动、静裂纹作用偏置效应的动焦散冲击实验

为了研究冲击荷载作用下脆性材料中运动裂纹与静止裂纹的相互作用，选取动态载荷下断裂行为与岩石材料类似且本身光学特性较好的有机玻璃（PMMA）作为实验材料，试件尺寸为220 mm×50 mm×5 mm，采用激光切割制作长度5 mm的预制裂纹和长度10 mm的静止裂纹，预制裂纹位于试件的底部边缘中心，静止裂纹的中心位于试件水平轴线。将静止裂纹偏置距离作为单一变量，采用数字激光动态焦散实验系统对含不同缺陷的PMMA进行三点弯曲实验，并结合几何分形理论研究不同偏置距离下运动裂纹的分形规律。实验结果表明:存在预制裂纹与静止裂纹的临界偏置距离（6 mm），该条件下裂纹轨迹对应的分形维数值最大，裂纹轨迹的规则程度最低，裂纹破坏形态最复杂。当预制裂纹与静止裂纹的偏置距离在0～6 mm时，裂纹Ⅰ起裂后垂直向上扩展一段距离，与静止裂纹交汇，并停滞一段时间后发生二次起裂，直至贯穿试件，偏置距离和交汇点竖向坐标值呈近似线性函数关系。偏置距离的存在不会影响裂纹Ⅰ的起裂时间和应力强度因子，但会显著减小裂纹Ⅱ的动态应力强度因子，且停滞时长随偏置距离的增大而逐渐缩短。当偏置距离大于临界偏置距离时，运动裂纹不再与静止裂纹交汇而是呈拱状向试件上边缘扩展直至贯穿，裂纹的起偏时间、起偏位置也会出现明显的滞后现象。

Dynamic caustics experiments on offset effects between dynamic and static cracks

To explore the interaction between dynamic and static cracks in brittle materials under impact load, polymethyl methacrylate (PMMA) was chosen as the experiment material, considering that the PMMA has good optical properties and its fracture behavior is similar to rock under dynamic load. The size of the specimens was 220 mm×50 mm×5 mm with a prefabricated crack of 5 mm in length and a static crack of 10 mm in length. The prefabricated crack was located at the center of the bottom edge of the specimen, and the center of the static crack was located at the horizontal axis of the specimen. Three-point bending experiments of different defects in PMMA had been explored by setting the static crack offset distance as the single variable with the digital laser dynamic caustic test system and the fractal law of dynamic crack at different bias distances was studied by combining with the geometric fractal theory. Researches show that when the offset distance is at a critical condition between prefabricated crack and static crack (6 mm in this experiment), the fractal dimension corresponding to the crack track is the largest, the regularity degree of the crack track is the lowest and the failure mode of the crack is the most complicated. When the offset distance is between 0 to 6 mm, crack Ⅰ propagates vertically and intersects with a distant static crack, then produces a secondary crack which penetrates the specimen after a period of stagnation, the linear function relationship between the offset distance and the vertical distance of the intersection point is then obtained. The existence of the offset distance does not affect the crack time and the stress intensity factor of crack Ⅰ, but it can significantly reduce the dynamic stress intensity factor of crack Ⅱ, the length of stagnation decreases with the increase of offset distance. When the offset distance is larger than the critical offset distance, the dynamic crack no longer intersects with the static crack, but extends to the upper edge of the specimen in an arch shape until it penetrates the specimen, and there is also a significant hysteresis in the cracking time and the position of the crack.