不同荷载条件下低孔隙率砂岩巴西劈裂试验的声发射特性

针对巴西圆盘荷载接触条件对巴西劈裂试验影响的问题，采用声发射监测系统开展线/非线荷载接触条件下低孔隙率砂岩巴西劈裂试验。直径为50 mm，厚度为25 mm的标准巴西圆盘按照同一种传感器三维布设方式布置8个Nano30传感器。在相同的荷载速率下，声发射监测Richter8系统对线/非线荷载两种荷载条件下的巴西圆盘进行准静态加载的波形信号连续记录。通过P波自动到时及网格坍塌搜索算法进行定位，在线/非线荷载条件下分别有1131和931个声发射事件被成功定位。圆盘的起裂位置均在圆盘非中心位置，对于非中心起裂的试验值可能低估了巴西抗拉强度。裂纹下半球极点密度投影分析表明，非线荷载条件下破裂面的局部扭曲程度大于线荷载。试样三维损伤演化结果表明，圆盘所受荷载面积大小，显著影响圆盘损伤累计的时间、释放能量的大小和裂纹扩展的稳定性。对有效声发射定位事件进行矩张量分解获取了两种荷载条件下各向同性部分（ISO）、纯双力偶（DC）和补偿线性矢量偶极成分（CLVD）频率百分比，并采用微裂纹破裂类型分类方法来定量分析震源机制，结果表明巴西劈裂对荷载条件并不敏感，两者均可以解释为近似平行于荷载方向上的张拉裂纹的萌生、扩展及贯通。 

Acoustic emission characteristics of Brazilian test for low-porosity sandstone under different load conditions

In view of the influence of the load contact conditions on Brazilian test results, the acoustic emission (AE) monitoring system was used to conduct a Brazilian test of hard and brittle low-porosity sandstone under linear/non-linear load contact conditions. The standard Brazilian discs with a diameter of 50 mm and a thickness of 25 mm were instrumented with a three-dimensional sensor array containing eight Nano30 sensors. All the discs were equipped with identical three-dimensional sensor arrays. At the same load rate, the Brazilian discs were quasi-statically loaded under both linear/non-linear loads. The Richter 8 acquisition system continuously recorded waveform signals from eight channels from load application to brittle failure. Under the linear/non-linear load conditions, 1131 and 931 AE events were successfully located by a P-wave automatic picking and collapsing grid search algorithm. Under the linear/non-linear load condition, the crack initiation points were both away from the disc center. For non-central crack initiation, the tensile strength test may underestimate the true value. A pole density analysis of the planes under nonlinear load conditions shows that the local distortion of the fracture is greater than that under linear load. The evolution of the 3D damage to the disc shows that the load area of the disc significantly affects the cumulative time of damage, amount of energy liberation and stability of the crack propagation. The moment tensor decomposition was performed on the effective AE events, and the isotropic (ISO) component, the pure double-coupled (DC) and the compensated linear vector dipole (CLVD) component frequency percentage were obtained. The classification method was applied to quantitatively analyze the focal mechanism. The results show that the Brazilian test is not sensitive to the load contact conditions, and the focal mechanism of both cases can be interpreted as the initiation, propagation, and penetration of the tensile and shear microcracks approximately along the load direction.