Residual stresses in aerospace structures reinforced with bonded crack retarders

Bonded crack retarder technology is an innovative concept to improve the fatigue performance of aircraft structures. Stiffening ‘straps’ are adhesively bonded to areas where potential fatigue cracking may occur. The straps retard the growth of fatigue cracks, by a combination of the local stiffening effect that reduces the crack driving force, and bridging in the wake of the crack. However, bonded crack retarder results in thermal residual stresses that may adversely affect the performance of the reinforced structure due to extensive fatigue loads in service. This is the first study where we have looked at the application of GLARE6/5 fibre-metal laminate as a bonded crack retarder onto a structural butt joint and simulated manhole mock-up assemblies containing cold-worked holes. Neutron diffraction was used for residual stress measurements. Results indicate that the strap-bonding process has no discernible effect on the magnitude of the compressive cold-working stresses. The use of bonded crack retarders should not, therefore, impair the benefits of cold working of fastener joints in aircraft structures.