赤間 誠

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。

鉄道レールのシェリングは、列車の車輪が繰り返し通過することにより発生する表面の転がり接触疲労き裂である。本研究では、シェリングの水平裂の進展について数値シミュレーションを実施した。成長は、ヘルツ接触圧が負荷される2次元傾斜き裂の応力拡大係数（K値）の境界要素解析に基づいて行った。このような2次元き裂を複数並列に配置し、その先端を結ぶことで、3次元半楕円水平裂のK値を近似的に求めることができる。この方法は、完全な3次元で解析された方法と比較して、計算速度で優位性がある。進展はモードI/II及びモードI/IIIの非比例混合モード試験のデータを用いて、Richardによって提案された等価K値範囲を用いるパリス型き裂進展則で求めた。分岐の方向はHourlier-Pineau規準で予測した。

A numerical model is developed to simulate the competition between RCF-initiated short crack propagation and wear in a wheel tread. The crack is assumed to initiate when the total accumulated plastic shear strain reaches the critical value. In the early crack growth simulations, the two-stage short crack growth model proposed by Hobson is applied. With regard to wear, the Archard model is adopted as the basis. The model is shown to operate properly when applied to the results of unidirectional twin disc tests. When an outward route comprising a 1000 km straight run is simulated based on the Shinkansen routes, it is found that the tread is hardly damaged. The results obtained from the rolling reversal fatigue tests show that if the rolling direction of the wheel is reversed, the delamination wear becomes dominant.