基于构型力理论的应变疲劳裂纹扩展驱动机理及控制参量

The stress intensity range ΔK and the experimental cyclic J-integral are the mainly used fatigue crack propagation control parameters. ΔK is limited to elastic and small scale yielding fracture mechanics. And the experimental cyclic J-integral is difficult to characterize the crack driving force due to the lack of theoretical basis. The configurational force theory is based on the movement of defects in materials, and the integral of configuratinal stress at infinite small area near crack tip is the crack driving force. Therefore, the study of strain fatigue crack propagation mechanism and control parameter based on configurational force theory has physical basis. In this research project, firstly, based on the configurational force theory and incremental plasticity J-integral, the theoretical calculation method of the control parameter for strain fatigue crack propagation will be established. Meanwhile, combining with the global strain field experimental technique, the experimental method for characterizing the control parameter will be established, and the theoretical calculation method will be verified. And then, based on the evolution laws of crack driving force, global stress strain field near crack tip, control parameter and microstructure near crack tip with the strain fatigue crack propagation, the driving mechanism of strain fatigue crack propagation will be revealed. At last, from the view of crack propagation driving mechanism, the prediction model of strain fatigue crack propagation based on the configurational force theory will be established. The control parameter and prediction model based on the configurational force theory can overcome the shortcomings of classical fatigue crack growth characterization methods, such as the lack of theoretical basis and inapplicability of elastic-plastic materials. And it can provide a new approach for the study of fatigue crack growth behavior.

疲劳裂纹扩展的控制参量多采用应力强度因子幅ΔK或实验循环J积分。ΔK限于线弹性小范围屈服，后者理论基础不足难以表征裂纹扩展驱动力。构型力理论源于材料的缺陷运动，以构型力张量在裂尖无限小区域积分为裂纹扩展驱动力，因此基于构型力理论对应变疲劳裂纹扩展驱动机理及控制参量研究具有物理基础。本课题首先基于构型力理论与增量塑性J积分建立应变疲劳裂纹扩展控制参量的理论计算方法；同时结合全局应变场实验技术建立控制参量的实验表征方法，实现理论方法的论证；然后基于裂纹扩展驱动力、裂纹尖端全局应力应变场、控制参量以及裂尖组织随应变疲劳裂纹扩展的演化规律，揭示应变疲劳裂纹扩展的驱动机理；最后从裂纹扩展驱动机理出发，构建基于构型力理论的应变疲劳裂纹扩展预测模型。本项目基于构型力理论构建的控制参量与预测模型能够克服经典疲劳裂纹扩展表征方法在理论基础和弹塑性材料应用的不足，为应变疲劳裂纹扩展行为的研究提供一个新途径。

经典疲劳裂纹扩展控制参量包括应力强度因子幅ΔK与实验循环J积分，前者限于线弹性小范围屈服，后者理论基础不足。裂纹尖端构型力为裂纹扩展驱动力，基于构型力理论对疲劳裂纹扩展行为研究具有物理基础。本项目围绕疲劳裂纹扩展驱动机理及控制参量，采用构型力理论、疲劳理论、断裂力学理论、疲劳试验与疲劳裂纹扩展试验、数字图像相关技术（DIC）、显微组织分析、有限元仿真等全方面多维度的研究方法，从疲劳裂纹扩展行为、裂纹尖端循环应变场、基于构型力理论的疲劳裂纹控制参量、疲劳裂纹扩展速率、疲劳裂纹扩展模型等方面开展系统研究，主要研究结果包括：. （1）基于疲劳试验、疲劳裂纹扩展试验，结合微观组织分析揭示了奥氏体不锈钢疲劳裂纹扩展规律以及疲劳失效机理。. （2）结合疲劳裂纹扩展试验与DIC方法实现了疲劳裂纹尖端循环应变场表征，并在裂纹尖端区域建立了循环应变环，不仅能够表征循环载荷下裂纹尖端的循环塑性区、单调塑性区、弹性区，还能够表征随疲劳裂纹扩展，弹性区发展为单调塑性区、循环塑性区的演化过程。. （3）结合有限元仿真和Matlab编程，获取了疲劳裂纹尖端区域的构型力分布规律，构型力的最大值集中于裂纹尖端附近，随着疲劳裂纹扩展构型力峰值逐渐增大，由此为疲劳裂纹扩展提供了驱动力。. （4）以单调塑性区边界和循环塑性区边界为围道积分路径，建立了基于构型理论的疲劳裂纹扩展控制参量，既能够从物理角度表征疲劳裂纹扩展驱动力，又能够从定量角度计算疲劳裂纹扩展速率，满足了理论研究与工程应用两方面的需求。. （5）基于构型力理论构建了具有物理意义的疲劳裂纹扩展预测模型，通过试验验证，能够有效预测疲劳裂纹扩展行为。. 通过本项目的研究，从疲劳裂纹扩展的本质即裂纹扩展驱动力出发，揭示了疲劳裂纹尖端的循环塑性变形、疲劳裂纹扩展机理、循环应变场的演化规律，在此基础上构建了基于构型理论的疲劳裂纹扩展控制参量，建立了疲劳裂纹扩展的预测模型，本项目的研究成果补充了疲劳裂纹理论知识，进一步完善疲劳知识体系，并且为保障含裂纹结构在疲劳载荷下的安全稳定运行提供理论基础。