Fatigue of welded joint is a scientific issue of great significance, because welding process always causes the loss of fatigue strength up to 40–60% for welded joint of the key components, resulting in a relatively poor reliability in service. In the present work, the fatigue behavior and damage mechanism of a medium-carbon steel welded joint were systematically investigated by microstructure observation and fatigue tests. It was found that, for the medium-carbon steel welded joint, fatigue crack mainly initiated at the welding defects, e.g., welding porosity, slag inclusion and heterogeneous microstructure with a poor mechanical property (i.e., the network-block proeutectoid ferrite). The competitive impact of various defects on the fatigue damage behaviors of welded joint was discussed, i.e., the competition between the volume defects (e.g., porosity and inclusions) and the heterogeneous structure. It is revealed that the smaller surface defects are more susceptible to fatigue damage due to the low constraint of plane stress state and higher stress intensity factor at high stress amplitude, while the larger internal defects easily cause fatigue damage at low stress amplitude because the driving force for crack propagation is larger than that of the damage case from the sample surface. Generally, the fatigue lives of samples with fatigue crack initiating from the weak phase are relatively longer, about 10 times as long as those of the samples with fatigue crack starting from the sample surface. In addition, the residual fatigue crack growth life, N_f, and the initial stress intensity factor (SIF), ΔK₁ at the volume defect tip exhibit a linear relation in the double logarithmic coordinate system. The present findings can provide a theoretical basis for the anti-fatigue design and fatigue life extension technology for the metallic component with welded structures.

焊接接头疲劳是一个具有重要意义的科学问题，因为焊接过程往往会导致关键部件焊接接头的疲劳强度损失高达40%~60%，从而导致服役可靠性相对较差。在目前的工作中，通过显微组织观察和疲劳试验系统地研究了中碳钢焊接接头的疲劳行为和损伤机制。研究发现，对于中碳钢焊接接头，疲劳裂纹主要起源于焊接缺陷，如焊接气孔、夹渣和力学性能较差的异质组织（即网状块状先共析铁素体）。讨论了各种缺陷对焊接接头疲劳损伤行为的竞争影响，即体积缺陷（例如，孔隙度和夹杂物）和异质结构。结果表明，在高应力幅下，由于平面应力状态约束较低，应力强度因子较高，较小的表面缺陷更容易发生疲劳损伤，而较大的内部缺陷在低应力幅下容易引起疲劳损伤，因为驱动力因为裂纹扩展大于样品表面的损伤情况。一般来说，从弱相开始疲劳裂纹的试样的疲劳寿命相对较长，大约是从试样表面开始疲劳裂纹的试样的10倍左右。此外，残余疲劳裂纹扩展寿命，结果表明，在高应力幅下，由于平面应力状态约束较低，应力强度因子较高，较小的表面缺陷更容易发生疲劳损伤，而较大的内部缺陷在低应力幅下容易引起疲劳损伤，因为驱动力因为裂纹扩展大于样品表面的损伤情况。一般来说，从弱相开始疲劳裂纹的试样的疲劳寿命相对较长，大约是从试样表面开始疲劳裂纹的试样的10倍左右。此外，残余疲劳裂纹扩展寿命，结果表明，在高应力幅下，由于平面应力状态约束较低，应力强度因子较高，较小的表面缺陷更容易发生疲劳损伤，而较大的内部缺陷在低应力幅下容易引起疲劳损伤，因为驱动力因为裂纹扩展大于样品表面的损伤情况。一般来说，从弱相开始疲劳裂纹的试样的疲劳寿命相对较长，大约是从试样表面开始疲劳裂纹的试样的10倍左右。此外，残余疲劳裂纹扩展寿命，而较大的内部缺陷容易造成低应力幅下的疲劳损伤，因为裂纹扩展的驱动力大于样品表面损伤情况下的驱动力。一般来说，从弱相开始疲劳裂纹的试样的疲劳寿命相对较长，大约是从试样表面开始疲劳裂纹的试样的10倍左右。此外，残余疲劳裂纹扩展寿命，而较大的内部缺陷容易造成低应力幅下的疲劳损伤，因为裂纹扩展的驱动力大于样品表面损伤情况下的驱动力。一般来说，从弱相开始疲劳裂纹的试样的疲劳寿命相对较长，大约是从试样表面开始疲劳裂纹的试样的10倍左右。此外，残余疲劳裂纹扩展寿命，N _f和初始应力强度因子（SIF），体积缺陷尖端的ΔK ₁在双对数坐标系中呈现线性关系。该研究成果可为焊接结构金属构件的抗疲劳设计和疲劳寿命延长技术提供理论依据。