载荷频率对车轴钢疲劳短裂纹行为的影响研究

The change of loading frequency can lead to the variation of the fatigue properties of material, and in turn affects the assessment results of fatigue reliability and safety for structure or components. However, relevent research on the mechanism and the regularity of related phenomena is still not mature and complete. Because of this, based on the replica tests on surface short fatigue cracks of smooth hourglass shaped specimens for railway LZ50 axle steel at several different loading frequencies, the influence of loading frequency on the initiation and the propagation of short fatigue cracks are investigated in this project. The essential reason of the phenomena that fatigue damage process changes at different loading frequencies is revealed from microcosmic viewpoint, and the mechanism of frequency effects on the behavior of short fatigue cracks is clarified. Furthermore, by introducing a loading frequency function, a new short fatigue crack growth model that includes multiple micro-structural barriers and the loading frequency is presented. The influence and its tendency of frequency effect on the reliability of structure safety assessment is illustrated by utilizing the new model. Thereby, the principle and the method for choosing the crack growth rate curves reasonably while performing safety assessment of axle is investigated. The results can provide the theoretical foundation for the probabilistic safety assessment and management for the key components of railway vehicles.

载荷频率的改变可导致材料疲劳性能变化，进而对结构或零部件疲劳可靠性与安全性评价结果产生影响。现有针对相关现象的机理与规律性研究尚不完善。鉴于此，本项目通过完成低频至高频范围内的多组LZ50车轴钢光滑漏斗形试样的疲劳短裂纹复型试验，对比研究载荷频率对材料短裂纹萌生与扩展行为的影响。从微观角度揭示频率变化导致疲劳损伤过程发生改变的本质原因，明确频率效应对短裂纹萌生与扩展过程的促进或抑制机理。进一步，引入"载荷频率效应函数"，建立一种在能反映疲劳短裂纹与各种微观结构障碍相互关系的同时，还可量化载荷频率影响的短裂纹扩展率新模型。基于该模型，阐明载荷频率效应对结构安全性评定结果可信度的影响及趋势；探索在进行车轴安全性评价时，合理选择材料裂纹扩展率曲线的原则与方法，为铁道车辆关键零部件的概率安全性评价与管理提供理论依据。

载荷频率是影响材料疲劳短裂纹行为的可能因素之一，本项目以LZ50车轴钢为对象，开展了15Hz、140Hz、160Hz和180Hz共4种不同载荷频率下的光滑漏斗形试样短裂纹复型试验和相关辅助试验，对比研究结果表明：(1) 在本项目试验涵盖的低常频与高频载荷范围内，载荷频率对短裂纹的萌生与扩展行为影响十分有限，载荷频率效应可以忽略；(2) 以主导有效短裂纹(DESFC)尺度为驱动力、包含多微观结构障碍影响的短裂纹扩展率模型能较好描述短裂纹扩展行为；(3) 尽管载荷频率对短裂纹扩展行为影响有限，但在DESFC 尺度首次突破晶界约束前的MSC 阶段，即短裂纹扩展出现第一次较显著降速前，较低载荷频率下短裂纹扩展率相对较高，试验后显微硬度提高较大；(4) 进行车轴安全性评价时，裂纹扩展率曲线的选取无需考虑载荷频率的影响。