基于疲劳损伤熵的点焊结构疲劳性能表征与累积损伤评估

The research is based on the background of railway vehicle welded structure service fatigue reliability. The concept of entropy that can measure the irreversibility of things is introduced into the fatigue research field of welded structure, which provides a new method for fatigue problem of welded structure. Research object of the project is dissimilar materials spot welded joints of stainless steel and carbon steel. By conducting high-cycle weekly load fatigue test of dissimilar materials spot welded joints, the temperature evolution rules of the dissimilar materials spot welded joints of stainless steel and carbon steel during the process of fatigue is explored. The model of the quantitative relationship between temperature gradient turning point and fatigue limit is established. What’s more, the relationship between the temperature evolution and energy dissipation in the process of fatigue are studied and established. And the fatigue fracture mechanism of dissimilar materials spot welded joints is revealed combing with the feature of macro morphology, microstructure and fracture analysis. On this basis, the temperature and energy are selected as the intensive quantity and extensive quantity to study and establish the fatigue damage entropy model that can characterize fatigue performance of spot welded joints, and cumulative damage variable evaluation model of dissimilar materials spot welded joints based on the fatigue damage entropy is established. The model can be used as the criterion for fatigue damage of welded structure at the measurable level, so as to realize the effective and reliable evaluation of the cumulative damage for the loading sequence effects.

本项研究以轨道车辆焊接结构服役疲劳可靠性评价为背景，将作为度量事物不可逆性特征的熵的概念引入到焊接结构疲劳研究领域，为焊接结构疲劳问题的研究提供了一种新的思路和方法。项目研究以不锈钢/碳钢异种材料点焊接头为对象，通过开展异种材料点焊接头高周循环载荷疲劳试验，探究不锈钢/碳钢异种材料点焊接头疲劳过程中的温度演化规律，建立温升斜率转折点与疲劳极限之间的定量关系模型；研究并建立疲劳过程中温度演化规律和能量耗散之间的关系，结合宏观形貌、微观组织和断口特征分析，揭示异种材料点焊接头的疲劳断裂机理。在此基础上，以温度和能量作为强度量和广延量，研究并建立用于表征点焊接头疲劳性能的疲劳损伤熵模型，以及基于疲劳损伤熵的异种材料点焊接头累积损伤评估模型。所建立的模型有望在可测量层面上作为焊接结构疲劳损伤的判据，实现对存在加载次序效应的变幅加载累积损伤的有效可靠评估。

项目以轨道车辆焊接结构疲劳性能表征和可靠性评价为背景，以不锈钢/碳钢异种材料点焊接头为主要研究对象，综合应用焊接结构学、热力学、金属学、统计学等理论和方法，开展基于疲劳损伤熵的点焊结构疲劳性能表征与累积损伤评估研究。为此，基于对疲劳过程中热耗散现象的研究和红外热成像原理，首先完成了SUS301L-Q235B异种材料点焊接头疲劳试验设计；通过对疲劳过程中接头不锈钢侧熔核及塑性环表面局部热点的温度监测，研究了异种材料点焊接头温升演化规律与疲劳极限间的定量关系，并在此基础上获得一种基于温升斜率转折点预测异种材料点焊接头疲劳极限的方法，极大提升了试验方法测试疲劳极限的效率；基于信息熵、信息增益率以及邻域粗糙集等智能方法，建立了碳钢与不锈钢电阻点焊接头疲劳性能影响因素的分析模型。在此基础上，研究并建立了疲劳试件温度演化规律与能量耗散之间的关系，建立了一种基于能量耗散理论的点焊接头疲劳寿命快速预测方法；提出了焊接结构疲劳损伤熵的概念，并参考广义热力学熵的构建法则，分别以温度和能量作为强度量和广延量，建立了表征点焊接头疲劳性能的疲劳损伤熵模型，提出了基于该模型的F-N曲线表达式，理论上该模型和F-N曲线表达式最少仅需三组断裂试件的疲劳数据便可得出；从连续损伤力学以及损伤变量构建法则的角度出发，建立了基于疲劳损伤熵的异种材料点焊接头累积损伤评估模型。试验研究结果表明：在给定的点焊工艺参数下，相同几何参数的点焊接头发生疲劳断裂时疲劳损伤熵为定值，且该值与载荷水平、加载频率、疲劳寿命等无关；所建立的累积损伤评估模型能准确地评估SUS301L-Q235B异种材料点焊接头多级加载情况下的疲劳损伤程度并有效地预测其疲劳寿命。研究提出的焊接接头疲劳性能表征方法，在碳钢激光焊对接接头和轨道车辆用Q460高强钢对接接头疲劳性能分析和表征的试验研究中得到成功应用。项目研究为实现焊接结构疲劳性能表征和累积损伤评估提供了新的理论和方法。