钢桥面板肋-面板焊缝疲劳裂纹的三维扩展及寿命分析

Fatigue life evaluation of rib-to-deck welded joints of orthotropic steel deck by using 3-D simulation of crack propagation

为研究钢桥面板肋-面板焊缝疲劳裂纹扩展机理,准确评估其疲劳性能,对肋-面板焊缝疲劳裂纹进行三维扩展分析及寿命预测。以天津海河大桥为工程背景,建立钢箱梁节段模型用于肋-面板焊缝疲劳应力分析,提取特征点的应力作为疲劳敏感区的局部三维断裂力学子模型的边界条件。将初始表面裂纹分别置于焊趾和焊跟处,采用Schwartz-Neuman交替法求解裂纹应力强度因子并进行扩展分析。研究结果表明:肋-面板焊缝疲劳裂纹为Ⅰ型裂纹主导的Ⅰ-Ⅱ-Ⅲ混合型裂纹,在疲劳扩展过程中不再保持平面;萌生于焊趾并向顶板扩展的疲劳裂纹是其主导疲劳失效模式;保证80%焊缝熔透率可以使肋面板焊缝具有较高的疲劳抗力,完全熔透焊并无必要。数值模拟结果和文献报道的试验结果吻合较好,说明所提出的方法能够准确模拟肋-面板焊接细节疲劳裂纹拓展特性,适用于疲劳问题研究。

In order to investigate fatigue cracks propagation mechanism in rib-to-deck welded joints of orthotropic steel deck and accurately evaluate their fatigue properties, 3-D simulation of fatigue cracks propagation and fatigue life prediction in rib-to-deck welded joints were carried out. Prior to the fatigue simulation, a FEA model of standard steel box girder segment based on the Haihe Bridge in Tianjin was used to analyze the fatigue stress at the rib-to-deck welded joints. The resulting stresses at the characteristic points were used as the force boundaries for a local 3-D fracture mechanics model of rib-to-deck welded joint. The initial surface cracks were placed at the weld toe and weld root respectively, and the stress intensity factors of the cracks were computed by using Schwartz-Neuman alternating method. The results show that fatigue cracks at rib-to-deck welded joints are of the mixed mode cracks of Model Ⅰ,Ⅱ and Ⅲ, but dominated by Model Ⅰ, which will no longer remain flat during the propagation process. The fatigue life of the rib-to-deck welded joints is controlled by the crack propagation along the deck thickness at the weld toe. The fatigue resistance of 80% partial joint penetration weld is higher, and weld melt-through is not desirable. The numerical results are in good agreement with the experimental results reported in the open literature, which proves that proposed method can accurately simulate the fatigue crack propagation characteristics of rib-to-deck welded joint, applicable to the research on fatigue problem.