An Application of Fracture Mechanics to the Fatigue Design of Welded Bridges

研究了应力比、焊接缺陷和板厚对焊接桥梁构件疲劳强度的影响，提出了桥梁设计时应考虑的因素。本文主要研究内容如下：研究了应力比对高拉伸残余应力场中疲劳裂纹扩展的影响。试件由600 Mpa级钢制成，为中心裂纹型，沿钢板中心含有纵向焊道。测量了几种应力比(-1~0.91)下的疲劳裂纹扩展速率和裂纹张开水平。在这些试验条件下，无论应力比如何，裂纹都完全张开，疲劳裂纹扩展速率和门槛值应力强度因子范围与应力比无关。利用断裂力学的概念进行疲劳裂纹扩展分析和疲劳试验，以定量地估计焊接缺陷(气孔)的大小和形状对部分穿透纵向…疲劳强度的影响更多的直角焊接接头。在这种分析中，气孔被认为是沿其壁面带有小初始裂纹的椭圆形空穴。通过对气孔疲劳裂纹萌生和扩展的密切观察，确定了初始裂纹的数目、间距、大小和形状。提出了用简化方法预测气孔疲劳寿命的等效圆形裂纹尺寸。将预测的疲劳寿命与试验结果进行了比较，研究了板厚对焊接接头疲劳强度的影响。对几何形状相似、肋板厚度相同的不同尺寸的横向角焊接头进行了疲劳试验。用断裂力学方法对疲劳裂纹扩展进行了分析。几何相似节点的疲劳强度随着主板厚度的增加而降低。但当肋板厚度小于主板厚度的60%时，相同厚度肋板节点的疲劳强度与主板厚度无关。较少

The effects of stress ratio, weld defect and plate thickness on the fatigue strengths of welded bridge components are studied, which should be considered in designing the bridges. This study is summarized as follows.The effects of stress ratios on the fatigue crack growth in a high tensile residual stress field are examined. Specimens, made of 600Mpa class steel, are center-cracked type which contain longitudinal weld beads along the center of the plate. Fatigue crack growth rates and crack opening levels are measured at several stress stress ratios (-1 to 0.91). Under these test conditions, cracks fully open regardless of the value of stress ratio, and fatigue crack growth rates and threshold stress intensity factor ranges do not depend on the stress ratios.Fatigue crack growth analysis using a fracture mechanics concept and fatigue tests are performed to quantitatively estimate the effect of weld defect (blowhole) size and shape on the fatigue strength of partially penetrated longitu … More dinal welded joints. In this analysis, a blowhole is regarded as a ellipsoidal cavity with small initial cracks along its wall. Number, interval, size and shape of initial cracks are determined by closely observed results on the fatigue crack initiation and propagation from the blowhole. The equivalent circular crack size for a blowhole is presented to predict fatigue life using a simplified approach. The predicted fatigue life is compared with experimental results.The effects of plate thickness on the fatigue strengths of welded joints are studied. Fatigue tests are carried out for transverse fillet welded joints of various size with geometrically similar configurations and with same thickness rib plates. Fatigue crack growth analysis using fracture mechanics is also performed. The fatigue strengths of geometrically similar joints decrease with the increasing of the main plate thickness. However, the fatigue strengths of joints with same thickness rib plate do not depend on the main plate thickness when the plate thickness of rib plate is less than sixty percent of the main plate thickness. Less