Optimization of Numerical Analyses for Maintenance of Fuselage Skins with Rectangular Repairs

Abstract

Greatly reduced inspection intervals of skin damage repairs pose a significant financial problem for aging commercial aircraft fleets. Such intervals for visual inspections are the result of simplified conservative repair substantiation analyses, based on the same crack propagation scenarios and curves that were established in the initial project development. These neglect the structural role of external repair ("doubler") and consider only the increase in hidden crack path. A more refined approach to reassessing inspection intervals after a repair may keep maintenance jobs in accordance with common C-check routines in most cases. This approach, based on new crack growth simulations for worst case scenarios that could occur at the region of repair, uses respective kinetic equation and new geometric stress intensity factor functions, obtained in additional FEM (Finite Element Method) analyses. In particular, for standard rectangular repairs, the number of possible geometric configurations is astonishing considering length, width, skin and "doubler" thickness, reinforced panel dimensions, and frame and stringer cross sections. This investigation deals mainly with defining a minimum sufficient number of intermediate crack length values for FEM analyses in each propagation scenario. A conservative but efficient definition of most relevant parameters for a new numerical analysis campaign is another important issue. The results obtained are helpful for the improvement of the operational efficiency and safety of an aging fleet.