To enhance automobile fuel efficiency, it is crucial to develop lightweight and compact components with improved strength. This research specifically addresses the increasing demand for higher fatigue strength in components that undergo surface hardening processes. The study focuses on the impact of refining the prior austenite (prior γ) grain size on the fatigue strength of low carbon steel sheets that have been processed through vacuum carburizing followed by quenching. To investigate this relationship, specimens were prepared under various quenching temperatures, and their plane bending fatigue strength was evaluated. The results indicated a clear trend: finer prior γ grain sizes generally correlate with improved fatigue strength. However, it was observed that when the maximum grain size fell below 47 μm, the enhancement in fatigue strength became minimal. Further analysis of the fracture surfaces revealed a significant shift in the initiation point of fatigue failure, transitioning from intergranular to transgranular fracture. This shift is crucial as it contributes to the overall fatigue behavior of the materials. The findings of this study underscore the importance of achieving a sufficiently fine prior γ grain size in vacuum carburized and quenched materials. Such refinement is essential for attaining stable and high fatigue strength, as it facilitates transgranular fracture initiation, ultimately leading to enhanced durability and reliability of automotive components.

Generally, the aerodynamic force coefficients of railway vehicles are determined through wind tunnel tests that simulate the interaction between the vehicle and track structures under turbulent boundary layer flow conditions. In most conventional crosswind wind tunnel tests, aerodynamic forces are measured on a stationary vehicle model placed on representative track structures. In actual operation, however, the relative wind direction acting on a train may vary owing to changes in natural wind direction, train acceleration or deceleration, or passage through curved track sections. To evaluate aerodynamic forces under such varying wind-direction conditions, this study measured the surface pressure distribution on a vehicle model mounted on a rotating turntable. Wind tunnel tests were carried out in the large low-noise wind tunnel of the Railway Technical Research Institute using a 1/40-scale model of a typical commuter train placed on a low-embankment structure. A turbulent boundary layer representing natural wind was generated upstream of the turntable. Surface pressures measured on the vehicle body were used to calculate the side force and lift force coefficients. The results obtained under rotational conditions, in which the relative wind direction changed continuously from 0-90°, were compared with those under stationary conditions corresponding to conventional tests. The results showed that no clear difference was observed in the side-force coefficient between stationary and rotational conditions. In contrast, the lift-force coefficient under rotational conditions differed clearly from that under stationary conditions, and its tendency also depended on the direction of rotation. Although the maximum difference in lift-force coefficient was notable, its influence on the critical wind speed for overturning was considered limited because lift force contributes less to overturning safety than side force. These findings suggest that wind-direction changes can affect the aerodynamic characteristics of railway vehicles.

In power generation using vibration energy to rotate an electro-magnetic coil, we encounter the problem that the force due to the coil current suppresses the coil rotation, vibration and power generation. This paper studies a method to avoid this problem by noting that the vibration of a rotor is promoted by internal damping. This method uses an electro-magnetic coil system such that it produces internal damping for the rotor vibration. To suppress the unstable vibration caused by the internal damping, another electro-magnetic coil system is applied to supply external damping. By increasing the external damping according to the increase in the internal damping, we can augment the powers produced by the two kinds of damping without raising the vibration amplitude (structural load).