高速列车车轮踏面局部滚动接触疲劳的机理研究

Rolling contact fatigue (RCF) of high-speed wheels has become one of the main concerns in the expanding Chinese high-speed railway industry, e.g., a new type of local wheel RCF was observed in a series of Chinese high-speed trains in 2012 and the cracks were found to propagate to a depth of 5-8 mm whithin about three months, being a great threat to running safety. This project will first conduct the geometry and material analyses of fatigued wheels, and measure the material behavior under high strain rate loading and the wheel-rail adhesion under high speeds. In the meantime, a 3-D transient wheel-rail rolling contact model will be developed by the explicit finite element method for high speeds, in which the influences of the rolling speed, the rolling behavior and the high-frequency vibrations are inherently included and the measured contact geometries, the strain rate dependent material model and the high-speed wheel-rail adhesion behavior can be introduced. Afterwards, based on the multi-body simulation results, parametric analysis of the transient rolling contact model will be performed to study the transient material responses under the rolling contact loading and to evaluate the material wear and plastic deformation. By further combining the numerical analyses and the microscopic observations and measurements of RCF samples, the initiation and propagation mechanisms of local RCF of high-speed wheels will be revealed, which builds a theoretical basis and provides a guideline for increasing the reliability of high speed wheels and reducing the maintainance costs. Finally, cases which are vulnerable to the local wheel RCF will be figured out and some counter measures will also be recommended.

高速列车的车轮滚动接触疲劳随着我国高铁运营里程的持续增加逐渐成为急待解决的问题，例如，2012年新发现的一种局部接触疲劳裂纹能在三个月内扩展到5-8 mm深，严重威胁行车安全。本项目首先针对疲劳车轮进行几何、材料分析，并测定高应变率下的材料本构和高速轮轨黏着特性。同时，采用显式有限元法建立三维弹塑性瞬态滚动接触模型求解高速轮轨滚动接触，其中考虑速度、滚动行为、高频振动的影响并引入实测接触几何、含应变率影响的材料模型和高速黏着特性。之后，结合高速车辆-轨道耦合动力学结果，应用滚动接触模型计算高速滚动接触载荷条件下的材料瞬态响应，评估表层材料的磨损、塑性流变，进一步结合疲劳试样的微观材料分析结果，揭示高速车轮局部滚动接触疲劳的萌生机理与裂纹扩展规律，为增加高速车轮可靠性、降低维护成本提供基础数据和理论指导。最后，找到导致高速车轮局部滚动接触疲劳的载荷条件及列车运行状态，提出减缓措施。

在现场调查测试、关键参数测定和数值模拟分析的基础上，本项目详细分析了我国高速动车组近几年出现的车轮局部滚动接触疲劳（局部疲劳）的主要损伤特点、萌生机理、发展过程及可能后果。跟踪、调研数据及试样分析结果表明，该损伤与材料缺陷无关，应该主要由踏面硌伤引发，相应案例实践中确有观测。此结论既与69%的局部疲劳发生在硌伤概率最大的动车组头轴上的事实相吻合，亦可解释镟修后不重现的现象。通过建立考虑材料弹塑性及应变率影响的三维瞬态轮轨滚动接触有限元模型，详细研究了高速滚动接触条件下局部疲劳由硌伤萌生的机理。结果表明，局部几何比较平滑的硌伤，会降低动车组的运行品质，但不足以引发局部疲劳；相对而言，局部疲劳更可能萌生于尺寸大于2-4 mm的局部几何尖锐的深硌伤处。裂纹的具体萌生位置应该不是硌伤表面，因为表面尖锐几何很容易被磨损和塑性变形改变，难以萌生裂纹，即使萌生了裂纹，形成小块材料脱落（较大块磨屑）的可能性最大，很难往材料内部扩展。与钢轨无法接触的硌伤底部，若其局部几何足够尖锐，会有充足的时间来萌生裂纹并扩展至材料内部，从而更易于萌生局部疲劳。鉴于此萌生机理，建议及时清理轨道上存在的可能造成较深硌伤的异物，从根本上解决问题，同时，改进现有维修标准，及时镟除车轮表面尺寸大于2-4 mm的深硌伤。若无法及时镟修，平均裂纹深度扩展速率约为0.18 mm/万公里，即运行约17天后，裂纹深度扩展0.5 mm。从避免局部疲劳的角度而言，没有必要对硌伤车轮实行限速。需说明，若任其发展，车轮局部疲劳裂纹会扩展到2.5~8 mm深，之后很可能向表面扩展，最终形成较大块剥离，不太可能会向内部扩展，即不会导致车轮崩箍等严重事故。上述研究成果，为更详细研究局部滚动接触疲劳现象指明了方向，也为实践中损伤车轮的维修提供了支撑。