Visualization and analysis of stress waves in blasting processes by a laser-shadowgraph method

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Dynamic fracture control in blasting is very important in underground excavation and demolition of concrete structures. It is necessary to examine the dynamic mechanism of stress waves, gases and cracks in blasting process for achieving high degree of fracture control. In this research, experimental results on visualization of stress waves in blasting by a laser-shadowgraph method are presented and a new method for fracture controlled blasting is suggested. The shadowgraph system using a Q switched ruby laser as a light source is set up. The system enables us to get a giant pulse beam at a desired time and to obtain the high spatial and time resolved instantaneous visualization photographs.Experiments are made on stress waves interacting with a circular hole, a circular arc-shaped free surface and a free surface with 90ﾟ corner. Model experiments are casrried out with the PMMA plate of 20 mm in thickness as a transparent material. From these experiments, the following results are obtained. The shadowgraphs show the behavior of reflected waves, deformations of the circular hole and the generation of converging shock waves in the hole by interaction of incident waves with the hole. In the case of the hole with two notches along the prpagation direction of the incident waves, it can be seen that the well-known shadow spots are produced in the vicinity of the notches. From the fact, it might be expected that utilization of the circular hole with notches is effective as the guide hole technique for achieving fracture control.