Research

Metal fatigue is the most common cause of failure in structures. For metals, fatigue cracks are generated by the formation of characteristic fatigue dislocation structures. The size of these structures is a few micrometers, which is independent of the material dimensions. This implies that this conventional fatigue dislocation structures cannot exist inside nano- and micro-sized materials. In other words, there is a possibility that nano- and micro-sized materials have a unique fatigue behavior. We conduct fatigue tests and detailed observations in transmission and scanning electron microscopes in order to clarify this. We have the world's leading technology for fatigue testing of nano- and micro-sized materials.

In this age of rapid development and evolution of nanotechnology, it is no exaggeration to say that the social infrastructure of humankind is supported by the unique functions of nano-sized materials. Although nano-sized materials have different properties from macromaterials (bulk) due to the significant surface effects, the details of their mechanical responses to various physical properties have not been clarified. Therefore, we are studying the responses of nano-sized materials under mechanical loading, focusing on ferroelectricity, magnetism, Mott transition, and so on. We have the observation equipment (transmission electron microscope, piezoelectric response microscope and magnetic force microscope) necessary for this research, and elucidate the phenomena using loading equipment we developed originally.

By making controlled artificial structures in materials, we can create "metamaterials" (materials beyond materials) with physical properties that do not appear in materials in nature. Today, it is possible to make metamaterials with nano- and micro-sized structures. On the other hand, it is necessary to investigate structures that exhibit new mechanical properties prior to the miniaturization of structures. Therefore, we design metamaterials that express characteristic mechanical properties (mechanical metamaterials), actually fabricate them using a 3D printer, and demonstrate their deformation and fracture behavior through experiments.

At a crack tip, there is a singular stress field in which the stress increases to infinity. The singular stress field also occurs at the junction between the interface of dissimilar materials and the free surface. Fracture phenomena in materials have conventionally been discussed using the mechanics of fracture under the continuum assumption (fracture mechanics), but when the material dimensions are reduced to the nanoscale, the singular stress field is reduced to an equivalent size by the similarity law. In this case, the number of atoms contained in this region is about several tens, and it is not clear whether the discussion under the continuum assumption is possible or not. We are trying to elucidate this experimentally and analytically. Recently, we have succeeded for the first time in the world in conducting fracture experiments from atomic-level line defects (dislocations) and clarifying fracture phenomena from atom-sized singular stress fields.