Fatigue crack growth in aluminium alloy structures
Fatigue cracks have been reported in the super structure of the surface ships in the navy. Repair techniques have been proposed and applied on compact tension specimens. Such techniques were; either by creating compressive residual stresses using welding anisotropy, local plastic deformation and spot heating. Or, constraining the crack tip using tightened bolt or steel/carbon fibre patches. Complete F.C.G. data were obtained for the metal in the as-rolled and welded conditions. A metallurgical study has also been made. Fractography, the influence of residual stresses and second phases have been studied. The study showed that superior crack resistance was obtained from spot heated specimens, which were very simple to prepare. Considerable retardation was also obtained using welding anisotropy, local plastic deformation and tightened bolts. Patching seems to provide limited improvement. The influence of compressive residual stress was found to improve the crack resistance. The influence of the inclusions was found to depend on their morphology. Medium sized and homogeneously distributed inclusions provided a retarding effect on the crack speed. More inclusion interaction was reported in the presence of residual stresses.