PSI - Issue 13

Koji Uenishi et al. / Procedia Structural Integrity 13 (2018) 652–657 Uenishi et al. / Structural Integrity Procedia 00 (2018) 000 – 000

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1. Introduction

For total or partial destruction of intricate solid structures, considering working hours and environmental load, technical methods based on dynamic fracture by blasting with explosives are very efficient but in practice, details of blasting demolition are still most often empirically designed and undesired results may be sometimes obtained. Thus, utilization of dynamic fracture tends to be avoided in densely populated areas (Uenishi et al., 2010). The purpose of this research, therefore, is to develop safer, more mechanics-oriented and more controllable dynamic fracture techniques, for large-scale operations in remote regions as well as for destruction of relatively smaller but very complex, damage sensitive three-dimensional structures in urban areas. Since a series of small-scale tests to use wave interactions by short ignition delay time of explosives for optimal fracture of magnetic mortar blocks showed no clear difference of the fracture between the cases with and without delays (Johansson and Ouchterlony, 2013), we simultaneously release energy from all blast holes (energy sources) placed in a brittle solid material and try to find favorable geometrical and loading conditions that control not only wave and crack propagation but also fracture pattern developed. 2. Controlled fracture development in a rectangular specimen: Field experiments Because of the difficulty in treating explosives in urban areas, in the previous series of field experiments, we have applied easy-to-handle electric discharge impulses (EDI) to fracture solid materials. As a result, Uenishi et al. (2014) have demonstrated that for controlled 3D fracture development in cylindrical concrete specimens (diameter 500 mm, height 500 mm) without reinforcing steel bars, it is effective to set (the centers of) blast and empty dummy holes on a straight line. Now, we destruct more realistic, larger-scale rectangular concrete specimens by EDI and try to clarify more preferable positions of blast and dummy holes for controlled dynamic fracture. We prepare ten specimens (900 mm  900 mm  300 mm) that have no reinforcing steel bars but different geometrical and loading settings with one or more blast holes (diameter 12 mm, depth 185 mm). In every blast hole, we place a cartridge containing a self-reactive liquid (deflagration agent) and connect it to the Electric Discharge Impulse Crushing System (EDICS) developed by Nichizo Tech, Inc.: The electric energy stored in a capacitor of the system is released in each cartridge in several hundreds of microseconds through an electronic switch and high pressure is produced by rapid liquid evaporation. We fill every blast hole with stemming material (silica sand). Here, we show the results of two typical cases. In the first case IC-09/00 (Fig. 1(a)), the specimen has two blast holes situated on a virtual central vertical plane. Each blast hole with the self-reactive liquid is surrounded by four empty dummy holes (diameter 18 mm, depth 300 mm) that are set at the corners of a square of side length 200 mm (see top view of Fig. 1(a); every blast hole is located at the center of the square). This specimen is geometrically symmetric with respect to the virtual central horizontal plane (at a height of 150 mm) as well as to the virtual central vertical plane containing the blast holes except for the stemming sections and screw holes drilled for the carriage of the specimen. The second case IC-10/00 (Fig. 1(b)), having the same geometrically symmetric character, is akin to IC-09/00 but now there exist three blast holes, each of which is at the center of the square of four empty dummy holes. We record the dynamic fracture process on the top surface induced by the application of EDI with a high-speed digital video camera (Photron FASTCAM SA5) at a frame rate of 50,000 frames per second. The relatively large specimen size, observation area (approximately 350 mm  850 mm) and the measured shear wave speed (2,400 m/s) suggest that this frame rate 50,000 fps is sufficiently large for tracing dynamic cracks that normally propagate much slower than shear waves. We describe here these two cases so as to emphasize the influence of a group of empty dummy holes on the directional stability of dynamic crack propagation. Indeed, superposition of the reflected waves from the dummy (and blast) holes will play a crucial role in guiding the propagation direction of a main tensile crack. Alternately charged (blast-dummy-blast- ...) holes may be more readily prepared, especially in a relatively small, two-dimensional laboratory specimens (Mohanty, 1990), but here, in place of setting alternately charged holes on the expected main crack path (Uenishi et al. 2014), we propose more geometrically intricate positions of blast and dummy holes in the real three-dimensional fracturing work. We do not place dummy holes on the expected main crack path or fracture plane but set them so that they can assist the main c r a c k t o c o n n e c t t h e b l a s t h o l e s o n l y .