Experiment Research On The Deformation Characteristics And Damage Variable Expression Of Marble Under Cyclic Load

In order to study the volumetric deformation,crack volumetric strain,capacity expansion,and deterioration damage characteristics of marble under cyclic loading and unloading test,an electro-hydraulic servo rock triaxial test system was used to perform stepwise and constant amplitude cyclic loading and unloading tests of marble under different confining pressures.The main research work of the thesis includes three aspects: stepwise cyclic loading and unloading test,constant amplitude cyclic loading and unloading test and damage variable discussion.(?)Stepwise cycle loading and unloading test study(1)proceeding from the stress-strain curve of marble stepwise cycle loading and unloading test,the related mechanical and deformation properties of marble are analyzed.With the increase of the number of cycles,the elastic modulus of marble increases first and then decreases,and the Poisson's ratio slowly increase first and then increase rapidly,increases from about 0.2 to about 0.5.the total volumetric strain and crack volumetric strain at each time are separated from the full curve,and the effect of each cycle on the volume strain and volumetric strain of the fracture is analyzed.It is found that a new phenomenon that the closing threshold of fracture,the threshold of crack initiation and the threshold of crack development almost coincide with the increase of the cycle times.It is noted that with the increase of the confining pressure,the proportion of the elastic volumetric strain in the total volumetric strain becomes smaller and smaller at the time of the peak stress,The proportion of crack volumetric strain is getting bigger.Relative to monotonic loading,cyclic loading and unloading causes marble to produce more irrecoverable secondary fractures.(2)Compare the dilatancy angle and expansion index of marble under different confining pressures in conventional loading and cyclic loading and unloading tests.When the confining pressure is small,the expansion indexes of monotonous loadingand cyclic loading and unloading test are similar.When the confining pressure is large,the expansion index of cyclic loading and unloading test is much larger than that of monotonous loading test,and the failure mode changes from shear failure under monotonous loading test to drum shear failure under cyclic loading and unloading test.The fitting formula for the expansion index of the monotonous loading test was changed to obtain a fitting formula that complied with the cyclic loading and unloading test.(3)The hysteresis loop of each loading and unloading curve is extracted and the energy of the hysteresis loop is computed.It is found that as the upper limit of the cyclic stress level increases,the shape of the hysteresis loop changes from a slender willow leaflet to a full shuttle,and the energy dissipating capacity and energy dissipation rate of the hysteresis loop also increase slowly then increase rapidly with the increase of the upper limit of stress level.(?)Constant amplitude cyclic loading and unloading test(1)The residual axial strain,residual volumetric strain,average elastic modulus and Poisson data of marble in constant amplitude cyclic test were statistically analyzed,and the rule of change with the number of cycles was studied.The variation of the residual axial and volumetric strain shows two different changing laws with the difference of confining pressure and stress ratio level: 1)The rock samples,which are destroyed in the cycle,the residual axial strain and the residual volumetric strain increases rapidly as the number of cycles increases and then increases slowly,rapid increase finally;2)The rock samples,which are not destroyed in the cycle,the residual axial strain and the residual volumetric strain of them increase firstly,then gradually slow down and remain stable with the increase of the number of cycles.The variation of the average elastic modulus and Poisson's ratio also presents two different changes with the number of cycles under different confining pressure and stress ratio level.1)The rock specimens that are destroyed in the cyclic test,the elastic modulus increase then decrease gradually with the increase of the number of cycles,but the Poisson's ratio increases rapidly then gradually slows down and finally increases rapidly with the increase of the number of cycles;2)Therock specimens that have not been destroyed during the cycle test,the average elastic modulus increases rapidly and gradually slows down with the number of cycles,and The variation of Poisson's ratio is similar to the average elastic modulus.(2)During the single loading process,the axial strain rate of marble decreases with the increase of axial strain and stress,and it shows a flat "U" rule of gradual increase,it gradually decreases first and remains relatively stable and gradually increases..By contrast,it is found that the axial strain rate of the marble at the initial stage of each loading can reflect the cracking process of marble.(3)The axial strain rate at the initial stage of each loading of marble in each test group was calculated.It was found that the axial strain rate at the initial stage of each loading of the marble specimens subjected to two different patterns of change with the number of cycles.The rock specimens that are destroyed in the cyclic test:1)When the confining pressure is low and the stress ratio is relatively large,it reduce first and increase rapidly with the increase of the number of cycles;2)When the confining pressure and stress ratio are relatively low,it reduce first,then gradually increase and slow down and finally increase rapidly with the increase of the number of cycles.The axial strain rate at the initial stage of each loading of marble samples not destroyed in the cycle shows two different variations with the increase of the number of cycles: 1)With the increase of the number of cycles,the axial strain rate decrease then increase gradually and keep stable;2)with the increase of the number of cycles,the axial strain rate reduce first then increase rapidly,after that gradually decrease again and remain relatively stable.(?)Discussion of damage variables(1)The shortcomings of existing damage variables for damage calculation are analyzed.A new calculation method of damage variable is defined.Combined with stepwise cyclic loading and unloading test and constant amplitude cyclic loading test,the practicability and superiority of new redefined damage variables are verified.(2)The failure mode of marble specimen are all shear failure modes,which damage caused by monotonic loading,final destruction with axial stress increase without occur in the process of circulation and cycles are destroyed by a few times.The damage variables of this type of rock sample are all less than 1.The powder particles were the most between the shear surfaces that who are broken under the cycle test,The powder particles were the least between the shear surfaces that who are broken under monotonic loading test.The failure modes of marble samples with a large number of cycles of failure are all “X” type failure modes,and there are a large number of fragments and granular powders,and the damage variables are all greater than 1.