Analysis of Residual Stress Effect on Fatigue Crack Propagation in Bonded Aeronautical Stiffened Panels

Fatigue crack propagation analyses are one of the most critical design drivers in wide areas of the structure of the modern transport aircraft, influencing both the initial structural design and the definition of the in-service maintenance plans. In order to prevent catastrophic failures during the operative life of an airplane, slow fatigue crack propagation in the principal structural elements must be assured to permit the damage detection before the reaching of the failure condition. Maintenance plans based on fatigue crack propagation analyses are thus fundamental to reach this goal. Recent test campaigns have shown that built-up bonded structures, such as stiffened panels, can provide high damage tolerance, with intrinsic slow crack propagation features. Bonded stiffener elements can indeed act as effective crack retarders. The fatigue crack propagation analysis of bonded stiffened panels needs a reliable tool that takes into consideration the significant stress field that can be induced by hot-bonding curing process of dissimilar skin and stiffener materials. In this work, a section of a possible real aeronautical fuselage structure is investigated nu-merically in order to characterize the influence of residual stress field on the fatigue crack propagation in the skin of a stiffened panel. The models consist of a flat aluminium skin with equally spaced bonded aluminium stringers and additional bonded doublers in the middle of the stringer bays. The cases of both aluminium and titanium doublers are investigated. The initial analysis was performed in order to asses the residual stress field induced by the stiffener hot-bonding curing cycle in the skin of both the panels investigated. This was the input for the successive crack propagation analysis, with the simulated crack propagating orthogonally to the stiffeners and the loading direction. It is shown that the hot-bonding process in the case of the titanium doublers can induce tensile residual stresses in the skin. This residual stress field can significantly accelerate the crack growth, reducing the crack retarding effectiveness of the titanium doubler. This result confirms the importance of the correct evaluation of residual stresses in damage tolerance assessments of bonded structures.