花岗岩材料在动静组合加载下的断裂机理研究

In the process of deep mining and deep digging, the top of sap subject to not only high levels of static pressure, but also dynamic loads such as explosions and micro earthquakes, which may cause disasters such as tunnel collapse. Recent reports show that the total strength of the preload static stress and the measured dynamic strength doesn't equal to neither the pure static strength nor the pure dynamic strength with similar loading rate, because the static load changes the micro cracks in rock, and then changes the mechanical properties. It is thus required to study rock fracture mechanics subjected to coupled static and dynamic loads. This project will set up the experimental techniques of coupled static and dynamic loads with the Split Hopkinson Pressure Bars (SHPB) system, such as the calculation of stress intensity factors in the crack tip with coupled static and dynamic loads. Two kinds of granite samples, the Cracked Semi-Circular Bending (NSCB) specimen and the cracked Chevron Notched Semi-Circular Bend (CCNSCB) specimen, are selected in this project. The micro cracks distribution and the micro crack opening process with preloads in the granite can be obtained by Micro-CT and acoustic emission (AE) methods. The crack initiation and propagation properties will be investigated with different preload static loading conditions. Together with the experimental results of quasi-static and dynamic experiment results, a constitutive model of Statistical Crack Mechanics (SCRAM) will be developed. This constitutive model can be embedded in the sub program of user's mat. In this way, the experimental process can be simulated by the commercial software LS-DYNA with our constitutive model. The constitutive model validate by comparing experimental results and the simulation results. The main aim of this project is to work out these two key scientific problems: the fracture toughness and fracture propagation properties of rocks subject to coupled static and dynamic loads, and the SCRAM constitutive model and the way to obtain its parameters.

在深部工事开挖过程中，坑道顶板不仅承受着很高的静载荷，还受到爆破、震动等动载荷的影响，容易诱发塌方等灾害。现有研究表明，在初始静载荷与动载荷叠加后，其强度准则不能简单叠加。这是因为静载荷导致岩石内部微裂纹发生变化，进而影响其力学性能。因此需要研究岩石材料在动静组合加载下的断裂机理，为工程设计提供理论依据。本研究首先建立基于霍普金森杆的动静组合加载实验方法，研究动静组合加载下裂尖应力强度因子的求解方法。选取一种花岗岩材料，采用两种不同预制裂纹方式的半圆盘三点弯试样构型，通过Micro-CT结合声发射实验技术观测静载荷下花岗岩内部微裂纹分布及变化规律；分析不同动静组合条件下裂纹的起裂韧度和传播机理。结合常规的力学实验，建立统计裂纹模型，并将计算预测结果与实验结果进行比较分析，验证模型的有效性。重点解决花岗岩动静组合加载下的裂纹起裂扩展规律研究和花岗岩统计裂纹模型及参数获取方法两个关键科学问题。

随着矿山开采深度增加和开采条件趋于复杂，以及越来越多的水利水电、交通、国防和基础物理实验等工程在深部和构造活动区兴建，岩石常受到地应力静载和爆破、震动等动载的影响。岩石材料在动静组合加载下的力学性能成为深部岩石工程所必须要考虑的一个关键问题。本研究首先根据实验需要和实验室现有条件，搭建了基于霍普金森杆试验平台的动静组合加载试验系统。研究选取了两种试样（NSCB和CCNSCB）分别进行了不同方面的断裂性能研究。通过Micro-CT实验技术观测花岗岩内部微裂纹分布规律。对试样进行了准静态及动态SHPB试验，提出了一种修正带几何缺陷试样的SHPB动态强度测试的方法，采用本研究提出的修正方法不仅能够极大的降低由于几何缺陷引入的测量误差，而且操作起来简单、可行。对于NSCB试样，其无量纲应力强度因子的计算是基于ANSYS有限元仿真平台建立二维模型，并对裂尖建立1/4节点模型进行数值求解。动态试验中施加的预应力分别是NSCB试样最大静载荷的0%、37%、74%，实验表明：岩石材料动态起裂韧度会随初始静载荷的增加而降低，但是总的断裂韧度会提高，即在有初始静载条件时，岩石材料的起裂会更加容易。对于CCNSCB试样，采用子模型法结合1/4节点模型进行建模和无量纲应力强度因子的数值求解，并对裂纹区域进行了更为精确的应力场模拟。三组试验施加的预应力分别是CCNSCB试样最大静载荷的0%、45%和90%，实验表明：初始静载荷对裂纹的传播过程并无影响，传播韧度与初始预应力无关。最后，建立了一种描述岩石材料的统计裂纹规律的本构模型，并将该模型嵌入LS-DYNA中去，该模型除了可以模拟岩石材料的力学性能外，还可用于模拟混凝土等准脆性材料的力学性能。