Experimental Research On Dependence Of Sandstonr Mechanical Properties And Failure Characteristic Load Rate

The impact of load rate changes induced by rock excavation,blasting vibration,tectonic stress,temperature or water level changes on the strength,deformation,damage and destruction of rocks cannot be ignored.The effect of load rate on rock mechanical properties and damage and failure characteristics should not be ignored.It has important practical significance for safe construction and rock engineering disaster prediction.The paper takes Junan sandstone as the research object,carries out conventional uniaxial loading and cyclic loading acoustic emission testing tests,and analyzes the full stress-strain curve,mechanical parameters,energy evolution,characteristic stress change law,and macroscopic failure mode of the rock at different loading rates.The influence of isomechanical characteristics;the rock fragments under different loading rates are screened and combined with fractal theory,from the perspectives of fragment size distribution and fragment mass distribution,the relationship between the load rate dependence of sandstone failure characteristics is obtained;In the process,acoustic emission technology was used for detection,and the influence of loading rate on acoustic emission parameters was analyzed;the GR relationship was used to study the characteristics of acoustic emission b-value under different loading rates;an acoustic emission space fractal dimension was proposed to quantitatively describe the rock damage state Methods.The research results obtained are as follows:Under conventional uniaxial loading,the strength of sand and gravel,the axial strain corresponding to the peak stress,the modulus,etc.,show an obvious dependence on the load rate,and they all increase with the increase of the load speed;the larger the load rate,except for the closure stress,The earlier other characteristic stresses appear,the closer the elastic energy curve is to the total energy curve,and the closer the dissipated energy curve is to the strain axis;U~d/U~e first decreases and then increases during the whole process,and the minimum value appears at the end of the elastic phase.Under cyclic loading,the"memory"of sandstone decreases with the increase of stress level,and increases with the increase of load rate;residual strain is negatively related to load rate,and is affected by load rate,stress magnitude and cycle times;different unloading Under stress,the greater the loading rate,the lower the input energy.When loading to failure,the greater the loading rate,the higher the input energy;as the stress increases,the ratio of the elastic deformation energy to the input energy first increases and then decreases.The plastic deformation energy and the input energy The ratio of is first decreased and then increased,and the maximum ratio of the plastic deformation energy to the input energy appears in the initial loading stage.The fragmentation degree of a rock sample is dependent on the loading rate.The larger the loading rate,the smaller the characteristic value of fragmentation,and the more fragmented the rock sample is.The distribution of fragmentation and the loading rate have fractal characteristics,and the fractal dimension increases with the load rate.The logarithm has a good linear relationship;the lower the degree of fragmentation,the smaller the fractal dimension,and the fractal dimension D_b is negatively correlated with the eigenvalue dm of the fragmentation.The loading rate has a greater impact on the fractal dimension of the fragmentation than on the eigenvalue of the fragmentation..Acoustic emission ringing is mainly concentrated in the compaction stage and the damage failure stage.With the increase of the loading rate,the amplitude of the elastic stage increases significantly.Increasing the load rate will weaken the phased phenomenon of the amplitude;the b value of the uniaxial loading elastic stage is at a relatively high level.During the damage and destruction stage,the b value will be greatly reduced.The b value is negatively linearly related to the logarithm of the load rate.The cumulative frequency calculation of the b value is more accurate than the differential frequency;the b value is between 2 and 4 under cyclic loading.,The smaller the load rate,the more obvious the b-value jump.As the load rate increases,the b-value jump occurs later or even disappears;the b-value drops briefly,which can be regarded as a large-scale change in rock cracks,and the b-value continues to fall.It is regarded as a precursor to rock failure;the spatial distribution of acoustic emission events has fractal characteristics,the dimension changes in the interval[2,3],and the dimension is negatively related to the load rate.The increase in the load rate affects the number of acoustic emission events in the rock sample.Spatial distribution is a dimensionality reduction process.The larger the dimensionality,the more uniform the damage spatial distribution,and the smaller the dimensionality,the more concentrated the damage spatial distribution.