PSI - Issue 28

Costanzo Bellini et al. / Procedia Structural Integrity 28 (2020) 667–674 Author name / Structural Integrity Procedia 00 (2019) 000–000

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specimen the first load drop was considered; in fact, this specimen did not present a sudden breakage as the all-titanium one, but after the first load peak there are other two of similar value, bringing the maximum displacement to 7.8 mm. Moreover, even after these peaks, there was not a sudden drop to zero, but other peaks at lower load value were observed. Being the dimension of the two specimens very similar, a comparison about the flexural rigidity of the structure was performed comparing the slope of the elastic part of the loading curves: for the all-titanium specimen, this rigidity was equal to about 325 N/mm, while for the CFRP-skinned specimen it was about 200 N/mm. It can be noted that there is a decrease of 62.5% in the flexural rigidity of the structure. In the light of the abovementioned results, it appears that the skins made of titanium gave a better flexural behaviour to the lattice structure. However, another parameter should be considered in the present study: the weight, that is an important specification for the structures to be designed for the aeronautic applications. In fact, the average weight of the all-titanium specimens, evaluated by means of a precision balance, was equal to 96.88 g, while that one of the hybrid specimens was 77.46 g. The weight decrease can be calculated equal to 20.1%, that is less than the rigidity decrease. Therefore, it could be concluded that the solution of the CFRP skin is not beneficial, as the rigidity loss is not counterbalanced by a significant weight loss. Indeed, a meaningful comparison should be carried out by fixing one of the parameters, the weight or the stiffness. In this study, the former one was chosen as reference and blocked, while an analytical survey was carried out to draw more significant conclusions. According to the ASTM D7250, the panel bending stiffness D can be calculated as ܦ ൌ ா ௕ ൫ ℎ య ି ℎ ೎య ൯ ଵଶ (1) where E represents Young’s modulus of the skin material, b the specimen width, h the thickness of the whole sandwich and h c the thickness of the core. The relation presented by Johnson and Sims (1986) is quite similar, but it takes into account the bending stiffness of the core too. However, this term can be neglected since preliminary bending tests, carried out on the lattice core alone, determined a low stiffness; moreover, the core was the same for both specimens, so the relevant contribution to the stiffness of the whole structure was the same for both kinds of specimens. Let now introduce the weight parameter: considering the masses of the different skins, for those made of CFRP skin it was 6.83 grams, while for the titanium ones it was 16.53 g, that is about 2.5 times the CFRP one. Therefore, to obtain the same weight of the whole structure, the thickness of the CFRP skins should be multiplied by 2.5. Considering Young’s modulus of printed titanium equal to 105 GPa, as found by Pirozzi et al. (2017), and that one of CFRP to 65 GPa, the bending stiffness D for the all-titanium specimen was equal to 82.7 Nꞏmm 2 , and that one of the hybrid specimen to 54 Nꞏmm 2 . The bending stiffness of the hybrid specimen with the same weight of the all-titanium one reached 162.3 Nꞏmm 2 , that was 86.2% higher than the all-titanium one, as it can be noted in Fig. 6. It can be concluded that, being the masses equal for both types of specimens, the lattice structure with CFRP skins was found to be more performing than the all-titanium one, highlighting the great potential of these hybrid structures. 4. Conclusion In the present work, the potentiality of hybrid structures was investigated and verified. In particular, the flexural behaviour of a sandwich structure composed of two skins of CFRP (Carbon Fibre Reinforced Polymer) and a titanium lattice core was evaluated and compared to that one of an all-titanium structure. The founding idea of the present activity is the manufacturing of a strong and light structure that combines the potentialities of metal additive manufacturing with the characteristics of composite material. Indeed, the mechanical properties of sandwich structures with composites skin and aluminium honeycomb core are well known and these parts are quite diffused in the aeronautical industry, but in the flat configuration only. The capability of additive manufacturing technology to produce complex shape parts is well known too, as well as the high strength to weight and stiffness to weight ratios of composite materials, so their coupling can lead to the production of complex shape, light, strong and stiff structures. The hybrid specimens were manufactured in a two-step process: first of all, the octet-truss lattice cores were produced through EBM (Electron Beam Melting) technology, then the CFRP skins were added through the autoclave vacuum bagging process. Instead, the specimens with both core and skins made of titanium were produced in a single step, using the same EBMmachine. The three-point bending test carried out on all the produced specimens highlighted