预循环应力对高速列车关键结构用铝合金材料疲劳裂纹扩展行为的影响

Effect of Pre-Cyclic Stress on Fatigue Crack Propagation Behavior of Key Structural Al Alloy Materials Used in High Speed Trains

对服役多年的高速列车关键结构用铝合金材料进行断裂力学性能测试,探究其稳态扩展区域和近门槛值区域的疲劳裂纹扩展行为,阐释服役材料力学行为的变化规律。鉴于服役材料疲劳裂纹扩展行为的折线特征,开展不同水平预循环应力作用下的断裂力学实验研究,剖析预循环应力对材料断裂力学性能"锻炼"效应的影响,建立合理的寿命预测模型,并与实测数据进行对比。研究表明:服役材料在稳态裂纹扩展阶段的疲劳裂纹扩展行为呈折线特征,近门槛值区域的延迟扩展行为是促使折线现象产生的原因;经历预循环应力作用后,材料疲劳裂纹扩展速率与应力强度因子范围(da/d N-ΔK)的关系曲线在稳态裂纹扩展的初期阶段呈现类似服役材料的折转行为;不同水平的预循环应力对材料断裂力学性能的"锻炼"效果不同,存在能够使材料达到最佳"锻炼"效果的预循环应力水平;对于折转型da/d N-ΔK关系曲线,本工作所建立的计算模型能够更加准确地预测材料的疲劳裂纹扩展寿命。

The complex cyclic loading is a "potential killer" affecting the service security of high speed trains. It is necessary to investigate the influence of cyclic loading on vehicle structures and explore potential methods to improve the service strength and life of structural materials. In this work, the mechanical property tests for key structural materials(Al alloy) that experienced years of service were described, the fatigue crack propagation(FCP) behavior at different stages were analyzed, and the changing pattern of mechanical behavior of material was demonstrated over time. Since the specimens showed turning characters on FCP behavior, the mechanical property tests for materials that subjected to different levels of pre-cyclic stress(PCS) were further carried out to analyses the "coaxing" effects of PCS and establish a more reasonable life prediction model for materials. It is found that a turning phenomenon or "turning" point is clearly shown in the early stage of the fitted curves for the specimens with service experience, which is mainly due to the delayed extension behavior in the region near the threshold;the curves of crack propagation rates and stress intensity factor(da/d N-ΔK) of specimens subjected to PCS show a similar turning phenomenon at the initial stage of steady-state crack growth to that of specimens with service experience;the "coaxing" effect of PCS on material is different for different PCS levels, and there is an optimal PCS for the "coaxing" effect;the model proposed in this study has higher accuracy in FCP life prediction for the da/d N-ΔK curves with "turning" character.