Issue 46

C. Bellini et alii, Frattura ed Integrità Strutturale, 46 (2018) 319-331; DOI: 10.3221/IGF-ESIS.46.29 321 be guessed, turned out to be the critical element of the structure. A unidirectional prepreg tape made of glass fibre and epoxy resin, whose parameters of interest are reported in Table 1, was chosen as material for this work. Figure 1 : Mould dimensions for one fifth of the structure . Property Value Resin weight content 33% Composite density 1850 kg/m3 Fibre weight 0.66 g/m Table 1 : Material properties. Mould design For the design of the mould required for the manufacturing of the isogrid structure, the manufacturing technology was considered in addition to the geometry of the structure, because the technology inevitably determined a geometric/dimensional variation of the ribs compared to the drawn ones. Therefore, the design phases of the mould and the subsequent optimization step proved to be fundamental for obtaining the appropriate quality of the produced parts; the mould, in fact, must guarantee the dimensions, the compaction and the surface finish of the final piece. A well-defined geometry and dimensions were assigned to the mould channels in order to guarantee the deposition of the composite material roving and to facilitate as much as possible the compaction and the subsequent extraction. The groove depth was chosen considering a uniform compaction and a rib thickness of 2 mm; therefore, the channel depth between two intersection point was 2 mm, while in the intersection point it was 6 mm, that is three times the thickness of the single rib. From a geometrical point of view, the groove bottom conceived in that manner was concave, therefore it had to be managed appropriately in order to reduce to a minimum the problems of compaction due to the fibre bridging; this means ensuring a smooth transition between these two thicknesses (2 and 6 mm). In this sense, the two chord points where the tape detached from the deposition surface were taken as reference; in particular, a distance between these two points of about 50 mm was chosen, as reported in Fig. 2. In this zone the channel thickness passed from 2 mm to 6 mm (in the intersection centre) and then returned to 2 mm. Operatively, this was achieved by a sweep cutting operation with a rectangular cutting profile guided by two spline curves, as visible in Fig. 3. A concave surface with a negative curvature was generated through this method, so an auxiliary compaction system, capable of making the fibres adhere to the groove surface during the material deposition, was required for managing the issues raised by surface geometry. In order to overcome this question, an innovative solution was studied which delegated this task to the deposition of the circumferential ribs. For this reason, the thickness variation of the circumferential grooves was designed taking into account this purpose. The circumferential ribs also presented a depth variation from 2 mm to 6 mm and, taking into account their role of compaction system, to create a zero curvature surface was decided, as visible from Fig. 4. In this way, the compaction of the helical spirals was facilitated since the first fibre layer. Mould production and preparation A block of epoxy resin was machined for the production of the mould for the lattice structure. This material was chosen since it had a coefficient of thermal expansion compatible with that of the composite material and it was convenient for the purpose of this experimental campaign. As concern the machining sequence, the dimensions of the raw block were 358 171 54