峰前峰后循环加卸载对砂岩动力特性的影响

Effect of Cyclic Loading and Unloading on Dynamic Characteristicsof Sandstone before and after Peak Load

为深入分析砂岩峰前和峰后不同阶段的动力特性规律,开展单轴分级循环加卸载试验,综合分析下限应力对砂岩峰前峰后不同加卸载阶段的表观弹性模量、滞回圈面积、阻尼比、阻尼系数及动弹性模量等相关动力参数的影响。研究结果表明:①岩样在峰后因内部损伤加深和累积,相同下限应力下滞回曲线位于峰前右侧,且应变量增幅和最大应变增幅均随下限应力的增加而逐渐减小,说明峰后岩样虽然强度已达到极限状态,但只要岩石未完全破坏失稳,其致密性仍可以在高应力条件下的循环加卸载中得到增强。②砂岩在峰前峰后循环加卸载下的变化过程,可以通过表观弹性模量曲线斜率a值的变化得以体现,低下限应力循环加卸载试验中,致密性增强系数a_(1)>致密性劣化系数a_(2),斜率a均为正值,轴向应力对岩石整体的压密作用明显强于劣化损伤作用;随着下限应力升高,a_(2)的降低速率高于a_(1)的升高速率,a值逐渐减小甚至变为负值,说明轴向应力对岩石整体的压密作用逐渐弱于劣化损伤作用。③4组不同下限应力条件下,岩样峰后的滞回圈面积、阻尼比、阻尼系数均高于峰前状态,而动弹性模量低于峰前状态,说明岩样在峰后循环加卸载过程中内部劣化损伤程度高于峰前。

To comprehensively examine the dynamic characteristics of sandstone at different stages before and after the peak,we conducted uniaxial graded cyclic loading and unloading tests.These tests aimed to analyze the impact of the lower limit stress on various parameters,including the apparent elastic modulus,hysteretic loop area,damping ratio,damping coefficient,and dynamic elastic modulus of sandstone during the loading and unloading stages.The research findings indicate the following:1)Due to the aggravation and accumulation of internal damage after peak,the hysteretic curve lies on the right side before the peak at the same lower limit stress.With an increase in the lower limit stress,the increments in strain and maximum strain gradually decrease.This suggests that despite limit strength has been reached after peak,cyclic loading and unloading under high-stress conditions can enhance the compactness of post-peak rock samples,as long as the rock remains intact and stable.2)The changes in sandstone under cyclic loading and unloading before and after peak load can be observed through variations in the sloping(a)of apparent elastic modulus curve.At small lower limit stress,the coefficient of compactness enhancement(a_(1))is greater than the coefficient of compactness degradation(a_(2));additionally,the slope a is positive,indicating that the compressive effect of axial stress on rock mass is significantly stronger than the effect of deterioration damage.As the lower limit stress rises,the rate of decrease in a_(2) is higher than that of a_(1),suggesting that the compressive effect of axial stress gradually weakens compared to that of deterioration damage.3)Under four different lower limit stress conditions,the hysteretic loop area,damping ratio,and damping coefficient are higher after the peak than before,while the dynamic elastic modulus after peak is lower than before.These findings suggest that the degree of internal deterioration damage in rock samples during cyclic loading and unloading after the peak is higher than that before the peak.