Research On Dynamic Mechanical Properties And Damage Mechanism Of Sandstone Considering Accumulated Damage Effect

In the process of deep coal mining,the stress equilibrium state of the surrounding rock is broken due to engineering disturbances such as roadway excavation and workface retrieval.The roadway surrounding rock will repeatedly experience the stress change process of loading and unloading.The complex in-situ environment and strong engineering disturbances will cause large areas of accumulated damage in the surrounding rock,which in turn weakens the physical and mechanical properties of the rock medium and induces rapid and discontinuous development and expansion of micro-fractures in the rock mass.In addition,deep mining projects are susceptible to dynamic disturbances such as underground impact pressures,rock bursts and earthquakes,making the surrounding rock highly susceptible to instability at higher strain rates.Therefore,it is of great practical significance to investigate the effect of accumulated damage on the dynamic mechanical properties of coal measures sandstone for the rock surrounding support and dynamic disaster prevention and control in deep mining engineering.In this thesis,coal measures sandstone is studied to clarify the accumulated damage effect of sandstone under cyclic loading and the dynamic damage mechanism of accumulated damage sandstone.The following research results were obtained.(1)Experiments on the accumulated damage characteristics of coal measures sandstone under cyclic loading were carried out.Combined with CT scanning technology,the change rules of fine-scale characteristic parameters of coal measures sandstone were clarified,and the mechanism of fine-scale damage evolution of coal measures sandstone under cyclic loading was revealed.Research shows that: cyclic loading action led to an increase in irreversible deformation of sandstone,producing a significant accumulated damage effect.The sandstone strength decreased linearly with the cycle number after cyclic loading,and the increase of cyclic load made its strength decrease more rapidly.As the cyclic load increased,the pore number and equivalent diameter changed from linear to exponential growth trend with the cycle number,while the difference between porosity and connected porosity gradually decreased.Connected porosity is a key parameter to describe the accumulated damage effect of sandstone caused by cyclic loading,and thus a quasi-static strength decay model based on connected porosity was proposed.The introduction of the relative change in connected porosity effectively characterized the accumulated damage effect of the sandstone.(2)Impact tests on coal measures sandstone under accumulated damage conditions were carried out.Combined with high-speed digital image correlation technology,the dynamic damage mechanism of coal measures sandstone under the cumulative damage effect was revealed.Research shows that: the dynamic strength of sandstone increased as a logarithmic function with the loading strain rate,while the sensitivity of the dynamic strength of sandstone to the strain rate gradually decreased with the increase of accumulated damage.A dynamic strength decay model based on the accumulated damage factor was established.The dynamic deformation modulus of sandstone gradually decreased with the increase of accumulated damage,while there was no significant relationship with the strain rate.For a given impact load,the presence of accumulated damage causes the sandstone to exhibit a higher strain rate.On the macroscopic level,the accumulated damage intensifies the sandstone fragmentation;on the fine level,the accumulated damage enlarges the surface roughness of the sandstone,indicating that the accumulated damage effect significantly influenced the dynamic deformation damage process of the sandstone.(3)Cyclic impact tests on coal measures sandstone under accumulated damage conditions were carried out.Combined with CT scanning technique,the dynamic damage evolution laws of sandstone during cyclic impact were obtained by introducing the connected porosity variation.Research shows that: it was found that the accumulated damage accelerated the internal damage of sandstone caused by cyclic impact,which made the impact numbers that the sandstone could withstand gradually decrease with the increase of impact load and accumulated damage.With the increase of accumulated damage,the decrease rate of peak stress and deformation modulus of sandstone with the impact number gradually increased.The maximum strain of sandstone gradually enlarged with the increase of impact load and impact number.During the cyclic impact,the sandstone underwent a staged damage evolution process of decelerated growth,stable growth,and accelerated growth.The accumulated damage effect significantly increased the damage level of the sandstone after each impact,while prolonging the decelerated and accelerated growth phases,and shortening the stable growth phase.(4)Based on the rock energy dissipation principles,the energy evolution law of coal measures sandstone with accumulated damage under impact loading was obtained,and the dynamic deformation and damage mechanism of damaged sandstone under impact loading was revealed.Research shows that: with the increase of impact load and accumulated damage,the transmitted energy coefficient of sandstone gradually reduced,while the reflected energy coefficient and absorbed energy coefficient gradually enlarged.The absorbed energy density showed a linear positive correlation with the accumulated damage.The higher the impact load,the greater the increase rate of absorbed energy density with accumulated damage.During the cyclic impact,the absorbed energy density gradually increased with the impact number,and showed a linear positive correlation with the strain rate.At the same time,the accumulated damage effect improved the sensitivity of the absorbed energy density with strain rate.The cumulative absorbed energy density gradually dropped with the increase of accumulated damage.In contrast,when the impact load was higher,the increase in cumulative absorbed energy density indicated an intensification of sandstone fragmentation.(5)The dynamic constitutive model of coal measures sandstone considering the evolution of accumulated damage was established.The process of micro-fracture development under impact loading was considered from a microscopic perspective.Combined with the accumulated damage factor,a damage constitutive model of coal measures sandstone under impact loads was established to accurately describe the effect of accumulated damage on the dynamic stress-strain relationship of coal measures sandstone.Based on the component model theory,a nonlinear viscoelastic constitutive model of sandstone under cyclic impact was established considering the dynamic damage evolution and nonlinear deformation characteristics during the cyclic impact.The applicability of the model was verified by tests,which accurately reflected the variation law of dynamic peak stress of sandstone with the cycle impact number.This dissertation contains 103 figures,15 tables,and 228 references.