Papers by Author: Toshihisa Nishioka

Abstract: Recently, some improvements made to machine performance have caused accidents as a result of impact fracture. These fractures were caused by unexpected dynamic loads. To suppress the damage in these accidents, it is necessary to clarify the dynamic fracture mechanism, many reports have been published on dynamic fracture phenomena [1, 2, 3]. Cast iron is used to repair some structural and mechanical parts following fracture accidents. The brittle behavior of cast iron is not desirable for preventing dynamic fracture. It is necessary to clarify the dynamic fracture mechanism of cast iron for the safety design and maintenance of structures. The dynamic behavior of deformation and fracture depends on the size of a structure. In some cases, an experimental approach using specimens at industrial scale is difficult. As a first step, dynamic fracture without a huge mass effect should be discussed. In this study, a normal sized three point bending specimen consisting of cast iron was used in dynamic experiment. An ultra-high speed camera was used to observe crack propagation. Some fractures were caused under eccentric loading, non-straight cracks propagated in this condition. According to the experimental results, the path and velocity of crack propagation were estimated. Fracture criteria were discussed from the results of numerical simulation. To simulate the behavior of crack propagation a moving finite element method based on Delaunay automatic triangulation was used. The prediction of fracture paths based on the fracture mechanics theory was demonstrated in these numerical simulations. The predicted fracture path agreed with the experimental fracture path.

Abstract: The finite element alternating method (FEAM), in conjunction with the finite element analysis (FEA) and the analytical solution for an elliptical crack in an infinite solid subject to arbitrary crack-face traction, can derive the stress intensity factor (SIF) of surface cracks by using the FEA results for an uncracked body. In the present study, the FEAM was applied to evaluations of SIF for noncoplanar multiple surface cracks. The SIF was evaluated for two surface cracks of dissimilar size, and three crack of the same size. The results suggested that the interaction is greatly affected by the relative crack size and negligible when the difference in the crack size is large enough, and the interaction can be evaluated by taking into account the adjacent cracks even if there are many cracks around them. Finally, the crack growth simulations were conducted and a possibility of the direct evaluation of influence of interaction between adjacent crack without using the combination rules was revealed.

Abstract: In transonic interfacial crack propagating fracture problem, the generation-phase simulations were done using the moving finite element method based on Delaunay automatic mesh generation. And the contact function based on the penalty method was newly developed to consider the crack face contact near the propagating interfacial crack tip. It was succeeded to visualize in 3-dimensions the Mach shock wave emanated from the propagating crack tip. And it was tried for the transonically propagating crack problem that solving the energy flows through the contact zone or along the Mach shock wave line emitted from the crack tip. The energy flow patterns into the crack tip were also visualized. Furthermore, from the values of the separated dynamic J integrals, it was found that the dynamic J integral is non-zero even for transonic fracture region and the most of the energy release rate is provided from the more compliant material epoxy.