significance to evaluate the tensile mechanical properties of rocks and clarify the mechanical mechanism of rock tensile failure to ensure engineering safety.To this end,this article conducts research on the testing methods of rock tensile mechanical properties and the micro mechanism of tensile failure from three aspects:the development of direct tensile testing equipment,laboratory tests,and numerical simulation analysis.The research results provide powerful technical means and important theoretical support for scientifically and reasonably evaluating the tensile mechanical properties of rocks.The specific research results are as follows:(1)A new type of rock direct tensile testing centering device has been improved and developed,including two parts:adhesive centering device and tensile dynamic centering device.Problem of local stress concentration in rock direct tensile testing has been effectively solved through bond correction and tensile correction.This device also has the characteristics of simple structure and convenient operation,and can be used as a universal device for conducting direct rock tensile tests,with wide promotion and application value.(2)Direct tensile tests were conducted on 8 types of rock samples taken from domestic key hydropower projects using the developed device.After effective testing,tensile mechanical parameters such as tensile strength(DTS)and ultimate tensile strain were obtained,as well as the total tensile stress-strain curve.Compared with the indirect testing of rock tensile strength(BTS)using Brazilian test,it was found that the direct tensile test showed a significantly lower rock tensile strength,and the ratio of DTS to BTS was between 0.60 and 0.71.(3)Based on the particle discrete element method,a three-dimensional rock micro fracture analysis model,the three-dimensional plane joint equivalent crystal model(FJ-GBM3D),was constructed by introducing plane joint contact into the equivalent crystal model.This model overcomes the problems of traditional particle discrete element models such as low compression to tension ratio and inability to characterize the mesoscopic intergranular and transgranular fracture processes of rocks.The model also exhibits nonlinear characteristics in the tensile stress-strain curve obtained from numerical simulation,which better matches the results of indoor mechanical tests.(4)Using FJ-GBM3D,discrete element numerical simulations were conducted on direct tensile tests and Brazilian splitting tests,and the evolution laws and spatial distribution characteristics of stress and micro cracks were obtained.The numerical simulation results provide a panoramic representation of the phenomena of intergranular fracture and transgranular fracture during the rock tensile failure process,revealing the microscopic damage and fracture mechanisms of direct tensile testing and Brazilian testing.A new insight has been proposed through the analysis of numerical simulation results,stating that a higher proportion of transgranular fracture in Brazilian tests is an important reason for BTS being greater than DTS.(5)Through numerical simulation under multiple working conditions,the influence of rock micro strength and micro structural factors on DTS,BTS,and their ratios was revealed.By using the grey correlation sensitivity analysis method,the ranking of sensitivity factors affecting rock DTS/BTS is calculated.The research results indicate that the mineral composition and microstructure heterogeneity of rocks are important factors affecting the differences between DTS and BTS.The research results scientifically answer the reason why the ratio of DTS to BTS varies with different rock types.