Bonded prestressed method for fatigue crack repair

icaf2023 Tracking Number 97

Fatigue life extension methods and fatigue crack repair methods have always been a vibrant research topic, seeking effective methodologies to significantly prolong the service life of aircraft. Bonding fibre reinforced epoxy composite patches have delivered superior fatigue crack growth life extension in comparison with mechanically joining metal patches. The longer fatigue life extension of bonding composite patches results from the bridging mechanism and the circumvention of new fatigue hot spots. However, tensile residual stresses in the parent metal structure due to mismatch of thermal expansion coefficients of composites and metal structures have adverse effects. A state-of-art fatigue crack life extension methodology is to bond prestressed patches. There are three fatigue crack retardation mechanisms in this system, namely added load path, bridging mechanism and compressive stresses due to prestressing. The emergency of an iron-based shape memory alloy (Fe-SMA) that exhibits excellent shape memory effect and generates notably high recovering stresses makes the bonded prestressed method for fatigue crack repair feasible to achieve. An experimental campaign has been carried out to study the feasibility of developing such a bonded prestressed fatigue strengthening method and to demonstrate its repair efficiency. The specimen configuration consists of a metal plate with a central crack repaired with Fe-SMA patches on both sides. Different activation methods to generate compressive stresses around the crack area in the metal plate using the shape memory effect of Fe-SMA were investigated. Different patch sizes were also studied. The repaired specimens were then subjected to tensile-tensile fatigue tests with beach marking technique. The a-N data were analysed and the role of bridging mechanism and prestressing were discussed. The experimental results demonstrate the superior repair efficiency the newly developed repair solution where the Fe-SMA is bonded and activated. The fatigue life can be extended several times and even complete crack arrest can be achieved. The great potential of bonded prestressed fatigue strengthening method to significantly prolong the service life of fatigue damaged aircraft structures is substantiated in this paper.