Measurement of Near-Tip Field of Fast Propagating Cracks at Bifurcation

(1)高速全息显微术应用反射式高速全息显微术成功地拍摄了快速扩展裂纹分叉瞬间的三张连续显微照片，当一条快速扩展的张开型裂纹分叉成两条裂纹时，三束脉冲红宝石激光相继振荡。激光脉冲照射试样，裂纹通过角度复用全息术在照相板上记录为三幅全息图。利用透镜辅助方法分别从全息图中重建出三个裂纹图像。空间分辨率大于180线/mm，帧间隔约为5微秒。(2)尝试分叉和微裂纹从高速全息显微镜拍摄的显微照片中，可以了解到分叉前和分叉后小裂纹的行为，当一个快速扩展的裂纹分叉成两个或多个裂纹时，会出现多个裂纹尖端，这些尖端在直径约为1 mm的小区域内同时扩展。其中一些停止传播，并成为尝试分支。然而，一些裂纹尖端继续扩展，并且它们生长成大的分叉。裂纹前缘的三维结构或微裂纹的行为可能对裂纹分叉起重要作用。(3)分叉点附近的应力场在分叉点处，快速扩展裂纹尖端附近的应力场偏离动态断裂力学理论预测的K场。这种偏离不仅发生在分叉之后，而且发生在分叉之前。分叉后的偏差是由于多个裂纹尖端的存在。另一方面，分叉前的偏差被认为是由于扩展裂纹尖端附近的微裂纹或尝试分支造成的。近端场偏离K场可能导致大尺度分岔。

(1) High-Speed Holographic MicroscopyReflection type high-speed holographic microscopy was successfully applied to take three successive microscopic photographs of fast propagating cracks at the instant of bifurcation.When a fast propagating crack of opening mode bifurcates into two cracks, three pulsed ruby lasers oscillate successively. The laser pulses illuminate the specimen and the crack is recorded as three holograms on a photographic plate by means of angle-multiplexing holography. Lens assisted method reconstructs the three crack images separately from the holograms. The spatial resolution is more than 180 lines/mm, and the frame interval was about 5 micro-seconds.(2) Attempted Branches and MicrocracksFrom the microscopic photographs taken by the high-speed holographic microscopy, one can know the behavior of small branched cracks just before and just after the main bifurcation.When a fast propagating crack bifurcates into two or more cracks, there appear several crack tips which propagate simultaneously in a small region about 1mm in diameter. Some of them stop propagating, and become attempted branches. However, some crack tips keep propagating, and they grow into a large bifurcation. Three dimensional structure of crack front edges or behavior of microcracks may play an important role for crack bifurcation.(3) Near-lip Field at BifurcationAt bifurcation, the stress field near a fast propagating crack tip deviates from the K-field which is theoretically predicted by the dynamic fracture mechanics. Such deviation occurs not only after bifurcation but also just before bifurcation. The deviation after bifurcation is due to the existence of several crack tips. On the other hand, the deviation just before bifurcation is thought to be due to the microcracks or attempted branches near the propagating crack tip. The deviation of the near tip field from the K-field may cause the large scale bifurcation.