Potentiality of SHM and PFA for a new design approach of low weight aircraft structures

Abstract

The object of this work is to show the potentiality of the application of SHM systems and PFA methodology, from the beginning of the design phase of wing box composite stiffened panels, for obtaining lighter structures respect to the current ones. The common industrial approach to satisfy the certification requirements (EASA AMC 20-29) for composite structures, based mainly on the application of high conservative knockdown factors to the material strength properties and/or performing extensive test campaigns, can lead to oversized structures and to an increase in costs and timing. The use of information coming from SHM systems and of the PFA methodology could exploit the full potential of damaged composite materials, in favour of a greater weight reduction. By detecting the damages thanks to SHM systems, the structure could be designed with higher design allowables (more reliable detection of BVID) improving the static strength for a reduced damage size detection.Two aircraft wing box composite stiffened panels have been preliminary designed under static compressive load, one panel at the wing root and the other one at the wing tip, according to the traditional industrial design approach. Then, they have been re-designed releasing some of the current conservative criteria, because they were considered resolved by SHM systems: no BVID knockdown factor, no notch material design allowables (only bonded joints and bonded repair are considered) have been applied. The new design has shown the potential weight reduction achievable, the design parameters and panel subparts to which the panel weight is more sensitive. The results of these analyses provide fundamental requirements for the SHM system definition in terms of “where to monitor and why”.Successively, in order to exploit the actual residual strength of impact damaged panels, PFA has been performed also on the stiffened panels considering a discrete damage model against the traditional design approach; the latter, based on the first ply failure design criteria and assuming the structure uniformly damaged by using reduced design allowable obtained at coupon level. The results show that a significant weight reduction is potentially achievable by using PFA, and the potentiality of this methodology as a valid design tool to investigate a new design philosophy based on the evaluation of the residual strength of panels with discrete damages.