Characteristics Of Acoustic Emission In Yellow Sandstone With Different Grain Sizes Under Uniaxial Compression

As a kind of natural and porous medium material,rock has different mechanical properties and acoustic emission response characteristics due to the different physical properties of various mineral components and the uneven distribution of microcracks.At the same time,acoustic emission technology has been widely used in rock engineering and other fields,but due to the complexity of rock materials,the theoretical research of acoustic emission lags behind the engineering application.Therefore,this paper selects yellow sandstone with different particle sizes to carry out acoustic emission tests under uniaxial compression.Through acoustic emission positioning,moment tensor inversion theory and microscopic imaging technology,the influence of mineral particle size and composition on the acoustic emission response characteristics of rock is analyzed.On this basis,the relationship between the existence of acoustic emission quiescent period and its particle size and mineral composition was explored,and compared with the samples without the relative quiescent period of acoustic emission,the evolution characteristics of microcracks before and after the relative quiescent period of acoustic emission were analyzed,so as to reveal the generation mechanism of the relative quiescent period of acoustic emission.The main research contents and results are as follows:(1)By means of optical microscope,X-ray diffraction,nuclear magnetic resonance instrument and other technical means,in different particle size of yellow sandstone microstructure and pore characteristics of the specimens were determined,the analysis of coarse grain yellow sandstone specimens of calcium cementation oolite granular texture,grain,fine grained yellow sandstone in the specimen of argillaceous cemented sand granular structure,at the same time,with the increase of structural rock particle size specimen showed a trend of small pores in the pore to change;The mechanical properties and acoustic emission characteristics of yellow sandstone specimens were studied by laboratory tests.The results show that the peak stress of yellow sandstone decreases gradually with the increase of particle size.(2)The moment tensor inversion theory was used to identify the AE focal mechanism during the test,and it was found that the failure mode of all specimens was dominated by tensile crack.With the increase of particle size,the proportion of shear crack increases gradually.(3)By comparing the AE Ib value and the average AE energy rate of yellow sandstone specimens with different particle sizes in the process of deformation and failure,the average AE energy rate of all specimens has the phenomenon of "surge" and "sharp drop" before the peak failure,and the maximum average AE energy rate gradually decreases with the increase of the rock mineral particle size.When the sandstone specimen is critical to failure,the AE Ib value decreases relatively.When the average AE energy rate appears "surge",the AE Ib value drops to the minimum value,indicating that the rock specimen is about to undergo instability failure.As the fluctuation trend of acoustic emission Ib value of coarse-grained yellow sandstone is different from that of medium-grained and fine-grained yellow sandstone,it is divided into different stages.(4)The analysis of different particle size of yellow sandstone specimen failure process AE response characteristics of coarse grain yellow sandstone does not appear in the process of deformation and failure phenomenon of AE relatively quiet period,while the coarse and fine grained yellow sandstone are multiple occurrences of the AE phenomenon of relatively quiet period,and with the decrease of the structure rock particle size,the first appearance of AE relatively quiet period of time "in advance".(5)The box dimension method was used to characterize the macroscopic cracks on the surface of the specimen after failure.It was found that the dispersion of the fractal dimension of the macroscopic cracks was gradually decreasing,and the particle size of the structure-rock mineral was negatively correlated with its fractal dimension.Combined with the macro-mesocrack evolution characteristics of yellow sandstone,it is revealed that when AE is relatively calm during the test,the mineral particles inside the specimen are mainly transgranular fracture mode.