高强铝合金接头疲劳全寿命分析，模拟及验证

Aluminum alloy is the first choice in lightweight design, and has been widely used in the CRH train body and aerospace equipment. The fatigue resistance of welded joints plays a significant role in structural safety design, heterogeneous microstructure and non-uniform geometric profile make the fatigue prediction of welded structures much more complicated. The initial crack length and initiation life is figured out by using the strain-fatigue bar tests and mathematical model. The crack initiation behavior in weld zone, heat affect zone and parent metal are also explored in detail. By the mesh insensitive structural stress, the crack propagation life is calculated based on master S-N curve. This approach finally yields the two stage fatigue life model of the high strength aluminum joint based on the strain fatigue and fracture mechanics. The influence of geometrical characteristics (angular distortion, lack of penetration, fillet leg size，notch radius) of cruciform joints on the crack initiation and propagation behavior are also studied, which could provide basic understanding in the fatigue life analytical modeling and geometric design optimization of the joint. In this work, the crack initiation and propagation behavior are quantitatively described in different regions of weld joints, which provides theoretical basis for fatigue design and optimization of the welded joints.

铝合金是轻量化设计的首选材料，在动车组、航天航空设备中被广泛采用。其抗疲劳能力是铝合金焊接结构服役安全的关键所在，然而接头的组织不均匀与结构几何不均匀造成疲劳研究异常复杂。本项目借助圆棒应变-疲劳试验与数值分析，明确裂纹萌生长度与萌生寿命，探索裂纹在接头不同组织中萌生行为。以萌生长度为起点，基于master S-N曲线计算裂纹扩展寿命。将应变疲劳与断裂力学相结合，深入探索十字接头几何特征(角变形、未熔合长度、焊脚尺寸及焊趾过渡半径)对裂纹萌生与扩展行为的影响，提出能够准确预测承载高强铝合金十字接头疲劳全寿命模型与最佳接头设计方案。本项目旨在阐明高强铝合金焊接接头各区中裂纹的萌生行为，定量描述接头几何特征对裂纹萌生、扩展行为的影响，为焊接接头疲劳设计与优化研究奠定理论基础。

铝合金是轻量化设计的首选材料，在动车组、航天航空设备中被广泛采用。其抗疲劳能力是铝合金焊接结构服役安全的关键所在，然而接头的组织不均匀与结构几何不均匀造成疲劳研究异常复杂。首先基于Shercliff-Ashby模型，分析焊接非均匀热过程对接头不同区域的影响，将焊接过程非线性加热过程引起的组织性能与接头强度建立计算模型，从不同区域加热的等效时间可计算其屈服强度，量化了铝合金接头的软化程度。研究了7N01铝合金焊接板材提取紧凑拉伸试样（CT试样）前后试样内的残余应力重分布规律，借助扩展有限元（XFEM）方法建立了含有各个长度静止裂纹的三维CT试样裂纹扩展模型根据Walker公式拟合试验获得的疲劳裂纹扩展数据，获得疲劳裂纹扩展速率随裂纹尖端平均应力强度因子K_m和应力强度因子幅值∆K的变化规律（Walker曲面）。借助Walker曲面分析无残余应力和含残余应力试样在外载荷作用下的疲劳扩展行为。研究结果表明，当试样含残余应力时，随着裂纹增长，可能出现疲劳裂纹扩展速率增加或者降低的情况，在裂纹尖端产生的应力强度因子为零之前，残余应力比较明显地影响疲劳裂纹扩展速率；当裂纹长度很大时，影响疲劳扩展速率的因素主要是外载荷。以萌生长度为起点，节点力法所求的应力场强度因子随裂纹长度的分布变化更符合真实的结果；将所求的应力场强度因子带入Pairs公式求解试件的疲劳扩展寿命，即为master S-N曲线计算裂纹扩展寿命方法。研究十字接头几何特征(角变形、未熔合长度、焊脚尺寸及焊趾过渡半径)对裂纹萌生与扩展行为的影响，提出能够准确预测承载高强铝合金十字接头疲劳全寿命模型与最佳接头设计方案，此计算模型可适用于任意类型焊接接头/结构的疲劳寿命预测。