Mixed-mode compression/shear fatigue crack arrest in steel members by prestressed retrofitting

• Applicant: Professor Dr. Elyas Ghafoori
• Subject Area: Structural Engineering, Building Informatics and Construction Operation
• Term: since 2025
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 567908955

The aging of steel structures is a major worldwide challenge, given the increasing service loads, harsh environmental conditions, and fatigue degradation. This calls for immediate action to preserve their integrity, ensure safety, and prevent catastrophic failures and loss of lives. Strengthening existing structures can contribute to their sustainability by increasing their service lives and minimizing the adverse impacts of the (re)construction works on the environment, society, and economy. The conventional strengthening methods of aging steel structures usually entail the use of heavy steel plates, which are not only difficult to fix but also prone to corrosion and fatigue failure. Recent investigations demonstrated promising results in the utilization of carbon-fiber reinforced polymer (CFRP) to enhance the fatigue performance of steel structures. However, the majority of the studies have been concerned with the tensile mode of crack propagation, whereas, in real structures, cracks are often subjected to combined loading and mixed-mode fatigue behavior. This project aims to address this issue by characterizing the failure mechanism of mixed-mode fatigue propagation in CFRP-steel composite systems, filling the knowledge gap on the effect of FRP strengthening of structural members subjected to combined cyclic axial and shear loads. This project focuses on fatigue crack growth not only under tension-shear but, in particular, on compression-shear loading conditions. The compression stress field can be a result of using prestressed CFRP strengthening. The project will develop a map of failure mechanisms in steel under mixed-mode conditions and provide recommendations on the fatigue repair of cracks using prestressed systems.To this end, a systematic theoretical and experimental program is envisaged, which encompasses the determination of stress intensity factors in mixed-mode conditions, examination of the structural response of cracked I-beam subjected to mixed-mode stress condition, and investigation of prestressed strengthening on the fatigue mixed-mode crack growth. Small-scale plate tests and large-scale beam tests will be performed to gain insight into mixed-mode fatigue crack growth. Analytical and numerical models, as well as experiments, will be conducted to assess the performance of inclined crack behavior after prestressed strengthening. Finally, the project will develop reliable retrofit strategies and practical recommendations for strengthening aging steel details. The results could potentially be applied to a variety of steel structures, in particular steel bridges but also cranes, wind turbine structures, pipelines, communication towers, and offshore support structures.

• DFG Programme: Research Grants