动, 静裂纹作用偏置效应的动焦散冲击实验

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.