Dynamic Mechanical Properties And Constitutive Relationship Of Granite Damaged By Cyclic Loading

As an ideal rock material for oil storage,nuclear waste disposal and underground air-raid shelter construction,granite has the characteristics of high strength,low permeability and density.At the same time,in the process of underground engineering excavation,granite rock mass is inevitably disturbed by mechanical construction,blasting and other factors,resulting in varying degrees of damage.The mechanical properties of damaged rock mass under dynamic load is one of the key factors affecting the safety of rock mass engineering and the stability of surrounding rock mass.Therefore,it is of great engineering practical value to study the acoustic emission characteristics,dynamic mechanical characteristics and energy evolution law of granite in the process of damage caused by cyclic loading,to ensure the safety and stability of rock mass engineering.In this paper,the granite samples in hunan province were studied by cyclic damage test and Hopkinson(SHPB)impact failure test to explore the dynamic mechanical properties and constitutive relationship of granite samples under different cyclic loading modes.The main research contents and conclusions are as follows:(1)Through cyclic damage tests with different stress amplitude and loading rate,the influences of damage degree on the Kaiser effect and Felicity effect of granite acoustic emission are explored,and a new calculation method of Felicity ratio is proposed.It is concluded that when the cyclic upper limit stress of rock reaches 60%of compressive strength,Felicity effect is significant while Kaiser effect almost disappears.(2)Based on the two physical quantities of wave impedance and acoustic emission cumulative ringing count,the relations of two different damage variables are defined.The experimental data show that with the increase of the upper limit stress and the decrease of the loading rate,the damage variable of granite samples increases,and when the upper limit stress reaches 60% of the compressive strength,the damage degree of rock changes from weak to strong.Based on the advantages and disadvantages of the two definitions,the damage variable defined by wave impedance is selected as the basis for analyzing the dynamic mechanical properties of damaged granite.(3)The relationship between dynamic compressive strength,peak strain,dynamic growth factor(DIF)and damage variables is analyzed through impact failure tests of granite samples with different damage degrees,and it is proved that there is obvious compressive strengthening effect under dynamic loading.With the increase of damage degree,the dynamic compressive strength of granite samples decreases as a whole.Under the condition of low damage,the microcracks generated by the earlier damage will be compressed,so the dynamic compressive strength will increase.Under the condition of high damage,the cracks generated in the sample will be rapidly expanded,so that its strength will deteriorate.(4)The energy evolution characteristics during rock impact and the fracture characteristics after rock damage are analyzed,and the incident energy,reflected energy,transmitted energy and absorbed energy are calculated quantitatively.It is concluded that the absorbed energy is positively correlated with the intensity of failure and the absorbed energy of rock samples is mainly used for crack propagation and test block breaking.When the damage variable is small,the reflected energy is less than the transmitted energy,but with the increase of damage degree,the reflected energy is gradually enhanced,and the transmitted energy is constantly weakened.(5)Based on the combination constitutive model theory,the Zhu-Wang-Tang constitutive model was improved,and the part representing the low-frequency response was replaced by the damaged body.By introducing Weibull distribution and HoekBrown criterion,the constitutive relation of the damaged body was derived.The constitutive relation of the complete combination model was deduced by series and parallel theory,and the applicability of the constitutive relation was proved by comparing the stress-strain curves through laboratory tests.