Transition from multi-center fracture to fragmentation statistics under intensive loading

Abstract

Statistics of transition from damage to fragmentation is studied based on the analysis of multi-scale mechanisms of nucleation and growth of cracks in hollow cylindrical (tubular) specimens made of Al2O3 ceramics. Specimens are loaded by underwater electrical wire explosion. Around 98% of the initial mass of the specimen was recovered as a fragments. The fragments were classified into two types: quasi-two-dimensional (2D) samples, the characteristic size of which d* was greater than (or equal to) the wall thickness of the tube d; and three-dimensional (3D) samples of size d* < d. The analysis of the fragment geometry allows us to determine the mechanisms responsible for the formation of 2D and 3D fragments. The 2D fragments are formed by the large cracks (Mott mechanism) as a result of tube extension in the radial direction. The 3D objects are formed due to instability of fast crack propagation, which leads to microbranching, as was shown in Sharon (1996). The 3D fragments size distribution is governed by the power law, which corresponds to the microbranch distribution. The study of the influence of the initial sample porosity indicates that the distribution of the porosity is described by the power function with an exponent slightly differing from that of the fragments distribution. We suppose that both the initial porosity and crack instability influence the formation of small fragments. The preliminary analysis of the initial porosity of the material shows us that the initial porosity has a direct effect on the formation of 3D fragments. According to the scenario, the initial defects (pores) provide the conditions for multi-site fracture of ceramics under high rate loading.