不同预静载硬岩在动力扰动下断裂特性的试验研究

Experimental study on fracture behaviors of hard rock under dynamic disturbance with different pre-static loads

为了研究不同预静载水平下硬岩受不同扰动频率载荷的断裂特性变化规律,采用红砂岩制作中心直裂纹半圆盘三点弯曲试样(Semi-Circular Bend,SCB),利用MTS Landmark电液伺服试验机,开展"不同预静载+动力扰动"下砂岩的断裂韧度测试试验。以岩石静载断裂载荷为基础,选取4个预静载水平(60%,70%,80%和90%的静载断裂载荷)作为初始静载状态,之后分别施加扰动载荷直至试样断裂。扰动加载方式采用正弦波载荷,加载波幅值依次为静载断裂载荷的40%,30%,20%和10%,设置扰动频率分别为1,10,20,30,40和50 Hz。研究结果表明,不同预静载水平下岩石的断裂韧度值较常规加载下都有不同幅度的减小,并且随着扰动频率的增加,岩石断裂韧度呈线性减小的趋势。但不同预静载水平之间的减韧效应存在差异性,预静载水平越高,减韧幅度越大。与常规加载条件下相比,预静载水平下的断裂位移整体具有减小的趋势。岩石扰动断裂点的分布区段具有明显的集中性,当预静载水平低于90%时,断裂点位于预应力水平之上的加载段,而当预静载水平为90%时,断裂点多数位于预应力水平之上的卸载段,并且基本上没有断裂破坏发生在预应力水平之下。断裂疲劳寿命随扰动频率的增加出现先增大后减小再增大的趋势,整体呈增加趋势。断裂破坏时间跟预静载水平、扰动频率之间的相关性不明显。综合考虑认为,扰动频率为10 Hz时达到相对最优的致裂效果。

In order to investigate the variation of fractures of pre-static loaded hard rock under the different dynamic disturbance, the fracture toughness tests of sandstone were carried out on three point bending specimen with a straight crack in the center of the red sandstone（SCB） using the electro hydraulic servo testing machine Landmark MTS. Four pre-static load levels（60%, 70%, 80% and 90% of conventional static fracture load） were selected. The dynamic disturbance tests were conducted by using the sine wave loading. The amplitude of loading were 40%, 30%, 20% and 10% of the static fracture load and the loading frequency were 1, 10, 20, 30, 40 and 50 Hz respectively. Compared with in the conventional static fracture tests, the fracture toughness of rock under different pre-static loads decreased and it decreased linearly with the increase of the disturbance frequency. However, the toughness decreasing effects under the different static pre-loadings are different. The higher the pre-static level, the greater effect of the toughness decreasing. Compared with the conventional fracture displacement, the fracture displacement under pre-static loadings decreased as a whole. The concentrated distribution of disturbance induced fracturing point was found. The fracturing point occurred in the loading stage above the pre-static level when the pre-static load level is less than 90%, while the fracture point occurred in the unloading stage above the pre-static load level when the level of pre-static load is 90%. With the increase of disturbance frequency, the fatigue life span increased firstly, then decreased and finally increased again with an overall trend of increasing. No obvious correlations between the time of fracturing failure, the pre-static load level and the disturbance frequency were found. The cracking effect is about optimal when the disturbance frequency is 10 Hz.