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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taken also into account (on the whole panel, and in other cases only on the skin or stringers alone). The analyses have given indications on the weight reduction respect to different knock-down factors scenarios, and also allowing, in the redesign of the panels, post-buckling regime between limit and ultimate design load. The results of these analyses have provided fundamental requirements for the SHM system definition in terms of what monitor and the amounts of weight reduction potentially achievable.

2.1. Reference panels

Fig. 1 shows a simple sketch of the geometry and loads of the reference panels. About the material system, a carbon fiber resin epoxy system, commonly used for aeronautical applications, have been considered. It is a unidirectional prepreg tape, with a nominal cured ply thickness of 0.186 mm. The design of the reference panels, according to the current design approach, has been performed by applying all the necessary knock down factors, necessary to take into account temperature (T) and moisture (M) effects, K T,M , B-basis statistical

reduction, K B-basis , and low energy impact damage, K BVID . The panels have been sized by linear analysis according to  minimum weight requirement and according to the following design criteria: 

no strength failure and no onset of buckling up to ultimate load.

The design has been developed by means of a stand-alone approach (no finite element solver), based on the use of a dedicated structural sizing code for composite structures, Hypersizer ® , typically used in the preliminary design phase by civil aircraft industries.

Fig. 1. Reference panels: geometry and loads.

Hypersizer ® is able to compute the load distribution that satisfies the condition of FBD force equilibrium for imposed geometry, shape, material, layup. The external loads per unit length are resolved internally into stress resultants on each analysed object (e.g. flanges, webs, etc.). Furthermore, the FBD state of internal stresses and strains for all panel segments is used to verify the equilibrium of forces and strain compatibility for the panel/beam as a whole. The strength and stability of these panels are analysed for the applied loads and boundary conditions, using both analysis approaches based on traditional industry methods and modern analytical/computational solutions (over 100 different failure analyses). The objective of the panel design and sizing is to minimize the weight with the added constraint to have all the margins of safety (stability and strength) positive. Fig. 2 shows the results of the design of the two reference panels. As expected, panel 1 and panel 2 are characterized by minimum margin of safety at ultimate load, at strength and local buckling respectively.