大厚度铝合金板穿透裂纹扩展机理及寿命预测模型研究

The in-depth study of three-dimensional (3D) crack propagation and fracture mechanisms, and the development of 3D damage tolerance assessment methods and models of metal plate with large thickness, are of great significance to ensure the safe operation of large aircraft and other large machinery and equipment. The applicant found that when the plate thickness increased to a certain dimension, the crack front appeared "saddle-shaped" morphology in the unilateral crack propagation test of 7050 aluminum alloy plates with different thickness. The existing fatigue crack growth mechanism and models can not describe this phenomenon, and there is a big risk in the damage tolerance assessment of thick metal plates based on the traditional models. In the project process, a combination of theoretical and experimental methods will be used, and the physical mechanism of “saddle-shaped” crack front in aluminum alloy plate with large thickness will be ascertained, the essential reason of thickness effect on crack growth rate along through-thickness crack front will be revealed, and calculation models and methods of 3D through-thickness crack growth rate and crack front morphology in thick aluminum alloy plates will be established, then the 3D damage tolerance assessment model of thick aluminum alloy plates will be built. The research results will enrich the theory system of 3D fatigue & fracture in the aspect of theory, and perfect 3D damage tolerance analysis and assessment methods of metal structure in the aspect of technology.

深入研究大厚度金属板三维裂纹扩展及断裂机理，发展大厚度金属板三维损伤容限评估方法及模型，对保证大飞机及其它大型机械设备的安全运行具有重大意义。申请人在开展不同厚度7050铝合金单侧裂纹板疲劳裂纹扩展试验中发现，当板厚度增大到一定程度时，裂纹扩展前沿出现了“马鞍形”形貌，这种现象无法用现有的疲劳裂纹扩展机理与模型描述，且基于传统模型对厚金属板进行损伤容限评定存在很大风险。本项目将采用理论和实验相结合的研究方法，探明大厚度铝合金板穿透裂纹前沿呈“马鞍形”形貌的物理机理，揭示厚度效应导致的穿透裂纹在沿厚度方向产生不同扩展速率的本质原因；建立一套大厚度铝合金板三维穿透裂纹扩展速率计算模型及扩展形貌计算方法；进而建立起大厚度铝合金板三维损伤容限评估模型方法。研究成果将在理论上丰富三维疲劳/断裂理论体系，在技术上完善金属结构三维损伤容限分析评估方法。

本项目主要开展了：①含曲线前沿三维裂纹体的弹塑性有限元分析；②板厚对裂纹扩展形貌、裂纹扩展寿命与剩余强度的影响规律；③不同厚度板三维穿透裂纹扩展形貌计算方法；④厚度效应导致的大厚度板穿透裂纹在沿厚度方向产生不同扩展速率的物理机理；⑤不同厚度大厚度铝合金板三维损伤容限评估模型等内容的研究。取得的主要成果包括：①揭示了疲劳裂纹扩展“马鞍效应”的物理机理；②建立了一套连续厚度板的三维穿透裂纹扩展形貌计算方法；③建立起连续厚度的大厚度铝合金板三维损伤容限评估模型。通过研究发现：①裂纹尖端应力沿厚度的分布不是均匀的，应力最大值及其位置与厚度有关。当板的厚度较小时，中面的应力最大,随着厚度的增加应力最大值的位置逐渐向板边缘移动，板越厚应力最大值的位置距板中面越远。②大厚度铝合金单侧裂纹板疲劳裂纹萌生非单一疲劳源，且非角裂纹扩展；③大厚度铝合金单侧裂纹板疲劳裂纹稳定扩展属于典型的韧性疲劳纹，随着试样厚度的增加，解理裂纹形式更加明显；④大厚度板中间区域裂纹扩展速率相比侧面裂纹扩展速率较低，是导致快速扩展区域呈现出“马鞍形”的直接原因。本项目研究成果在理论上丰富了三位疲劳/断裂理论体系，在技术上完善了金属结构三维损伤容限分析评估方法。