Issue 46

C. Bellini et alii, Frattura ed Integrità Strutturale, 46 (2018) 319-331; DOI: 10.3221/IGF-ESIS.46.29 328 prepreg. Moreover, two head and base trajectories were added in order to obtain a homogeneous stratification in these two zones. It is evident that the optimized stratification turned out to be more advantageous than the first one. Step Trajectory Position Step Trajectory Position Step Trajectory Position 1 1-4 Above 8 1-8 Below 15 5-10 Below 2 4-0 Below 9 8-2 Above 16 10-4 Above 3 0-6 Above 10 2-7 Below 17 4-11 Below 4 6-10 Below 11 7-7 Head 18 11-3 Above 5 10-7 Above 12 7-4 Above 19 3-1 Below 6 7-10 Below 13 4-9 Below 20 1-1 Base 7 10-1 Above 14 9-5 Above Table 5 : Optimal stratification sequence. a) b) Figure 9 : Return trajectories below the mould: a) first stratification sequence, b) optimized stratification sequence. From a first analysis it was found that the thickness in the intersection area was less than 6 mm, that was the expected thickness; in fact, it was equal to 4.3 mm. Consequently, by calculating the difference between the nominal thickness, that is the desired one, and the obtained thickness, it was possible to determine the quantity of material necessary to get to the nominal thickness. This difference was equal to 1.7 mm. In more details, the after cure thickness of a single layer of prepreg was determined by dividing the obtained thickness by the number of layers, that in the intersection zone was equal to 60; in such manner, a layer thickness of 0.072 mm was determined. Then, the number of missing layers in the intersection zone was calculated by dividing the difference previously determined by the layer thickness; as a result, it was found that 24 tape layers had to be added to the stratification to obtain the desired thickness of 6 mm, that means 8 layers for each rib. Indeed, the single rib thickness was correct, while only the thickness in the intersection point was not as desired, so the correct solution consisted in redesigning the mould by varying the depth of the grooves in this area. Before performing this design change on the mould, a simple modification was made to the groove intersection point, in such a way as to preliminarily verify whether the optimization strategy was correct. Therefore, another test was made by modifying the depth of intersection points by inserting a Teflon ® sheet with a thickness of 1.7 mm, as visible in Fig. 10, and laying down 20 layers of prepreg, as stated in the paragraph “Design of tape stratification plan”. Another batch of 5 structures was manufactured and tested to verify the effectiveness of the preliminary optimization strategy. At the end of the cure process, the parts were visually inspected to understand whether the problem found on the head and base circumferences had been solved. As visible in Fig. 11, the bad stratification in the intersection points of the abovementioned circumferences disappeared, since the void spaces between fibre bundles were absent. Moreover, the thickness measurement confirmed a rib thickness of about 4.3 mm, as expected. Calcination tests were carried out on these structures too, according to the strategy presented in the previous section. From the test results, reported in Table 6, it can be noted that the compaction level is satisfactory both in the intersection nodes and in the ribs (between two intersection nodes), since the fibre content was high while the void content was low, denoting a high quality level of the material and, consequently, a structural strength in line with expectations. However, the table show that the compaction level of the transition zone (specimen 6) was not as expected, since a too high void content, more than 5% that is the