盐雾腐蚀及局部动态塑性失稳条件下接触疲劳损伤机理研究

Offshore wind turbine bearings, which are subjected to rolling contact fatigue, fail in the way of surface pitting and spalling at very early stage of service life due to the effect of salt spray corrosion and transient overloading caused by strong wind and typhoon. Based on the experimental testing and inspections on micro and macro levels and theoretical analysis, the damage mechansim of rolling contact fatigue which is siginficantly influenced by salt spray corrosion and locally dynamic plastic instability is invesigated. Rolling contact fatigue tests in salt spray will be conducted to clarify the formation and development mechanism of surface pitting which is produced under the inter-action of corrosion and sliding wear. Through studying the critical strain rate and energy conditions for adiabatic shear band formation, shear band evolution and its relationship with crack initiation in multiple high strain rate impact tests, the essence of subsurface white etching structure (WES) and its accumulative damage law will be revealed. Intermittent sliding rolling contact fatigue will be carried out to investigate the contribution of sliding wear and contact fatigue to the surface and subsurface damage. Based on this, a characteristic depth of subsurface crack will be proposed and the crack propagation condition will be developed. Therefore, a relationship between the spalling depth and the rolling contact fatigue life can be predicted. It is expected that the present study will give a better evaluation of rolling contact fatigue properties of offshore wind turbine bearings and their influence factors. Effective measures of preventing early failure can then be suggested, which is of great importance to ensure safe and reliable operation theoretically and practically.

海上风机齿轮箱轴承在接触疲劳载荷下，由于受到盐雾腐蚀和强风台风等瞬态过载的影响，过早地在接触表面出现材料点蚀和剥落而导致失效。采用宏微观试验和理论分析相结合，研究盐雾腐蚀环境中局部动态塑性失稳条件下接触疲劳的损伤机理。通过盐雾腐蚀滑移接触疲劳试验，阐明腐蚀和滑移磨损交互作用下接触表面点蚀的形成和发展机理；借助反复冲击试验中绝热剪切带形成的临界应变率和能量条件、损伤演化及其与裂纹萌生的关系，揭示局部动态塑性失稳条件下接触亚表面白色蚀刻组织（WES）损伤的本质特征和累积损伤规律。采用间歇滑移接触疲劳试验，研究滑移磨损和接触疲劳载荷各自的贡献机制和大小。提出亚表面裂纹的特征深度及扩展条件，从而预测材料剥落的深度和接触疲劳寿命的关系。通过本课题的研究，可更准确地评价风机齿轮箱轴承的接触疲劳性能及其影响因素，以便提出合理预防过早失效的措施，这对风机齿轮箱安全可靠运行具有重要的理论意义和实际应用价值。

风机齿轮箱轴承的失效严重影响齿轮箱的可靠运行。本项目研究了轴承钢在盐雾腐蚀下表面点蚀的形成、接触亚表面白色蚀刻区域（WEA）的本质、准静态下剪切带的形成和发展、亚表面裂纹沿残奥晶界的扩展、纳米层状晶对轴承钢摩擦性能的影响。结果如下：.1）点蚀的形成和发展主要受滑移量的影响。小滑移条件下试样表面分布大量点蚀，裂纹从点蚀坑底部或边缘萌生扩展，其深度与接触区最大剪应力范围一致。大滑差率下点蚀被磨掉，盐液、磨削和润滑油混合后，润滑油被污染，磨损占主导作用；.2）发现并证实了WEA中发生了严重的非晶化和奥氏体化。马氏体轴承钢WEA的形成取决于局部微观组织，受外界载荷条件的影响不大。该工作首次报道了疲劳载荷可诱导大量非晶和奥氏体相变的产生，这为疲劳载荷下材料的相变提供了重要依据；.3）在剪切-压缩试样准静态条件下产生了大量的剪切带，该结果为高应变率和高温升不是剪切带产生的必要条件提供了直接的证据。WEA和剪切带都是大剪切塑性变形下应变高度局域化的结果。揭示了WEA的本质是剪切带，而非接触疲劳独有的特征，这为理解接触疲劳失效机理提供了重要依据；.4）接触疲劳亚表面裂纹沿着残奥晶界开裂，即轴承钢中残奥的存在是亚表面裂纹的主要根源之一。这证实了WEA不是因裂纹面相互摩擦机制而产生；.5）轴承钢干滑动磨损下材料内部温升可达600°C，在机械主导热辅助机制下形成纳米梯度片层状组织。但润滑滑动磨损下温升不超过200°C，纳米片层状组织在塑性变形中发生明显的晶粒粗化。不同于软金属材料，轴承钢的摩擦系数不会因形成纳米片层状组织而减小。因此，对表面材料纳米化并不能改善摩擦性能；.6）从新的角度解释了超高周疲劳亚表面夹杂处FGA形成的机理。也即，FGA是塑性应变累积塑性变形诱发晶粒细化的结果。该工作的重要意义在于将接触疲劳WEA和超高周疲劳FGA统一起来，他们都是剪切应力主导下的塑性应变累积的结果。材料微观组织对疲劳载荷亚表面失效起决定性因素。