Mechanical And Acoustic Response Characteristics Of Sandstone Cracks In Stable Growth Stage With Different Water Conten

The upper limit and lower limit of the stable crack propagation stage are the crack initiation stress and damage stress,respectively,both of which are characteristic stress thresholds of rock deformation response.The former represents the end of the new crack incubation stage,and the latter indicates that the new crack growth enters an unstable state.Therefore,the stable crack growth stage is an important transition period in the rock damage and destruction process,which is of great significance for characterizing the mechanical behavior of rock.At present,most studies on rock damage focus on the whole process of the pre-peak stage,but there are few studies on the evolution law of the stable crack propagation stage and the mechanical properties of the evolution process under different water-containing states.This paper takes dry,natural and water-saturated sandstone as the research object.Firstly,the crack initiation stress and damage stress were measured by the volume strain method,and further combined with the incremental cyclic loading and unloading test,the plastic deformation,energy evolution,ultrasonic velocity,acoustic emission source mechanism and its spatial evolution law in the crack stable growth stage were comprehensively analyzed,the main research contents and conclusions are as follows:（1）Under uniaxial compression,the average peak strength of sandstone samples decreases with increasing water content.The results obtained by the volumetric strain method are as follows:The average values of cracking stress in dry,natural,and saturated states are 50.41%,40.17%,and 38.89%of the peak stress,respectively,and the average values of damage stress are 79.01%,70.68%,and 67.02%of the peak stress,respectively.（2）The results of the cyclic incremental loading and unloading test show that with the increase of the stress level at the unloading point,the axial residual deformation of the sandstone sample continues to increase,and the increase rate first decreases,then stabilizes,and then increases.Among them,the stable growth stage is basically located in the stable crack expansion stage,and the above rules are not affected by the water state.Based on the determination of the characteristic stress range by the volume strain method,the starting point and the end point of the stable stage of the axial residual deformation increase are taken as the crack initiation stress point and the damage stress point,and the residual strain corresponding to the crack initiation stress and damage stress of the dry sample is the lowest,They are about 2.65×10^-3 and 3.45×10^-3 respectively;the residual strains corresponding to the crack initiation stress and damage stress of saturated samples are the highest,about 4.01×10^-3and 4.55×10^-3 respectively.（3）Under the same water content state,with the increase of the stress level at the unloading point,the elastic energy density,dissipated energy density,elastic energy ratio and elastic energy index gradually increase;while the loading and unloading elastic modulus and the ratio of dissipated energy gradually decrease.The above indexes all decrease with the increase of water content.（4）Under the same water content state,as the stress level of the unloading point increases,the wave velocity at the unloading point continues to increase;after unloading,the wave velocity at the initial point of reloading continues to decrease.With the increase of water content,the wave velocity of the unloading point and the starting point of reloading after unloading both decrease.（5）With the increase of stress level at the unloading point,the event rate and energy rate of acoustic emission increased,but the number of accumulative acoustic emission events and energy decreased with the increase of water cut.For sandstones in different water-bearing states,the acoustic emission RA and AF values show that when the stress level at the unloading point is low,tensile cracks dominate;at the end of the stable crack propagation stage,the proportion of shear cracks increases,and then the acoustic emission event rate increases significantly.（6）The results of acoustic emission source analysis show that as the stress level at the unloading point increases,the spatial position of new cracks gradually accumulates toward the main fracture surface,and the degree of accumulation and the number of cracks both decrease with the increase of water content.