Issue 46

C. Bellini et alii, Frattura ed Integrità Strutturale, 46 (2018) 319-331; DOI: 10.3221/IGF-ESIS.46.29 326 Figure 8 : Specimens selection for the interlaminar shear strength test. R ESULTS t the end of the cure process, each lattice structure was extracted from the mould and a visual inspection was performed on the part as preliminary quality test. It was found that there were some points more involved for the tape inversion phase, in particular in the point 10 and 4 of Fig. 6. This created during the stratification phase a greater difficulty in depositing the tape, with the eventuality of neck-in or twist of the tape. Furthermore, the designed stratification sequence foresaw no trajectories to cover the head and base circumferences; as a consequence, the intersection points along these circumferences presented some issues, as shown in Fig. 9. Figure 9 : Defect due to lack of circumferential head trajectory. Analysing the produced structures, it was found that the tape had not undergone a homogeneous compaction; in particular, it varied in the proximity of the intersection point, where two helical trajectories intersected a circumferential one; the calculation of the groove depth in those points was made considering a triple stratification in the intersection, and therefore it had a triple depth than the other parts of the grooves. That is, a depth of 2 mm was considered in the zones with a single rib, while a maximum depth of 6 mm was adopted for those points where there was the intersection of the three ribs. After the demoulding of the lattice structure from the curing tool, the thickness of the ribs was measured by means of a digital calliper: the thickness of a single ribs was found equal to 2 mm, while in the intersection zone the measured thickness was 4.3 mm. Moreover, a visual analysis showed that the grooves tracts with variable depth were too long, that is the shift from 2 mm to 6 mm takes place in a too gradual manner. In fact, the tests showed a lack of compaction with a large displacement of the fibres in the radial direction and towards the inside of the groove, with a noticeable rib hollow in those areas. After the preliminary quality tests, the calcination tests were carried out for acquiring further information on the material compaction level in the different areas of the mould grooves. The results of these tests, that are reported in the Table 3, showed that the resin had a higher volumetric percentage in some parts of the structure than in others. Such a result could be expected but not to the extent that it occurred; in fact, according to the prepreg characteristics, the nominal resin A