基于DIC方法与声发射的花岗岩断裂过程区范围研究

Research on the range of fracture process zone of granite based on DIC and acoustic emission

岩石在断裂过程中容易在裂纹尖端产生断裂过程区(fracture process zone,FPZ),断裂过程区的几何特征决定了岩石材料断裂特征和宏观力学行为,直接影响工程岩体的安全性和稳定性。为研究花岗岩断裂过程区的孕育特征及演化过程,在MTS液压伺服试验机上对带切口半圆盘试样进行三点弯I型加载试验。借助数字图像相关方法(digital image correlation,DIC)和声发射(acoustic emission,AE)定位技术对宏微观尺度下I型裂纹尖端的断裂过程区孕育过程进行研究。采用自编程的声发射信号识别与定位方法,得到微裂纹事件空间分布密度阈值和能量分布,确定微观尺度上断裂过程区的范围和外边界,结合宏观DIC方法得到岩石的断裂过程区的临界应变值εFPZ。同时,使用Irwin-Bazant模型确定试样完全发育的断裂过程区长度,并与试验结果进行对比。结合试样波速,进一步得到裂纹端部临界张开度解析解。结果表明:(1)试验所用岩石断裂过程区临界应变值为1.8‰左右;(2)断裂过程区长度理论值与试验值吻合良好,对于试验所用岩石,断裂过程区的理论长度为5.2~8.7 mm,试验结果为5.0~8.0 mm;(3)断裂过程区的边界可认为是沿裂纹扩展方向声发射能量由高能量突变并维持在低能量的位置。

Fracture process zone(FPZ)is easily generated at the crack tip during the fracture process of rock.The geometry feature of FPZ determines the fracture properties and macro-mechanical behavior of rock.It significantly influences the safety and stability of the rock mass in practical engineering.Mode I three-point bending tests using notched semi-circular bend specimens were performed on MTS testing machine to study the nucleation characteristics and evolution process of the FPZ in granites.To understand the FPZ nucleation process at the tip of the mode I crack,the digital image correlation method(DIC)and the acoustic emission(AE)localization were employed.Micro and macro failure behaviors in rocks were obtained.According to the self-programmed acoustic emission signal identification and localization method,the spatial distribution density of AE events and AE energy were obtained.Then the zone and boundary of the micro-scale FPZ were determined.After which the critical strain valueεFPZ of the FPZ was obtained in cooperation with the DIC method.The Irwin-Bazant model was utilized to determine the fully developed FPZ length of the sample,and the experimental value was compared with the theoretical one.Combining the wave velocity of the sample,the theoretical solution of the critical crack opening displacement(COD)at the crack end is obtained and compared with the experimental results.The results indicated that:(1)the critical strain value of FPZ of the rocks used in the experiment is about 1.8‰.(2)The theoretical value of the length of FPZ is in good agreement with the experimental value.For the rocks used in the experiment,the theoretical length of the FPZ is 5.2–8.7 mm,and the experimental result is 5–8 mm.(3)The boundary of FPZ can be regarded as the position where the AE energy suddenly changes from high to low along the crack propagation direction and remains at low energy.