赤間 誠

ABSTRACT Rotary‐bending fatigue tests were carried out using induction‐hardened (IH) and press‐fitted (PF) specimens with a non‐IH axle component to evaluate the effects of induction hardening and press fitting on the fatigue behavior of railway axle steel up to the very‐high‐cycle fatigue (VHCF) regime. Induction hardening increased the fatigue limit by about 350 MPa. Fractures in the IH specimens were classified into three types based on their location and mode. These types were successfully explained using local fatigue strength, stress distributions, and microstructural observations. In contrast, press fitting reduced the fatigue limit by about 200 MPa due to stress concentration and fretting‐induced surface roughening.

Rolling contact fatigue (RCF) and wear are the primary types of damage found in rails and wheels, and these often compete with each other. This paper presents a comprehensive review of studies on RCF and wear of rails and wheels, focusing on their competition. First, RCF and wear in actual rails and wheels are discussed. Next, theory and models for RCF cracks are presented—from crack initiation, through short and long crack growth, to crack branching and branch crack growth. Then, different wear forms, wear regimes, and their theories and models are introduced. Several studies analyzing the competition between RCF and wear are discussed. Finally, current gaps or problems of the studies on RCF and wear of rails and wheels are identified, and recommendations for future work are provided. This review aims to assist researchers who investigate and address the problems associated with RCF and wear of rails and wheels.

A railway wheel flat occurs when the wheel locks and slides along the rail, forming a white etching layer (WEL) just under the flat surface where rolling contact fatigue cracks often form that can propagate into the wheel and spall during service. This study comprehensively investigated the influencing factors on wheel crack propagation that lead to spalling. Experiments were conducted to test wheel specimens with WELs of various dimensions, and simulations were performed to evaluate the stresses near the WELs as well as the rate and direction of crack propagation.

Rotating bending fatigue tests were performed to investigate the effect of induction hardening (HQI) on the very high cycle fatigue (VHCF) properties of the axle steel. In addition, finite element analyses (FEA) and scanning electron microscope (SEM) observations were conducted to elucidate the fundamental behavior. The results obtained are summarized as follows; (1) It was found that the fatigue strength of the axle steel was improved by almost 300 MPa in VHCF region by HQI, (2) A lot of specimens of the HQI had fractured in VHCF region, therefore, the consideration of VHCF properties should be taken into account in the design of railway axles, (3) Crack initiation sites can be classified into three forms; the surface of the HQI layer in the minimum section, the base metal beyond the HQI layer in the axial direction, and the internal base material in the minimum section. These can be reconfirmed through the comparison between the local fatigue strength distribution and the bending stress distribution obtained by FEA.

列車は一般的に折り返し運転を行い，終着駅に到着すると進行方向が逆になるため，車輪の転がり方向も逆になる。そのため車輪の回転方向を反転させることで，車輪踏面のき裂進展や摩耗がどのように変化するかを調べることは重要である。本研究では，転がり接触疲労（RCF）き裂および摩耗の挙動に及ぼす影響を調べるため，様々な条件下で二円筒摩耗疲労試験を実施した。また，これらの試験結果を明らかにするため，有限要素解析（FEA）を実施した。潤滑条件下では，すべり率1%の場合，車輪試験片の転がり方向を逆転させると累積摩耗量が増加する傾向があった。一方乾燥条件下では，逆転前後で摩耗量にほとんど変化は見られ なかった。FEA により，車輪試験片を逆転させた場合，RCF き裂に潤滑油が閉じ込められ，き裂面間の摩擦係数が低下し，せん断モードき裂が進展して疲労摩耗が増加することが示唆された。