Structures, propagation characteristics of stress waves and dynamic fracture behavior in blasting

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It is necessary to obtain a complete understanding of dynamic behavior of stress waves and cracks in the blasting process for the development of fracture control methods in rock excavation and partial demolition of concrete structures. In this study, model experiments were carried out to examine structures, propagation characteristics of stress waves and dynamic fracture behavior in blasting process using PMMA plates, mortar specimens and electric detonators. The interactions of stress waves with the notched circular hole were examined and crack propagation along the control fracture plane was observed. In these experiments, the dynamic behavior of stress waves in blasting processes was measured by means of strain measurements. The crack propagation along a control fracture plane was observed by means of a high-speed video camera. The following results were obtained.(1)Peak values of the radial compressive strain in the PMMA specimen produced by the incident stress wave arriving at six strain gauges placed between 20〜110mm near the charge hole decay from about 40000〜5000μ strains. The propagation velocity charges from about 3000m/s〜2800m/s and the velocity is larger than the P wave velocity. Peak values of the circumferential strain in PMMA decay from about 6000〜2000μ strains. The images of the high〜speed video camera system show that the crack originating from the charge hole propagates with the velocity of 100〜200m/s.(2) On the basis of these experimental results, the fracture plane control experiments using the time controlled blasting method were made. The radial cracks generated from the charge hole didn't penetrate the controlled fracture plane and the controlled fracture plane acts to arrest these radial crack.(3) The X-ray computed tomography (CT) system, which use the X-ray beam, was used to observe crack structure in mortar specimens. The radial cracks produced near the blast hole were shown by the X-ray CT images.