Akama Makoto

In this study, we conducted a series of twin-disc wear and fatigue tests and FE simulations of the tests. The results are as follows. In the wet conditions, the wear rate increased when the rolling direction of the wheel disc was reversed. In contrast, in the dry conditions, the wear rate was much higher than in the wet conditions, however, it changed little before and after the reversal. The Archard model could not explain the increase in wear rate after the reversal in wet conditions, because the model does not take into account the delamination wear induced by the fluid. Some discs after the test were sectioned and photographed to observe the crack morphologies and microstructural changes by optical metallography. In wet conditions, when the wheel disc was reversed, the cracks clearly tended to propagate parallel to the surface, and the resulting long and thin wear sheets delaminated. In this case, the crack is filled with fluid just before its mouth reaches the contact area between the wheel and rail discs, whereupon the mouth is sealed. Therefore, the fluid is trapped in the crack and acts as a lubricant to reduce the friction coefficient, and cracks tend to propagate easily and large delamination sheets are apt to form. In order to explain the increase in wear rate after the reversal under wet conditions, CTSDs were obtained by FE analyses. In the wet conditions, the ΔCTSD after the reversal was almost twice as large as that before the reversal. This means that the amount of crack propagation after the reversal is higher than that before the reversal. In contrast in the dry conditions, the ΔCTSD is almost the same before and after the reversal. Therefore, the amount of crack propagation before and after the reversal is considered to be almost the same.

本研究では，高周波表面焼入れを施した車軸鋼相当試験片を用いた超高サイクル領域までの回転曲げ疲労試験を行ってS-N曲線を作製した。同時にFEM解析を行い，試験結果を考察した。またSEMによって破面観察を実施した。その結果，未焼入れ材の10^7時間強度はほぼ300MPaであるのに対して，焼入れ材の同時間強度はほぼ700MPaであり，高周波焼入れによって疲労強度は，ほぼ400MPaも向上すること，焼入れ材では10^7サイクル以上で破断した試験片がかなり存在し，表面焼入れを行っている新幹線車軸は超高サイクル領域での疲労特性の解明が不可欠であることなどがわかった。