考虑热切割残余应力的钢桥面横隔板疲劳损伤机理研究

Thermal cutting has been adopted to improve the shape of the cutouts of the diaphragms in existing orthotropic steel decks. It is a reasonable maintenance technique that can relieve the stress concentration under vehicle loads and reduce the fatigue stress range. At the same time, the high-level residual stress caused by thermal cutting may rouse the risk of secondary fatigue cracking of cutouts of the diaphragms.. This project is aiming at this new problem. Focusing on thermal-cutting residual stress, the fatigue damage mechanism will be studied for the diaphragms of existing orthotropic steel decks. The main contents are as following. (1) The generating and coupling mechanism of thermal-cutting residual stress can be investigated based on component tests and numerical simulation. The distribution model of the thermal-cutting residual stress will be established for the fatigue analysis of diaphragm cutouts. (2) The initiation and propagation mechanism of fatigue cracks in diaphragm cutouts will be studied through fatigue tests. With considering the combined action of thermal-cutting residual stress field and vehicle-induced stress field, the fatigue analysis method will be proposed based on the total strain energy method of strain-based fatigue theory. (3) The reducing mechanism based on ultrasonic impact treatment will be studied for the thermal-cutting residual stress. Then, the fatigue tests, in which the reducing effects of the thermal-cutting residual stress is involved, will be carried out. At last, the fatigue life method will be presented for diaphragms of existing orthotropic steel decks. This program will provide knowledge to fatigue maintenance of existing orthotropic steel decks.

通过热切割对在役钢桥面横隔板弧形切口的形状进行改进，是缓解其在车辆荷载下的应力集中，减小疲劳应力幅的合理维护方法。然而，由此产生的高水平热切割残余应力使得横隔板弧形切口存在二次疲劳开裂的风险。. 本项目针对这一新问题，开展考虑热切割残余应力的钢桥面横隔板疲劳损伤机理研究，主要包括：（1）基于构件测试和数值模拟揭示热切割残余应力的产生与耦合机理，建立适用于横隔板弧形切口疲劳分析的热切割残余应力场分布模型；（2）通过疲劳试验研究横隔板弧形切口疲劳裂纹的萌生与扩展全过程机理，考虑热切割残余应力场和车载应力场的共同作用，基于应变疲劳理论中的总应变能法提出横隔板弧形切口的疲劳分析方法；（3）揭示基于超声波冲击的热切割残余应力场消减机理，进而开展热切割残余应力消减后的疲劳构件试验，建立在役钢桥面横隔板疲劳寿命预测方法。研究成果可为在役正交异性钢桥面疲劳维护提供科学依据。

早期对正交异性钢桥面疲劳损伤机理的认识不足，某些钢桥面横隔板弧形切口的形状设计不合理，应力集中严重，使得此处在服役短时间之后就出现大量疲劳裂纹，成为正交异性钢桥面疲劳开裂的重灾区，横隔板弧形切口的疲劳开裂机理与维护加固有待开展研究。本项目开展的主要研究和成果包括：（1）分析了横隔板-U肋焊接过程温度场及残余应力场，建立残余应力与车载应力叠加模型，提出了可考虑焊接残余应力的疲劳评估方法。明确了弧形切口的疲劳开裂机理，横隔板与U肋的焊接过程导致弧形切口边缘产生了高水平的残余拉应力，在残余拉应力与车载压应力的作用下，产生疲劳开裂。以某悬索桥钢箱梁为例，提出的疲劳评估方法可以准确计算弧形切口的疲劳寿命，计算疲劳寿命约为8年，和工程实际开裂寿命8-9年基本吻合。（2）针对横隔板弧形切口母材残余应力变化过程，提出了焊接-热切割-超声冲击处理全过程数值模拟流程，考虑U肋-横隔板焊接、横隔板弧形切口热切割与超声冲击处理弧形切口3个步骤，研究了残余应力场的变化规律。研究表明，U肋-横隔板焊接在弧形切口下起弧点产生残余拉应力，高达103MPa；横隔板热切割过程在弧形切口处产生的残余拉应力达340MPa；超声冲击处理后，残余应力峰值减小，超过屈服强度的塑性残余应力得到明显消减，可消减至25MPa。（3）提出了热切割优化弧形切口形状和冷贴加固板材的弧形切口综合疲劳维护方案，建立了某悬索桥钢箱梁的节段有限元模型，计算了弧形切口维护前后的车载应力响应，研究表明，该维护方式可以有效改善应力集中并降低弧形切口的车载压应力峰值。（4）针对正交异性钢桥面板顶板-U肋焊缝疲劳开裂问题，提出在顶板表面粘贴小尺寸增强板材的疲劳加固方法。采用碳纤维增强复合（CFRP）板和钢板2种疲劳加固板材，开展钢桥面局部区域足尺模型疲劳试验，研究表明：顶板表面粘贴4mm厚CFRP板和钢板，焊缝疲劳寿命分别提高87%和196%，双层4mm厚CFRP板叠层粘贴加固后，焊缝疲劳寿命理论上可提高4.9倍。

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