Research On Rock Mechanical And Acoustic Characteristics And Damage Evolution Law Under Different Loading Rates

The influence of loading rate caused by excavation,blasting vibration and structural compression on rock strength and deformation cannot be ignored.It is of great engineering significance to study the influence of loading rate on rock mechanical properties and acoustic emission characteristics.In this paper,the red sandstone and marble were taken as the research objects,and the uniaxial compression acoustic emission monitoring test with loading rate ranging from 0.001 to 0.1mm/s was carried out.The influence of loading rate on rock mechanical parameters,whole stress-strain curve,macro failure mode,micro fracture mechanism,acoustic emission basic parameters and main frequency were analyzed.G-P algorithm was used to calculate correlation dimension,and fractal characteristics of acoustic emission energy time series were analyzed.Finally,the numerical simulation of rock uniaxial compression fracture process was carried out by RFPA^2D software.The main conclusions are as follows:（1）The whole stress-strain curve of red sandstone has gone through five stages under uniaxial compression:compaction stage,linear elastic deformation stage,microcrack branching and stable growth stage,microcrack unstable growth stage to failure stage and post peak failure stage.When the loading rate is less than 0.05mm/s,the post peak stress-strain curve is mostly step drop shape,and it is smooth and steep curve when more than 0.05mm/s.The failure mode of red sandstone is less affected by loading rate,and the three modes of tensile failure,shear failure and tensile shear combined failure are randomly distributed.From the SEM of red sandstone fracture,we can see the mirror area,foggy area and sawtooth area.With the increase of loading rate,the area of mirror area is reduced,the sawtooth area is rougher and the dislocation is obvious.The loading rate increases from 0.001mm/s to 0.1mm/s,the peak strength of red sandstone increases first and then decreases,which can be expressed by two degree polynomials?_c（28）50.46-21.51lgV-5.98（7）lg V（8）².The whole stress-strain curve of marble has experienced four stages:compaction stage,elastic deformation stage,yield stage and failure stage.The rise slope of the front section of the stress-strain curve is large,while the decline of the back section is slow and has a certain residual strength.The failure modes of marble can be divided into two types:shear failure and tensile shear failure,which are more inclined to shear failure at higher loading rate.The fracture surface of marble is flat and smooth,and the number of cracks is less.With the increase of loading rate,the fracture surface becomes rough,accompanied by the appearance of crack branches.When the loading rate increases from0.001mm/s to 0.1mm/s,the peak strength of marble increases gradually,which can be expressed by two degree polynomials?_c（28）77.93（10）20.57lgV（10）3.75（7）lg V（8）².（2）With the increase of loading rate from 0.001mm/s to 0.1mm/s,the AE peak ring count rate and peak energy rate of red sandstone and marble gradually increased,and the AE cumulative ring count and AE cumulative energy first increased slightly and then decreased significantly.In the early stage of loading,the main frequency of acoustic emission of red sandstone is mainly concentrated in 110¹26 kHz and 136¹51 kHz.When it is close to failure,it will expand to 80¹29 kHz,136¹89 kHz,210²53 kHz,295³00 kHz and 311³31 kHz.The phenomenon that the main frequency of acoustic emission signal increases and tends to be dense can be regarded as the sign of fracture instability of red sandstone.The main frequency of acoustic emission of marble is mainly concentrated in the four frequency bands of 100¹28kHz,138¹57 kHz,281³26 kHz and 360³86 kHz.With the increase of loading rate,the main frequency concentrated in the post peak failure stage.（3）The correlation dimensions of AE energy at different loading rates all show a trend of increasing first and then decreasing,indicating that red sandstone and marble are in an orderly process from uniaxial loading to peak strength.With the increase of loading rate,the stress level at the peak point of correlation dimension of AE energy of red sandstone shifts from（40-50%）₍（8） to（30-40%）₍（8）.The stress level at the peak point of correlation dimension of AE energy of marble decreases first and then increases.（4）Under different loading rates,the characteristic stress value determined by AE method is smaller than that determined by crack strain model method.The normalized characteristic stress determined by the two methods is the closest when the loading rate is 0.001mm/s,and the difference increases with the increase of loading rate.The loading rate increases from0.001mm/s to 0.1mm/s,the normalized characteristic stress decreases generally.The damage variable curve D₁ calculated based on AE ringing count has the same change trend as that D₂calculated based on AE energy,and the damage acceleration points at different times can correspond well.At the same time,the damage variable D₁ is larger than the damage variable D₂.AE ring count is more suitable for the calculation of rock damage model than AE energy.（5）The stress-strain curve,AE events curve and rock failure mode simulated by RFPA²DD software are in good agreement with the results of laboratory test.With the increase of loading rate,the damage time in the rock is gradually shortened.The acoustic emission signal in a single loading step is more,and the acoustic emission activity is more intense The spatial evolution characteristics of acoustic emission of rocks under different loading rates can be divided into three stages.In stage I,the acoustic emission signal increases slowly from nothing,and appears randomly in various positions of rocks.In stage II,the acoustic emission signal is distributed intensively,and the transformation from disorder to order is completed gradually,at this time,macro cracks have appeared in rocks.In stage III,the acoustic emission signal is concentrated on the main fracture surface.