Papers by Author: Norio Kawagoishi

Abstract: In order to investigate the effect of radical nitriding on fatigue strength of spheroidal graphite cast iron FCD400, rotating bending fatigue tests were carried out at room temperature. Fatigue strength was increased by nitriding. Fracture started from microshrinkages or abnormal graphites in the same manner as that of as cast iron. However, fracture mode was different according to the stress level. That is, fracture occurred from specimen surface at high stress levels and it occurred from defects in subsurface at low stress levels. The cause of these results was the hardened layer and the compressive residual stress, which suppressed the initiation of the surface crack.

Abstract: Effect of high humidity on growth mechanism of a fatigue crack was investigated for an extruded bar of an age-hardened Al alloy 7075-T6 which had the marked texture of plane (111). Fracture in high humidity occurred by the growth of a shear mode crack under high stress levels, though a crack propagated in a tensile mode under low stress ones, macroscopically. Many voids and slip planes were observed on the fracture surface yielded by the shear mode crack. Especially most of the fracture surface was occupied by voids where the crack was small. The void percentage decreased with increasing in the crack depth. Fracture surface yielded by the shear mode crack was a plane (100). The growth direction of the shear mode crack to the cross section of specimen was about 55° corresponding to the angle composed by this plane and the texture of plane (111). These results suggest that the shear mode crack was related to microstructure, stress and environment. The growth mechanism of the shear mode crack assisted by hydrogen was proposed based on the results of the acceleration of crack growth and the formation of voids in high humidity.

Abstract: In order to study the effect of post-ECAP (equal channel angular pressing) mild annealing treatment on fatigue strength of ultrafine grained (UFG) copper, the stress-controlled fatigue tests of UFG copper with and without post-ECAP annealing treatment were performed for a wide range of fatigue life. The evolution of surface damage and growth behavior of surface cracks were monitored by plastic replication technique. The effect of post-ECAP annealing on the fatigue strength was discussed through the observation of changes in surface morphological features.

Abstract: In order to investigate the effect of humidity on fatigue strength of an extruded and age-hardened Al alloy 7075-T6, rotating bending fatigue tests were carried out using plain specimens in environments of controlled relative humidity of 25%, 50%, 75% and 85% and distilled water. The cross section of the alloy has a marked texture of (111) plane. Although fatigue strength was decreased by high humidity, the decrease by high humidity was very small when the humidity was lower than about 60% -70% and fatigue strength was largely decreased over the humidity. Both of initiation and propagation of a crack were accelerated by high humidity. In high humidity, a crack propagated in a shear mode macroscopically and it was ductile in company with many glide planes and voids microscopically. That is, the propagation was not a tensile mode with brittle facets even in water. The shear mode propagation inclined about 35° to the extruded direction and fracture surface was (100) plane, meaning that the shear mode propagation of a crack was mainly caused by the marked texture of the alloy. The propagation mode of a crack was affected by not only environment but also stress level.

Abstract: This paper presents a system of 3-D micro structure measurement that uses an optical fiber probe of 5 µm in diameter. The probe is deflected when it comes into contact with a measured surface, and this deflection is measured optically. In this research, we optimize design parameters of optical system using ray tracing, and a prototype of the measuring system is fabricated on trial to verify the simulation results. Then, its measuring accuracies are examined by using the basic experimental apparatus. As a result, it is clarified that the resolution of the fiber probe is better than 10 nm. Also, the utility of this system is confirmed by measuring the shape of a 600 µm diameter ruby sphere.

Abstract: Ultrasonic and rotating bending fatigue tests were carried out using plain specimens and specimens with a small blind hole for an extruded and age-hardened Al alloy 7075-T6 in different environments in order to investigate the effect of humidity on fatigue strength and fracture mechanism. Fatigue strength was decreased by high humidity under both tests. The effect of humidity on fatigue strength was larger in ultrasonic fatigue. The humidity affected both of crack initiation and propagation processes. Crack propagated in tensile mode then changed to shear mode macroscopically in all environments under ultrasonic fatigue, though it was only in tensile mode under rotating bending fatigue. These differences in fracture mechanism related to the difference in environmental effect on fatigue strength in both tests.

Abstract: Ultrasonic and rotating bending fatigue tests were carried out for aged and nitrided Ni-base super alloys to investigate the effects of loading frequency and nitriding on fatigue strength. Loading frequencies were 19.5 kHz under ultrasonic and 50 Hz under rotating bending, respectively. Fatigue strength under ultrasonic was higher than that under rotating bending in both alloys. Moreover, in both tests, fatigue strength was improved by nitriding. The increase in fatigue strength by nitriding was large in ultrasonic fatigue. These results were discussed through the successive observation of fatigue process at specimen surface and fracture surface observation.

Abstract: In order to investigate the effect of nitriding on the crack initiation and propagation behavior of Ni-base super alloy, Alloy 718, rotating bending fatigue tests were carried out until 108 cycles at room temperature. By nitriding at 500°C for 12h, compound layer of about 5μm in thickness was formed and the initiation of a fatigue crack was strongly suppressed causing the increase in fatigue strength. A crack initiated in brittle manner at the compound layer in all of fractures. However the crack propagated in ductile manner controlled by the property of the base alloy. That is, there is no or little influence of nitriding on the crack growth rate of the alloy.

Abstract: In the present study, a unified crack growth law based on the small fatigue crack growth law was investigated using specimens with different configurations and loading types for the annealed 0.42% carbon steel. Then, a convenient prediction method of fatigue life was proposed. This small fatigue crack growth rate was uniquely determined by the modified small crack growth law, , despite of changing for specimen configurations and loading types. The constants of can be estimated by an empirical equation of without stress/strain gradient. Considered the effect of stress gradient for different specimen configurations and loading types, the stresses under bending loadings and specimen configurations was calculated from that under push-pull loading. Therefore, the fatigue life of the specimens with different configuration and under different loading types can be simply estimated through the fatigue crack growth law based on the only tensile strength of carbon steels. The availability of proposed method was confirmed experimentally by the results of several carbon steels.

Abstract: Fatigue properties of a squeeze cast Al-Si-Mg alloy were examined under rotating bending and the effect of humidity on the crack initiation and propagation behavior was investigated in controlled humidity. The fatigue strength was decreased in high humidity (RH85%) in comparison with the one in low humidity (RH25%). Cracks initiated at the early stage of fatigue life from slip bands and eutectic Si particles in many cases and partially casting defects, and most of fatigue life was occupied by the growth life of a small crack regardless of humidity. The propagation of a crack were promoted by humidity, especially the growth rate of a crack larger than about 100 µm, which propagated in tensile mode, was strongly accelerated.