Romano, Fulvio (2015) Design of composite stiffened panels by new design criteria and progressive failure analysis. [Tesi di dottorato]

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The objective of this work is the investigation of a new design methodology for the design of composite stiffened panels aiming to reduce the conservatisms of the traditional design and to obtain lighter structures respect to the current ones. In particular the work has evaluated the amount of the weight reduction that can be obtained releasing some traditional limitations and constraints used with the traditional design approach. The new methodology is based on the assumption of the presence of structural health monitoring (SHM) systems, and on the application of the progressive failure analysis (PFA) methodology. With reference to a typical regional aircraft wing box, it has been evaluated the benefit on the weight reduction achievable by redesigning, under compressive load, two stiffened panels representative of the upper skin of the wing box (at wing root and toward the wing tip), releasing some of the conservatisms of the traditional design, because resolved by SHM systems. The identification of the design variables that more affect the weight saving, and definition of the design requirements for the SHM systems, have been performed. The PFA has been applied in order to evaluate the actual residual strength, under compressive load, of impact damaged stiffened panels, considering several discrete damage models with the full mechanical strength properties of the material. The results of these analyses have been compared with those ones of the traditional design; the latter is based on the FPF criteria and on the use of material design allowables degraded because of BVID (Barely Visible Impact Damage) evaluated at coupon level (panel wholly damaged). A simplified design approach, based on the equivalence between the BVID and the hole 6.35 mm (1/4 inch) in diameter, as suggested by industrial guidelines for the preliminary design, has been investigated. All the numerical activities of this study have been supported and verified by correlation with experimental results of CAI (Compression After Impact) tests. In conclusion, in order to assess the real degradation of the load carrying capability at panel level in the presence of BVID, the impact damage induced on CAI coupon has been substituted to an equivalent portion of the skin of a stiffened panel. These analyses have had the scope to evaluate the benefits of a design allowable obtained at panel level rather than at coupon level.

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