基于扩展有限元和循环内聚力模型的榫连结构微动疲劳裂纹萌生与扩展研究

Due to the fretting fatigue crack, the fracture failure of dovetail joints in the aero-engine service results in incalculable loss and disaster for aircrafts. The disagreement of how to define the boundary between initiation and propagation stage still exists currently. The existing mechanical models cannot describe the damage evolution of these two stages simultaneously, they need to combine with another model to predict the total fatigue life of the fretting fatigue crack, thus the computational efficiency is inefficient. The present proposal aims to establish a cyclic cohesive zone model (CCZM) which can describe initiation and propagation of the fretting fatigue crack uniformly, and combine it with the extended finite element method (XFEM) to simulate the whole formation process of the fretting fatigue crack numerically, meanwhile the computational difficulty of the conventional finite element method due to the mesh structure will be resolved. Firstly, a multi-axial fatigue criterion will be developed to determine the initiation position and propagation direction of the fretting fatigue crack within the framework of the XFEM. Secondly, a damage evolution equation of the material will be established by taking the effects of the external cyclic load, the contact pressure, the friction and other main influence factors into account, based on which the gradual change process of crack propagation from fretting fatigue to plain fatigue should be described. Finally, based on the established CCZM, the initiation and propagation life of the fretting fatigue crack will be predicted. The model could provide an effective theoretical analysis method for assessing the fretting fatigue life of dovetail joints in aero-engines.

微动疲劳裂纹导致的航空发动机榫连机构在服役期间的断裂失效给航空界造成了不可估计的损失与灾难。目前对如何界定微动疲劳裂纹的萌生与扩展阶段仍然存在分歧,现有力学模型很难同时描述这两个阶段的损伤演化规律，需要结合其他力学模型才能进行微动疲劳裂纹的全寿命预测，从而计算效率很低。本项目旨在建立一个可以统一描述微动疲劳裂纹萌生与扩展的循环内聚力模型，并结合扩展有限元对裂纹的整个形成过程进行数值模拟，同时也将克服传统有限元计算受网格结构影响的困难。首先，在扩展有限元法的框架下，构建判断微动疲劳裂纹的萌生位置与扩展方向的多轴疲劳准则；其次，建立可以量化循环外力、接触压力、摩擦力等微动疲劳主要影响因素的材料损伤演化方程，予以描述裂纹扩展从微动疲劳到常规疲劳的渐变过程；最后，基于所建立的循环内聚力模型完成对微动裂纹萌生与扩展寿命的预测，从而为航空发动机榫连结构的微动疲劳寿命估算提供有效的理论分析方法。

微动疲劳裂纹导致的航空发动机榫连机构在服役期间的断裂失效给航空界造成了不可估计的损失与灾难。目前对如何界定微动疲劳裂纹的萌生与扩展阶段仍然存在分歧，现有力学模型很难同时描述这两个阶段的损伤演化规律，需要结合其他力学模型才能进行微动疲劳裂纹的全寿命预测，从而计算效率很低。本项目旨在建立一个可以统一描述微动疲劳裂纹萌生与扩展的循环内聚力模型，并结合扩展有限元对裂纹的整个形成过程进行数值模拟。.项目研究内容概括如下：（1）对弹塑性情形下的Ti-6Al-4V合金微动疲劳接触区域的受力进行有限元分析，考察接触压力、切应力以及远端循环载荷的影响。（2）针对判断裂纹萌生的多轴疲劳准则进行了比较，最终确定采用Smith-Watson-Topper（SWT）参数作为裂纹萌生判据，结合不同的循环内聚力模型分别对低周和高周疲劳裂纹萌生与扩展进行数值模拟。（3）设计并搭建了一套液压式微动疲劳试验加载工具。（4）构建了描述疲劳裂纹扩展高载迟滞效应的循环内聚力模型。（5）将循环内聚力模型的应用范围拓展高速率疲劳裂纹扩展的数值计算。（6）详细总结并探索了将各类力学模型与扩展有限元法相结合，以及构建既可以描述微动疲劳损伤演化过程又可以跟踪裂纹形成路径的计算方法。（7）利用细观损伤力学模型研究了片层结构β钛合金的沿晶裂纹扩展过程。.基于上述研究内容，定性或定量的建立了基于循环内聚力模型对微动裂纹萌生与扩展寿命的预测的方法，为航空发动机榫连结构微动疲劳的初步设计提供可行的分析途径。此外，构建的细观损伤力学模型的计算框架可以为片层结构β钛合金制成的断裂关键构件的热处理过程提供指导方案。