PSI - Issue 5

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

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For the panel 1 , critical at strength, the results of the redesign show that a weight reduction is achievable up to 24.45% (ideal case of no degradation for BVID), Fig. 4 (a); the use of a SHM system could allow, therefore, the design of lighter panels depending on its specific capability and accuracy.

Fig. 4. Weight vs compressive design value. (a) Panel 1; (b) Panel 2.

The results obtained for panel 2 , critical at buckling, show that no weight reduction is achievable by incrementing the design allowable, and consequently also by using SHM systems, Fig. 4 (b). It is important to highlight that these results are sensitive to panel geometry and loads. For example, for not high loaded structures the weight reduction could be limited by minimum gage conditions and no further advantages could be obtained by increasing the material design allowable. Moreover, other elaborations have shown that weight reductions are not potentially achievable monitoring only the skin, against instead monitoring the whole panel or only the stringers. The catastrophic failure of a composite structure, i.e. its ultimate load-carrying capability, rarely occurs at the load corresponding to the FPF; composite structures usually fail due to the propagation or accumulation of local failures as the load is increased. Initial failure of a layer within the laminate can be predicted by applying an appropriate failure criterion; the subsequent evaluation of the failure propagation requires the use of advanced failure propagation models. PFA methodologies, founded on implicit solution methods and nonlinear analysis, are finalized to predict, the damage propagation (after the damage initiation, FPF) up to the collapse load of a composite structure. These methodologies have been implemented in many both commercial FE codes and in-house codes, Knight et al. (2002), Orifici et al. (2008), Schuecker and Pettermann (2008), Lin and Hu (2002). In this work, PFA has been carried out by using MSC Nastran ® ; Hashin criteria to detect the lamina failure, the immediate (instantaneous unloading) degradation model, Pure Full Newton-Raphson iteration method for the solution of nonlinear finite element equations, and automatic adaptive load stepping procedure in order to minimize the time step dependence. The PFA has been applied, for computing time saving, referring to a subpart of the panel 1 (sized at strength): a 2-stringers panel. A discrete damage has been introduced in the skin of the panel and keeping the mechanical strength properties of the material without degradation for BVID, in contrast to the traditional design: whole panel uniformly damaged by applying a knock-down factor K BVID , and FPF design criteria. For the research objectives of the present work the discrete damage has been induced only in the skin of the panel, and no delaminations and disbonding skin-stringers have been considered. First, a FE model of a 2-stringer panel has been built by using LS-DYNA ® software for the simulation of the impact test: 50 joule (BVID cut-off energy level) impact damage has been induced on the skin (28-plies), in the middle of the stringer bay. Then the LS-DYNA ® model, with the damage distribution induced by the impact analysis, was 3. PFA design approach