Issue 46

C. Bellini et alii, Frattura ed Integrità Strutturale, 46 (2018) 319-331; DOI: 10.3221/IGF-ESIS.46.29 320 In a work of Frulloni et al. [1], the design and manufacturing of isogrid structures made of composite materials is the best answer to this demanding task as composite materials present high resistance/weight ratio, while isogrid structures provide excellent performances for thin-walled components subjected to buckling failure; it is not without reason that this kind of structure are increasingly used in aeronautical and aerospace applications. These structures are composed by an isogrid lattice, formed by helical and circumferential ribs, surrounded by a thin skin: in such manner, the final product consists in a lightweight structure characterized by high mechanical performances. In the past years, there have been a lot of examples of aerospace and aeronautic parts conceived following this philosophy: fuselage structures, payload adapters and inter-stage structures have been produced resorting to large scale lattice cylinders; moreover, they have been implemented for Skylab Orbital Workshop module, Delta carriers and fuselage structures by Boeing, as reported by Dawood et al., Totaro and Zheng et al. [2-4]. Several studies in the literature deal with the mechanical performance of lattice structures. The buckling failure modes of lattice structures made of composite material were studied by Totaro [5-6]. In particular, he applied analytical models on both triangular and hexagonal cells system, after those models were experimentally verified on a curved lattice panel with an axial compressive load by Totaro et al. [7]. Lattice structures failure was predicted by other researchers too, that prepared several models. The higher mechanical performance of lattice structures in comparison with sandwich or stringer ones was proved in different work by Sun et al., Vasiliev et al. and Zheng et al. [8-11]. A FEM model was applied by Sui et al. on a one-dimensional lattice truss composite structure to analyse the compression failure mechanism, finding out that shorter truss fail for strut fracture, while the longer one for global buckling [12]. Instead, the characteristic failure mode of lattice-core sandwich structures was local buckling and delamination, as stated by Fan et al. [13]. According to Wang and Abdalla, lattice composite structures have higher damage tolerance characteristics [14]. The fabrication of isogrid structures made of composite material is very complex, as defects can arise that cause the part discarding or the part failure during service [15]. The forming technology, the necessary equipment and the process parameters must be determined with care, since they strongly affect the quality of the produced parts [16]. In particular, the mould shape has to be carefully designed since the part presents a complex geometry, due to the presence of ribs. In fact, a common defect that usually occurs is a bad compaction of the ribs, which involves porosity and low mechanical strength. An issue that commonly rise during manufacturing of isogrid structures is connected to the rib intersection, where the fibres cross-over. In fact, in these nodes there is three time the amount of fibre than in the other zones, giving rise to excessive build-up, that causes compaction problems and stress concentration points, as indicated by Young [17]. In these points the stress level can reach too high value and provoke a premature structural failure. Moreover, the non- uniformly distributed load caused by build-up can give rise to cure induced deformation of the lattice structure, because of CTE (Coefficient of Thermal Expansion) mismatch between fibres and resin [18]. A simple method to lessen the build-up and, consequently, its negative effects is represented by fibres offsetting: a few millimetres shift of the circumferential ribs is sufficient to reduce the overlapping from three plies to two. On the contrary, with this method small empty spaces are created at the centre of the nodes, which must be filled with resin during the cure process, as asserted by Kim [19]. The target of this research concerns the manufacturing of isogrid following an innovative process design methodology, that was verified through experimental structural tests on the produced parts. In particular, the mould groove geometry was defined in order to avoid part getting stuck in the mould and to obtain the right compaction degree. Then, different experimental tests were carried out to determine the quality of the produced structures and so the suitability of the designed mould. Finally, some actions were undertaken to avoid the problems rose in the first production run, on the basis of the quality tests. M ATERIALS AND METHODS he rib geometry was designed according to the Vasiliev’s theory in a previous work by Sorrentino et al. [20], in which a cylindrical isogrid structure was studied. In particular, the rib thickness was defined equal to 2 mm and the rib width to 5 mm; other geometrical parameters of the lattice structure were: triangle width equal to 94.25 mm, triangle height to 83.33 mm, helical angle to 60.51°, structure diameter to 300 mm and structure height to 338 mm. However, only a sector equal to one fifth of the structure was considered for the purpose of this work, as visible in Fig. 1. As a first approach for the realization of the structure, after having designed the mould and chosen the stratification sequence, the lattice structure alone was produced, without the skin. This procedure made it possible to optimize, as further described below, the production process as well as the isogrid structure itself. If the part had been made directly with the skin it would not have been possible to analyse the dimensions and the real compaction of the ribs, which, as can T