Takashi Konishi’s scientific contributions

The continuous-type acoustic emission (AE) from nuclear-grade isotropic graphite under compressive loading was examined to make clear the deformation behavior of the graphite. The type of AE for the graphite increased gradually with increasing strain in the initial stage of loading followed by the decreasing behavior for further loading to the failure point. A model was proposed to explain qualitatively the phenomenon, which was based on dislocation piled up at grain boundaries. To clarify the back stress effect at grain boundaries oxidized specimens were prepared. The type of AE for oxidized specimens increased monotonically up to the failure point. The model proposed was supported by the experimental results. Moreover, the qualitative relationships of total plastic strain rate and plastic strain rates by slip deformation and by microcracking as a function of total strain were given from the AE behavior.

Acoustic emission (AE) was measured on four kinds of isotropic graphites under tensile and compressive loading. Observed AE events can be divided into two types, with low and high relative energy. The events with low relative energy were initially observed under a low stress level, continued to the fracture of the specimen. On the other hand, the events with high relative energy were observed from an even higher stress level. AE events with low and high relative energy corresponded to cleavage between basal planes within the filler grains and to microcracking in the binder region, respectively. The stress at which an AE event with high relative energy was first detected, the AE onset stress, was closely dependent upon the fracture stress of the graphite. Empirical equations related to the AE onset stress and the fracture strength under tensile and compressive loading were proposed to help in estimation of tensile and compressive strengths of a polycrystalline graphite using an AE technique.

The effects of porosity on mechanical properties and deformation behavior of four isotropic polycrystalline graphites were studied. The pore size distributions of the graphites were measured using a conventional mercury penetration technique. The average pore radius of ISO-88 graphite was about one-tenth of that of ISEM-1, IG-11 or IG-15 graphites. Young's modulus of the graphites decreased with increasing porosity. The stress-strain curve of each graphite was measured in its lateral and axial directions. Young's modulus of graphite decreased with increasing load. The plastic strain at a given compressive load was calculated from the stress-strain curve and the initial gradient of the unloading curve at the load. The ratio of lateral plastic strain to axial plastic strain for the graphites was less than 0.5, indicating that the volume of the graphites decreased during compressive loading. By assuming that the volume change was caused by contraction of pores, plastic strain associated with contraction of pores was calculated from the axial plastic strain and lateral plastic strain by slips along the basal planes. The plastic strain increased with increasing axial plastic strain and porosity of graphite.