Investigations On Energy Evolution Law And Damage Constitutive Model Of Pre-heated Granite

With the development of deep engineering such as the mining of underground mineral resources,tunnel excavation at high-ground temperature,exploitation and utilization of geothermal resources and rock breaking assisted by high temperature,the influence of temperature on hard rock engineering cannot be ignored.The increase of surrounding rock temperature will aggravate the frequency and intensity of rockburst disaster,and the problems of high stress,high ground temperature and high rockburst are increasingly prominent in deep engineering.The essence of rock failure is the comprehensive result of energy storage,dissipation,surplus,and release.Therefore,this study is based on the laboratory test and theoretical analysis of research methods,granite specimens of different temperature has been carried out a series of conventional uniaxial compression and single-cyclic loading–unloading uniaxial compression tests,in order to reveal the function mechanism of temperature affects the rockburst proneness for hard rock from the perspective of energy storage;analyze high temperature damage characteristics and build damage constitutive model of rock from the perspective of energy dissipation.The main research contents and conclusions are as follows:(1)The variation characteristics of stress–strain curves of granite samples under different temperatures and unloading stress levels are analyzed.It is found that high temperature makes the compaction stage of specimens longer and the yield stage more obvious.The post-peak stress drops rapidly in all specimens,indicating that the granite specimens still exhibit strong brittle failure within 700 °C.The peak compressive strength and elastic modulus of granite specimens first increase then decrease with the increase of temperature,while the peak strain increases significantly.The temperature effect of peak compressive strength of granites is discussed,and three temperature ranges with obvious strength changes are defined,which are 20 °C–300 °C,300 °C–400 °C,and above 400 °C.(2)The relationship between the total input strain energy,elastic strain energy and dissipated strain energy of pre-heated granite specimens under different stress levels is investigated.It is found that there is a strong linear relationship between elastic strain energy and total input strain energy,and between dissipated strain energy and total input strain energy.It is verified that temperature does not change the essential property of granite with linear energy storage and dissipation laws,and high temperature only changes the compressive energy storage and dissipation capacities of granite specimen.(3)A new method is proposed to quantitatively determine the equivalent elastic modulus under loading and elastic modulus under unloading of rock specimens under uniaxial compression.According to the linear relationship between the square of unloading stress and the three strain energies,it is proved that the variation of strain energy during the uniaxial compression of granite specimens after high temperature follows the linear energy storage and dissipation laws.(4)The discriminant results of ejection mass ratio outside the indenter and four rockburst proneness indeices of granite specimens with different temperatures are compared.The quantitative results of the residual elastic energy index and the qualitative results of ejection mass ratio outside the indenter are basically the same,which realized the mutual verification and unification between qualitative and quantitative evaluation.With the increase of temperature,the rockburst proneness of granite specimens firstly increases and subsequently decreases.The rockburst proneness of granite specimens at 300 °C is the strongest.The granite specimens below 500 °C all have strong rockburst proneness,while the specimens at 700 °C have medium rockburst proneness.(5)A unified damage constitutive model for pre-heated granites under conventional uniaxial compression as well as a unified damage constitutive model under uniaxial compression with single-cyclic loading–unloading uniaxial compression are established.The accuracy and reliability of the unified damage constitutive model were verified by using the test curves for granites of conventional uniaxial compression under different temperatures and single-cyclic loading–unloading uniaxial compression at different unloading stress levels.The unified damage constitutive model can accurately reflect the change characteristics of the test curve,greatly improve the accuracy of the stress–strain curve in compaction and quasi-elastic deformation stages,the parameters are clear,the form is simple and easy to use.