Low Weight Design Of Impact Damaged CFRP Stiffened Panels by New Design Criteria And PFA

The object of this work is to outline new design criteria and analysis approaches for a low weight design of composite stiffened panels. The common industrial approach to satisfy the current certification requirements (EASA AMC 20-29) for composite structures, based 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. Nowadays a new design methodology, based on the incorporation of SHM (structural health monitoring) systems into composite structures, is under consideration aiming at exploiting the full potential of damaged composite materials in favor 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. Under this topic, two wing box stiffened panels, one critical at strength and another one critical at buckling, have been sized under static compressive loads according to classical design approaches and criteria (reference panels). In the first part of the work a sensitivity analysis, finalized to assess the influence of BVID allowable on the panels’ weight, have been performed. The two reference panels have been re-designed releasing the BVID allowable both on the whole panels and on some of their subcomponents (skin, stringer, etc.), in order to evaluate the weight reduction that could be potentially reached by reliable SHM systems. The results of these analyses provide fundamental requirements for the SHM system definition in terms of “which parameters needs to be monitored and where”. Successively, in order to exploit the effective residual strength of impact damaged panels, progressive failure analysis has been performed considering a discrete damage model against the traditional design approach that is based on the first ply failure design criteria on uniformly damaged panel. PFA has been performed on the panel considering a new simplified design model of BVID by simulating this kind of damage with an equivalent hole. This approach will allow to simplify, in the future, the numerical models simulating the low energy impact effects with high computational time savings. It will be possible to avoid the implementation of induced damage models to determine the laminate residual strength after impact, but simply model the laminate with the appropriate hole in the same BVID location. Some results of this approach are shown in this work supported by some impact and CAI tests on different layups and thicknesses.