Experimental Study On Dynamic Characteristics Of Rocks With Different Strength Under Cyclic Loading

Shallow bedrock site is one of the common engineering sites,especially the nuclear power structure is usually given priority to when the site is selected.At the same time,in order to achieve the national "carbon peak" and "carbon neutral" development strategy,the construction of nuclear power facilities increased demand.Because of the frequent occurrence of earthquakes in our country,the mechanical properties of rocks under earthquake are very important for the safety of nuclear power structures in shallow bedrock.Therefore,the cyclic loading and unloading tests on rock specimens of different strength are carried out by using the microcomputer servo rock triaxial testing machine TAW-2000.The three rocks are soft rock(mudstone),soft rock(white sandstone)and hard rock(red sandstone)respectively.The cyclic load is cosine wave of 0.2Hz-2Hz,and the test confining pressure is 0.5 MPa and 1MPa.The main research results and conclusions are as follows:(1)Under cyclic loading,shear failure is the main failure mode of the three rocks,but there are still some differences.For mudstone,when the surrounding rock is 0.5MPa,with the increase of loading frequency,the failure mode transits from shear failure to splitting failure.For red sandstone,the longitudinal cracks increase obviously with the increase of loading frequency.The frequency and confining pressure have no obvious effect on the failure mode of white sandstone.(2)Compared with the static compressive strength,the dynamic compressive strength of white sandstone and red sandstone has been significantly improved.Relatively speaking,the dynamic compressive strength of white sandstone has increased more,up to 71%,and red sandstone has less increased,up to 56.7%.,And the dynamic compressive strength of mudstone decreases first and then increases with the increase of cyclic load frequency.Under the same confining pressure,as the loading frequency increases,the dynamic compressive strength of mudstone generally increases,the white sandstone first decreases and then increases,and the dynamic compressive strength of red sandstone generally increases first and then decreases.(3)The shape of hysteresis curve of three kinds of rocks is different.The hysteresis curve of mudstone is similar to "parallelogram" and has obvious plastic section(no deformation recovery during unloading),while the hysteresis curve of white sandstone and red sandstone is similar to "boat" and has obvious slope abrupt change during unloading.(4)The dynamic elastic modulus of mudstone decreases with the increase of loading amplitude,while the dynamic elastic modulus of white sandstone and red sandstone increases with the increase of loading amplitude,showing significant strengthening characteristics.The maximum dynamic elastic modulus of white sandstone and red sandstone occurs when the dynamic stress amplitude reaches about 70% of the compressive strength of rock.The damping ratios of three kinds of rocks decrease gradually with the increase of loading amplitude,and finally tend to be stable.(5)The relationship between the actual maximum dynamic shear modulus of the three kinds of rocks and the dynamic shear modulus calculated according to the elastic theory is obtained.The Davidenkov viscoelastic constitutive model based on sand is introduced to describe the dynamic characteristics of the rock,and using the results of the triaxial dynamic test of the rock under cyclic loading,the value ranges of the three rock constitutive model parameters are given.