多轴应力下基于临界面法的缺口件疲劳寿命预测研究

Components in aerospace and other fields usually undergo multiaxial complex loading in service. Even if the far field is uniaxial stress state, the root of the notch is still in the state of multiaxial stress. A precise and reliable design methodology capable of accurately performing the fatigue assessment in the presence of stress/strain raisers under multiaxial fatigue loading is needed urgently to meet the requirement of designing light and long-life structures. This project intends to perform a multi-scale study on fatigue behavior of notched specimens of TC4 titanium alloy under multiaxial proportional and nonproportional loading by comprehensively applying the experimental research method, theoretical research method and numerical simulation method. Firstly, micro-mechanism of fatigue damage of notch under mixed mode loading is investigated based on the experimental observation on micro-morphology and in situ observation of stageⅠcrack initiation and propagation. Secondly, based on the concept of the critical plane, the driving force of fatigue crack initiation and early propagation of notched specimen under mixed mode multiaxial loading is studied. Then, the influence of the stress/strain gradient field and the plastic zone at the notch root on the fatigue crack initiation and early propagation is investigated based on the local strain energy density in a given control volume close to the notch root, and the control parameter of fatigue damage of notched specimen under multiaxial fatigue loading is proposed. Finally, a new method for estimating fatigue lifetime of notched components under multiaxial fatigue loading is devised. This research is aimed at provide theoretical foundation for structural design and life assessment of components of aircraft, engines, vehicles and other structures.

航空航天等领域结构件往往处于多轴复杂载荷下服役，即使远场为单轴应力状态，在缺口根部仍处于多轴应力状态。为满足结构轻质、长寿命设计要求迫切需要提出精确可靠的多轴应力下缺口疲劳寿命预测方法。本项目拟通过试验研究、理论研究和数值分析相结合的方法，对TC4钛合金缺口件在多轴比例及非比例载荷下的疲劳行为进行多尺度研究。以对多轴载荷下钛合金缺口件试验观测为基础，揭示缺口在复合模式载荷下的疲劳损伤微观机理，基于临界面概念对缺口根部在复合模式载荷下裂纹萌生及早期扩展的驱动力进行探讨，基于能量法研究缺口根部应力/应变场及塑性区对裂纹萌生及小裂纹扩展影响，提出多轴应力下缺口疲劳破坏的控制参量，最终建立多轴应力下缺口疲劳寿命预测方法。研究结果可为航空航天飞行器、发动机和车辆等工程结构的承力构件结构设计及寿命评估提供理论基础。

为了满足先进飞行器轻质、长寿命、高可靠性的发展需求，增材制造金属结构受到了航空领域的广泛关注。本项目通过试验研究、理论研究和数值分析相结合的方法，对激光选区熔化316L不锈钢多轴缺口疲劳寿命预测方法进行了研究。揭示了激光选区熔化316L缺口件在复合模式载荷下的疲劳损伤机理，建立了符合损伤机理的临界面定义，明晰了缺口根部塑性区与寿命之间的映射关系，基于缺口根部虚拟能量参数，最终建立多轴应力下缺口疲劳寿命预测方法。研究成果可应用于增材制造先进飞行器结构设计及结构寿命评估，为提高先进飞行器服役寿命及可靠性提供基础支撑。

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