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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more used for the realization of modern aircraft. However, not only the material but also the architecture improves the strength to weight ratio of a structure. In these regards, the lattice structure and the sandwich ones are an example of how the right combination of material and geometry can boost the performance of the structure. A sandwich structure is composed of two stiff face sheets, called skins, that have to resist to the external load, and a core, that present a cellular configuration and it is used to increase the moment of inertia and the bending stiffness, as defined by Hou et al. (2018). Honeycombs and foams have been used since many years as core material, but lattice structures present higher weight efficiency, as stated by Li et al. (2011). In the years, many research activities have been carried out to study the mechanical behaviour of the lattice structures made of composite material. Jishi et al. (2016) produced different kinds of lattice structures through a lost mould process, changing the diameter of the struts. The compression tests carried out on specimens extracted from the produced structures highlighted that the struts with smaller diameters failed in buckling, while those ones characterized by larger dimensions crushed, absorbing more energy. Anitha et al. (2018) analysed through FEM simulation the structural behaviour of grid stiffened panels made of carbon fibre and glass fibre reinforced polymers. They showed that the carbon fibre panel was stronger and stiffer than the glass fibre one, and the latter was able to sustain lower buckling load and point load compared to the former one. Xu et al. (2015) studied the graded lattice core structures, analysing the bending behaviour and the relevant failure modes. They tested panels with different lattice geometry and found that the failure mechanism was influenced by the geometric parameters: thin skins and thick structs led to the failure of the skin, instead thick skin and thin structs led to buckling of the core; therefore, an optimal design for graded lattice core sandwich panel was proposed. Bellini and Sorrentino (2018a and 2018b) produced and studied a cylinder made of composite material and reinforced by an isogrid lattice structure. They found a high strength to weight ratio, but the manufacturing process was quite complicated and induced several defects in the lattice materials. Therefore, a new design of the mould was necessary to warrant the right compaction degree of the material and, consequently, to obtain a more reliable structure. To wrap the lattice structure with skin is important to evenly distribute the loads over the entire surface of the structure, avoiding the stress concentration. If the structure is made of metal and it is produced by additive manufacturing process, the easiest strategy to achieve this goal consists in the building of the skin together with the lattice core; in such a manner, both the skin and the core should be made of the same material. This represents a very practical solution from a manufacturing point of view since the whole part can be produced within a single process; however, it may not be the best one in terms of performance, especially in terms of strength/weight ratio. Therefore, a new kind of hybrid structure is defined, which consists in a core made of additively manufactured metal lattice and composite material skins, which is a very lightweight material characterized by high strength. Although the hybrid structures are already widely used for structural applications in flat configurations (e.g. sandwich), the demand for high performances hybrid components characterized by more and more complex shapes (three dimensional and not axisymmetric geometry), makes necessary the study, the development and the optimization of additive manufacturing technologies to produce them. The idea at the base of this research is the production of a hybrid structure able to combine the qualities of additively manufactured metal parts with those of composite material. Indeed, the combination of different kinds of material, as the metal and the composite material, is not so recent. An example of such matching is the FML (Fibre Metal Laminate), that is constituted by metal sheets alternately stacked with composite material layers. This kind of material possesses high structural properties, as demonstrated by the works of Bellini et al. (2019 and 2020), Romli et al. (2017) and Xu et al. (2017). However, there are few research activities as regards the combination of metal lattice structures and composite material. Hwang and Steeves (2015) analysed the behaviour of a hybrid structure consisting in a titanium lattice and polymer foam, studying the effect of the foam on the limitation of the lattice trusses buckling. They optimized the cell density of the core to improve the compressive and shear strength, finding that tetrahedral cores without foam were the best solution. Wang et al. (2015) proposed a sandwich structure presenting an aluminium lattice core and CFRP (Carbon Fibre Reinforced Polymer) composite skins to reach high levels of energy absorption. They carried out impact tests, evaluating the damage mechanism, that was found consisting in the matrix cracking, fibre breakage and delamination of facesheets, coupled to the core struts buckling. Zhang et al. (2013) studied the quasi-static behaviour of a sandwich structure presenting a core made of aluminium and skins made of CFRP, finding that the pyramidal truss aluminium core presented a very high potential of energy absorption. Di Caprio et al. (2019) presented a numerical tool to design hybrid structures with metal lattice core and composite material skin. This tool