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

• Applicant: Dr.-Ing. Majid Farajian
• Subject Area: Mechanical Properties of Metallic Materials and their Microstructural Origins
• Term: from 2014 to 2020
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 271548146

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.

• DFG Programme: Research Grants
• International Connection: Italy, USA
• Co-Investigator: Professor Dr. Peter Gumbsch
• Cooperation Partners: Professor Ali Fatemi, Ph.D.; Professor Dr. Paolo Ferro; Herman F. Nied, Ph.D.