强烈地震下多高层钢框架焊接结点裂纹萌生与扩展疲劳寿命的预测和评估

It is crack initiation and propagation that is a typical earthquake damage for multistory and high-rise steel frame welded beam-to-column connections under strong earthquake excitation. There have been numerous studies to investigate the elastoplastic fatigue properties of damaged structural elements subject to inelastic cyclic loading, however, the limitations of these different involved approaches is that fatigue life assessment is implemented based on element rotation and displacement correlated to the welded beam-to-column connections, hence, the accurate method of fatigue crack initiation and propagation life assessment of welded beam-to-column connections is elusive. The purpose of this project, from the perspective of plastic strain, is to explore and develop a more accurate method for fatigue crack initiation life prediction and create a preliminary framework for fatigue crack propagation life assessment. The contents needed to discussed in detail of the project is:1)the multi-scale computational method for plastic strain state of multistory and highrise steel frame welded joints under strong earthquake excitation; 2)the fatigue life S-N curve for weld material of welded joints; 3)the computational method of fatigue crack initiation damage parameter based plastic strain critical plane approach and its response amplitude spectrum of welded joints; 4)the plastic strain fatigue crack initiation life prediction formula of welded joints under strong earthquake excitation; 5)the computational method of fatigue crack propagation life of welded joints under strong earthquake excitation. Including that the key scientific issues is: computation of the cycle amplitude spectrum which is used to characterize the plastic strain fatigue damage parameter under multiaxial low cycle non-proportional loading, and analysis of influence factors in the cycle amplitude spectrum correlating to fatigue crack initiation life.

强裂地震时多高层钢框架结构梁柱焊接结点的裂纹萌生与扩展是一种典型的震害。目前国内外的研究仅局限于对与焊接结点相联的梁柱杆端转角和位移的抗弹塑性累积疲劳性能的评估，无法做到对焊接结点裂纹萌生和扩展疲劳寿命的精确评估。本项目从塑性应变疲劳的角度探讨研究强震下多高层钢框架焊接结点裂纹萌生疲劳寿命较精确的预测方法和裂纹扩展寿命初步的评估方法。项目需详细探讨：1）强震下多高层钢框架焊接结点塑性应变状态的多尺度计算方法；2）焊接结点焊缝材料疲劳寿命的S-N曲线；3）焊接结点焊缝裂纹萌生塑性应变疲劳临界面法的损伤参量及其响应幅值谱的计算方法；4）强震下焊接结点焊缝危险点裂纹萌生塑性应变疲劳寿命的预测公式；5）强震下焊接结点裂纹扩展的计算方法。其中的关键科学问题是：多轴低周非比例加载条件下表征塑性应变疲劳的损伤参量的响应幅值谱的计算及响应幅值谱中影响裂纹萌生疲劳寿命计算的因素分析。

由于焊缝处塑性应变的累积疲劳损伤，强烈地震下多高层钢框架结构梁柱焊接结点常会出现焊缝处的裂纹萌生及裂纹向梁和柱内扩展甚至断裂的震害。以往的研究主要集中在对宏观尺度的焊接结点与梁柱杆端的塑性疲劳而展开的，缺乏对于结点焊缝细节的研究，而这正是强烈地震下钢框架梁柱焊接结点塑性疲劳破坏的关键所在。本项目就此展开研究，创建了强烈地震下多高层钢框架焊接结点焊缝裂纹萌生塑性应变疲劳寿命评估的系统化方法。主要的研究成果包括：.① 确定了多维应变状态下建筑钢材焊接试件焊缝处裂纹萌生疲劳寿命的S-N曲线，并对现有的临界面FS准则进行修正，提出了一种既考虑非比例附加强化效应又考虑加载路径影响的规格化的非比例影响因子；.② 试验验证焊接残余应力的松弛规律，证明焊接结点焊缝进入塑性应变状态时，焊接残余应力的影响可以忽略不计；.③ 提出强烈地震下多高层钢框架焊接结点焊缝塑性应变的多尺度分析方法，确定强震时梁柱焊接结点焊缝的精细化应力应变场，证明焊缝疲劳危险点处于三维塑性应变状态，为非比例加载工况；应用临界面FS准则，提出了强烈地震下多尺度钢框架梁柱焊接结点焊缝裂纹萌生疲劳寿命的精细化分析方法；.④ 完成了梁柱焊接结点十字形足尺试件的试验，验证了箱型柱-工字型梁焊接节点焊缝的疲劳破坏机理及裂纹萌生判别标准