PSI - Issue 5

F. Romano et al. / Procedia Structural Integrity 5 (2017) 721–728 Author name / Structural Integrity Procedia 00 (2017) 000 – 000

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 FPF criteria (not considering damage evolution after first ply failure),  material allowables reduced for BVID at coupon level; consequently the structural component is considered wholly damaged. This approach has as consequence the not exploitation of the full potential capability of the real structure in presence of discrete damage scenario. The PFA methodology, instead, is properly conceived to predict in addition to the damage initiation (FPF), the damage propagation up to the collapse load of a composite structure. This methodology is finalised to fully exploit the real residual strength of impact damaged structures, with advantages to achieve a lower final structural weight, against the traditional design, Knight et al. (2002), Sleight (1999), Moas and Griffin (1997). This kind of analysis foresees a discrete damage model that is introduced in the FE model of the structure and preserving the full mechanical strength properties of the material (no BVID degradation). The PFA allows to analyse also discrete maps of damages, representatives of real damage scenarios of the structure. Practically, in the advanced design platforms that are under consideration in the last years, the residual strength analysis represents the prognosis evaluation of the structure, and in conjunction with a SHM system (diagnosis), it constitutes the highest level of a comprehensive SHM system, Herszberg et al. (2007). This work shows the results of a feasibility study concerning the evaluation of the potential benefits deriving from the assumption in the design of the applicability of an off-line SHM system, in place of the visual inspection technique, and by using the PFA methodology. The activities have been performed on the basis of know-how achieved in previous and analogous works, Romano and Mercurio (2017), Romano et al. (2016), Romano (2015), Borrelli et al. (2015), Romano et al. (2015), Borrelli et al. (2013). The reference items of this work are flat stiffened panels, representatives of the upper skins, loaded in compression, of a regional aircraft wing box.

Nomenclature ADL

Allowable Damage Limit BVID Barely Visible Impact Damage CAI Compression After Impact FBD Free Body Diagram FE Finite Element FPF First Ply Failure K Knock-Down MS Margin of Safety PFA Progressive Failure Analysis SHM Structural Health Monitoring TF Total Failure UL Ultimate Load

2. Design by SHM

Two composite panels representatives of the upper skin, one located at the wing root (sized according to strength criteria) and the other one toward the wing tip (sized according to buckling criteria) have been considered. For the research objectives, the panels have been supposed flat and only compressive loads have been considered for their design. These panels have been initially designed according to the current industrial design approach; the panels so designed, and in particular their structural weight, represent the reference baseline . Successively the panels have been redesigned, by considering the potential information given by a reliable SHM system. In particular, a weight sensitivity analysis, finalized to assess the influence on the panel weight of the knock down factor for BVID, applied to the material strength allowables, has been performed. In detail, the two reference panels have been redesigned considering intermediate values of the BVID knock-down factor related to the different SHM sensitivity (different ADLs). In addition, partial monitoring conditions have been