The practise of identifying corrosion damage, air-blasting to remove loose material, grinding to remove corrosion, shot peening to increase the fatigue properties and finally back assembly onto the aircraft for many years has governed aircraft operators under the term "find and fix". This empirical practise inevitably causes thickness and hence load bearing reduction while can locally overstress components.To avoid that a rule of thumb is implemented making sure that no more than 10% of the initial thickness of the material has been lost.Otherwise, the component should be replaced.It is not difficult to understand that such practise primarily lies within the expertise of the person performing the repair as well as the geometric complexity of the damage.If the corrosion damage is situated at a location which is difficult to grind or, and to peen then most certainly the quality of the repair will vary.The problem can have significant implication under cyclic loading considering that the residual stresses induced by shot peening will undergo continuous redistribution as part of their relaxation process, while irregular thicknesses can transform the design philosophy used for that component from safe life to fatigue damage tolerance.Herein, such repairs can start fatigue cracks following local stress raisers, due to stiffness loss especially when close to stiffeners or by redistributing shear strains when close to joints.The Ultrasonic Impact Treatments process is employing continuous ultrasonic vibrations at the ultrasonic transducer output end strengthened with hard materials (carbide-containing alloys, artificial diamonds etc.) and being in direct and generally continuous contact with the treated surface.During impact the near surface of the material experiences high strain rates as well as heating.The first is responsible for plastically deforming the material.The induced compressive residual stresses are significantly more stable than those induced by shot peening due to the fact that high rate straining of aluminium alloys generates energetically stable dislocation cells.Heating, on the other hand is responsible for reimbursing the loose material by melting the corrosion oxides.Hence, the final repair does not produce thickness reductions. The table below shows the results from exposing aluminium alloy 2024-T351 under ASTM exfoliation conditions for 36,48 and 72 hours prior to static testing.
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