Study On Strength Characteristics Test And Damage Variable Calculation Method Of Rock-like Materials With Multiple Parallel Fractures

There are a large number of defects such as joint and microfracture inside the rock mass.The expansion and penetration of defects are the key reason for the strength degradation and engineering instability of rock mass.A large number of engineering practices show that the instability and failure of rock mass are closely linked to the size and geometric distribution of defects.Therefore,it is of extremely important theoretical value to study the defect expansion and its strength and deformation characteristics.Previous experimental studies on the mechanical properties for fractured rock mass were mainly aimed at the situation that the number of fractures was few(generally less than 3),while studies on the actual engineering containing multiple parallel fractured rock mass were relatively few.On the other hand,many scholars have tried to use the damage variable to establish the correlation between intact rock and fractured rock mass,and have proposed different calculation methods.However,these methods were rarely verified by experiments.In view of this,uniaxial compression tests in this paper of fractured rock mass specimens with different angles(0°,30°,45°,60° and 90°)and different numbers(1,3,5 and 7)were carried out by using similar materials.According to the test results,the variation laws of strength and deformation characteristics of specimens under different crack angles and crack numbers were revealed.The initiation and failure modes of intact specimens and single fracture specimens were analyzed.Based on the failure mode of specimens,an appropriate crack initiation model was established by using the existing mixed mode fracture criterion in fracture mechanics.By comparing with the existing damage variable calculation methods,an improved damage variable calculation method was proposed with the combination with damage mechanics and fracture mechanics.The main contents of this paper were as followed:(1)Uniaxial compression tests were carried out for different fractured specimens.Based on the test results,the existence of cracks had a significant impact on the mechanical properties of the specimens.Under the same number of cracks,the compressive strength ?and elastic modulus E increased with the increase of crack angle;Under the same crack angle,? and E decreased with the increase of crack number.(2)Based on the studies of crack initiation and extension modes of fractured specimens,single fractured specimens of 0°,45° and 90° and multiple fractured specimens of 45° and 60°showed tensile failure mode.The 30° and 45° single fractured specimens showed tensile-shear composite failure mode,and the rock-bridge coalescence mode was mainly tensile wing crack coalescence mode.The average initiation angles of wing cracks from 0° to 90° were 121.7°,97.3°,82°,68.3° and 0°,respectively.With the increase of crack angles,the crack initiation angle decreased gradually.The relative error calculated by the maximum circumferential strain fracture criterion was basically within 10 %,so it was more reasonable to use this method to calculate crack initiation angles.(3)Based on the comparison between experimental results and six calculation methods in theory,a damage variable expression of fractured rock mass was deduced from the perspective of strain energy in fracture mechanics.The calculation results were in good agreement with the damage variable law of strength characteristics,and the relative error was basically within 10 %.