| In the process of blasting construction,the rock mass in the range of working face are destroyed or stripped under the effects of the blasting stress wave;Although the rock mass at a certain distance from working face are not destroyed under the effects of blasting stress wave,it would be damaged,then destroyed by the effects of multiple blasting stress wave.In this process,the dynamic mechanical properties of rock under the effects of single stress wave and the damage evolution and strength attenuation characteristics of rock under the effects of multiple stress waves are involved.In recent years,the relevant scholars have made comparatively extensive research on the mechanical properties of rock under the effects of single stress wave,but the study of damage evolution and the changes in mechanical properties under the effects of multiple stress waves are obviously insufficient.In this paper,sandstone was used as the test object to study its dynamic mechanical properties and damage evolution under stress wave.The separate Hopkinson pressure bar(SHPB)device is an effective method to study the rock under stress wave,but it should be consistent with one-dimensional stress wave assumption and stress uniform assumption during the test.Considering that the sandstone has the characteristics of fine particle composition,the incident and transmission bar with the diameter of 50 mm and the length of 2000 mm were selected as bar system to satisfy the one-dimensional stress wave assumption.And stress uniform assumption requires a matching effect between the bar of SHPB device and the specimens.Based on the physical process of SHPB test,it was found that the wave impedance ratio between the specimens and the bar was the main factor that affects the stress uniformity.And based on it,the reflection-transmission times of the stress wave in the specimens was obtained when the stress balance was established in the specimen under different wave impedance ratio.And the conclusion was made that the aluminum alloy bar was the suitable system for the sandstone impact test,and the wave impedance ratio between the sandstone and aluminum alloy was 0.63 and the stress balance was established at 24?s by calculation.Wavelength and amplitude are two major parameters of stress wave,which affect the dynamic mechanical properties and the damage evolution law of sandstone in the test.In this paper,five kinds of stress wavelengths,which are 0.5m,0.8m,1.2m,1.6m and 2.0m,were selected and the single impact and cyclical impacts tests were carried out to obtain the dynamic mechanical properties and the relationship between damage evolution and stress wave parameters of sandstone.In the single impact test,the dynamic mechanical properties such as dynamic peak strength,maximum strain,loading strain rate and impact damage,and the relationship to stress wave parameters were observed at different wavelengths;In the cyclical impacts test,the dynamic mechanical properties such as dynamic peak strength,maximum strain,loading strain rate and damage evolution,and the relationship to impact times were observed at different wavelengths under the same incident energy.The test results were summarized and the following conclusions were drawn:(1)Under the single impact,the dynamic constitutive curves of sandstone were divided into four stages: linear elastic stage,new fracture formation stage,plastic reinforcement stage and unloading stage.The damage modes of sandstone were mostly splitting damage,which was the tensile stress damage of Poisson Effect,with multi-valve damage or even crushing damage.The dynamic strength of sandstone had a good correlation with strain rate,and the fitting relationship is (?_d=?_s/9.66+36.398exp(89.9(?)/(?)_s).The dynamic strength increase factor(DIF)of sandstone was in the range of 1 to 1.91.(2)The acoustic velocity variation of the sandstone before and after the impact was adopted as the representative values of damage,and the test results showed that under single impact,the damage of sandstone and stress wave amplitude were exponentially correlated and influenced by the change of wavelength,and using the model based on exponential function to represent the damage evolution,which was D=?·exp(?·?_i/?_s)+?.In this model,the increase of ? value makes the curve slope increase smoothly within the range of the whole incident wave amplitude,and the physical meaning of ? is the early damage rate factor under the influences of stress wavelength;the increase of ? makes the late damage development rate increase with the increases of radiation amplitude value,and its physical meaning is the late damage rate factor under the influences of incident wave amplitude.And the values of ?,? and ? in the model parameters: with the increase of the wavelength,the value of ? and ? both increase but ? decrease.(3)By 500 mm,800mm,1200 mm,1600mm and 2000 mm these 5 kinds of wavelength impact test,the damage threshold of sandstone was 45 J which was obtained by the damage threshold representative method of sandstone that using incident energy as the unified representative value.Under single impact,with the increase of the wavelength,the amplitude of the incident wave at the beginning of the damage decreased gradually,which resulted in the decrease of the minimum impact velocity and the increase of damage development speed of sandstone.The shorter the wavelength was,the slower the damage development rate was,and the higher the wavelength was,the faster the damage development rate was.At the same time,the increase of the wavelength also affected the increase of damage with the incident stress wave amplitude,and the shorter the wavelength was,the smaller the damage increment under the same incident wave amplitude increment was,and the denser the points in the coordinate system of was;the higher the wavelength was,the greater the damage increment was,and the more sparse the points in the coordinate system was.(4)Under cyclical impacts,the increase of impacts times made the reflection wave amplitude and the area enclosed by the reflection wave and the abscissa axis gradually increase,while the transmission wave amplitude and area enclosed by transmission wave and the abscissa axis gradually decrease.The analysis of the constitutive curve under cyclical impacts showed that with the increase of impact times,the peak stress decreased and the strain increased gradually,and with the increase of impact times,the elastic stage of sandstone decreased gradually,and the decreased amplitude increased with the increase of wavelength.The peak strength showed convex decrease in the linear type with the impact times,and with the increase of the wavelength,the slope of descending-section curve increased.The relationship between the strain rate and the impact times was exponential growth,and the maximum strain was positively correlated with the impact times and the higher the wavelength,the greater the maximum strain was.In the form of damage,only one fracture surface of sandstone specimens under cyclical impacts was the main difference with the fracture form under single impact.(5)Under cyclical impacts,the cumulative damage showed the form of rapid rise-steady development-rapid rise,and through the inverse transformation of the Geppitz Function to express the characteristics of the three stages of the sandstone accumulative damage,which as the model of sandstone damage evolution under cyclical impacts,with the expression:D=?-?·ln(ln(?)).The ? controls the length of the steady development stage,and its physical meaning is the influence factor of the cumulative damage affected by the wavelength;? controls the size of the independent variable in the later rapid rise stage,and its physical meaning is the influence factor of the cumulative damage affected by the impact times.When the relative impact times are used as independent variables,? is the fixed value of 1;the value of ? affects the initial cumulative damage in the first rapid rise stage,and its physical meaning is the regulatory factor of the cumulative damage.(6)According to the similar mechanical properties of rock and concrete,the feasibility of the HJC model used for rock dynamic simulation was described.The parameters of HJC model were divided into five parts: basic mechanical parameters,limit surface parameters,pressure parameters,damage parameters and rate effect parameters,and the test method of constitutive model parameters of sandstone were proposed.(7)The HJC constitutive model parameters of the sandstone were obtained by the uniaxial static test,three axial static load test,single impact test and repeated impact test,and these parameters are used in the SHPB simulation calculation.The calculated results showed that the incident wave was in accordance with the one-dimensional wave theory,and the stress-strain curve elastic stage,the dynamic peak strength and the maximum strain of specimens were consistent with the test results.It was shown that the dynamic constitutive model of sandstone can describe the dynamic mechanical properties under the impact. |
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