Numerical Incorporation of the Damaging Effects of Residual Stresses in the Multiaxial Fatigue Assessment of Welded Components and Structures

焊接接头在多轴载荷作用下的使用寿命评估由于其疲劳行为的各种影响因素，仍然存在较大的不确定性。焊接残余应力场在多大程度上影响多轴应力状态，从而影响疲劳损伤机制仍然是一个有争议的问题。这是因为在以往的焊缝多轴疲劳分析研究中，要么是焊接试样的应力解除，要么是在疲劳裂纹起裂处假定了很高的拉伸残余应力，而没有进行任何实验测定。评估残余应力的另一个困难是，这些应力在应力过程中会发生变化，因此即使精确确定初始残余应力也可能只是疲劳评估的第一步。上述情况导致现行规范在焊接结构的疲劳荷载设计中推荐了保守的评估方法，并将其归咎于可疑的高应力。减少这种设计保守性的科学和工程解决方案是开发一种有效的计算方法，以便在疲劳评估方法中纳入焊接残余应力场的破坏影响。因此，拟议的研究项目将解决这一问题，并有两个主要目标：焊接残余应力对管状焊接试件轴向和多轴疲劳裂纹行为影响的观察与实验研究。在数值上将残余应力的损伤效应纳入适当的疲劳损伤模型，以便准确评估焊接接头的寿命。S355J2H管状焊接试样计划在焊接状态和去应力状态下进行轴向、扭转和轴-扭转载荷（LCF、HCF）下的疲劳试验。将计算得到的焊接残余应力场数值整合到临界疲劳损伤模型中，评估焊接试样的疲劳寿命，并与实验结果进行对比。通过开发一种计算方法来量化残余应力对焊缝疲劳寿命的影响，并为设计指南提供适当的建议，可以充分发挥建筑材料的轻量化潜力。项目成功完成后，将有适当的多轴载荷下焊缝疲劳评估的工程指南和法规，并实际考虑残余应力对使用寿命的影响。

The evaluation of the service life of welded joints under multiaxial loading is still associated with large uncertainties due to various influencing factors on the fatigue behavior. The extent to which the welding residual stress field influences the multiaxial stress state and consequently the fatigue damage mechanisms is still a matter of debate. The reason for this is that in the previous studies for the multiaxial fatigue analysis of welds either the welded specimens were stress relieved or very high tensile residual stresses at fatigue crack initiation sites were postulated without any experimental determination. A further difficulty in the evaluation of residual stresses is that these are subject to change during the stress, so that even a precise determination of the initial residual stresses may represent only the first step towards the fatigue assessment. The circumstances described have led to the fact that in the design of fatigue loaded welded structures conservative assessment methods are recommended in the current regulations, for which the suspected high stresses are made responsible. A scientific and engineering solution to reduce this design conservatism is the development of a validated computational approach in order to incorporate the damaging effects of the welding residual stress field in fatigue assessment methods.The proposed research project will therefore address this issue and has two main objectives:1. Observational and experimental studies on the influence of welding residual stresses on the axial and multiaxial fatigue crack behavior in tubular welded specimens.2. Numerical incorporation of the damaging effects of residual stresses into a proper fatigue damage model for accurate life time assessments of welded joints.Tubular welded specimens out of S355J2H are planned to be fatigue tested under axial, torsional and axial-torsional loading (LCF, HCF) in as-welded and stress relieved conditions. Through numerical incorporation of the calculated welding residual stress fields into critical fatigue damage models, the fatigue life of the welded specimens is going to be assessed and compared with the experiment. By developing a computational approach for the quantification of the influence of residual stresses on the fatigue life of welds and the appropriate recommendation for design guidelines, the lightweight potentials of construction materials could be exhausted. After a successful completion of the project, appropriate engineering guidelines and regulations for the fatigue assessment of welds under multiaxial loading with a realistic consideration of the residual stress influence in service life would be available.