Retardation of fatigue cracks in welded structures through laser shock peening

icaf2023 Tracking Number 122

Aeronautical structures are often subjected to cyclic loading and can therefore fail due to fatigue. In most cases, fatigue cracks develop and propagate from critical areas, so-called stress concentrators, where the highest tensile stresses are present. When a fatigue crack has developed during operation, it has a significant impact on reducing fatigue life. To extend fatigue life, compressive residual stresses can be introduced in the critical areas to reduce the crack-driving tensile stresses and retard fatigue crack growth or even stop existing cracks. In this study, laser shock peening (LSP) is investigated as a promising technique to introduce deep compressive residual stresses in metallic aerospace materials. One application scenario of LSP is demonstrated on a welded stiffened panel representing a part of a fuselage structure, where the technique was successfully applied for the retardation of skin cracks. The skin-stringer AA2024-AA7050 T-joints were realized through stationary shoulder friction stir welding, a variant of the conventional friction stir welding process. It was shown, that application of LSP led to a 400 % increase in fatigue life. Another positive application scenario of LSP to restore the fatigue life of laser-welded AA6056-T6 butt welds with already existing surface fatigue cracks is discussed. By applying LSP to surfaces of specimens with fatigue cracks, the fatigue life could be restored to the level of specimens in the as-welded condition. A similar positive effect of LSP was demonstrated on AA2024-T3 specimens with a fatigue crack originating from overlap joints manufactured using a solid-state joining process. The results of the study show that LSP is an efficient method for extending the fatigue life of structural components with small surface cracks. In this context, LSP can be used to improve the fatigue performance of components where fatigue cracks may occur in critical areas such as welds. Therefore, LSP can be used as a prophylactic residual stress engineering technique to extend the fatigue life of critical structures in aging aircraft where fatigue cracks have not yet reached the detectable size. In this regard, LSP could reduce the required safety margins (safety factors) of the fatigue-critical component or structure, thereby reducing its weight.