Issue 46

C. Bellini et alii, Frattura ed Integrità Strutturale, 46 (2018) 319-331; DOI: 10.3221/IGF-ESIS.46.29 327 contents differed considerably from those detected by experimental tests. The reason for this discrepancy was partly due to the particular piece geometry and partly to the fact that the mould grooves presented different depths, that created areas with greater compaction than others. With regard to the interlaminar shear strength tests, the relevant results are shown in the Table 4. Specimen Resin content [%] Fibre content [%] Void content [%] Average St. Dev. CoV Average St. Dev. CoV Average St. Dev. CoV% 1 39.24 1.23 3.13% 59.13 1.38 2.34% 1.63 0.04 2.45% 2 45.47 1.95 4.29% 38.66 1.68 4.34% 15.87 0.73 4.58% 3 35.73 1.18 3.30% 59.28 1.20 2.02% 4.99 0.13 2.57% 4 41.45 1.83 4.41% 46.54 1.83 3.93% 12.01 0.60 4.98% 5 36.81 1.21 3.29% 59.25 1.43 2.42% 3.94 0.09 2.36% 6 48.86 2.24 4.58% 40.30 1.99 4.94% 10.84 0.50 4.63% Table 3 : Calcination test results. Specimen Average [MPa] St. Dev. CoV 1 54 14.4 26.69% 2 67 11.4 17.07% 3 66 18.1 27.42% 4 65 16.1 24.69% 5 66 12.7 19.18% Table 4 : Interlaminar shear strength test results. As concerns the interlaminar shear strength, the values obtained in the areas far from the intersection points fall within the specifications of the considered material (ILLS>50 MPa), so the designed manufacturing methodology can be considered suitable for lattice structure production. However, in the intersection points the rib thickness measurement and the calcination test found some differences in terms of rib thickness between the nominal structure and the produced one, which were due to the fact that the prepreg resin underwent irregular transactions during the curing phase, filling more some areas rather than others. Therefore, some improvements must be implemented in the mould design to warrant the attainment of high quality standards in all the areas of the lattice structure. Preliminary process design optimization The isogrid structure produced with a mould designed according to the abovementioned consideration presented some issue; therefore, some improvements in the mould design and in the stratification sequence were necessary to obtain high quality parts. First of all, in order to avoid the separation of the fibre bundles in the head areas reported in Fig. 9, an optimized stratification sequence was defined taking into account the described problems; the new sequence is shown in Table 5. In this case it is observed that, by slightly changing the stratification philosophy previously adopted and laying down the helical trajectories at first and then the circumferential ones, there was a better distribution of the return trajectories below the mould. The result of this optimization operation can be seen from the comparison between the return trajectories obtained with the first sequence, visible in Fig. 10a, and those obtained from the optimized one, reported in Fig. 10b. Therefore, the problem related to the accumulation of tape in certain particular points was solved with the optimized sequence, thus making the changes of deposition directions less critical without the risk of twist and neck-in of the