Issue 46

C. Bellini et alii, Frattura ed Integrità Strutturale, 46 (2018) 319-331; DOI: 10.3221/IGF-ESIS.46.29 325 typical issues that can be solved only with a robotic filament winding system, as that proposed by Sorrentino et al. [21]. Nevertheless, a satisfactory stratification quality was reached, even if the deposition operation lasted about 3 hours, that was a quite long time and it justify the implementation of an automatic system. At the end of the stratification process, a debulking phase, that is a pre-compacting action suitable to obtain less porosity, was carried out by vacuum bagging before the polymerization process. In this phase the stratified tape in the mould was compacted with a vacuum of -0.8 bar to make the tape adhere as much as possible to the bottom of the groove, thus increasing material consolidation, surface finish and structural solidity. For executing the debulking operation, to prepare a vacuum bag was necessary, similar to the one required by the cure process. First of all, the mould with the laid down material was wrapped with a release film to prevent it from sticking to the other ancillaries. Subsequently a layer of breather cloth was added, which was a transpiring material allowing a better distribution of vacuum on the mould surface and, consequently, increasing the homogeneity of the compaction. Finally, the vacuum bagging film, in plastic material, was deposited on the breather cloth, completing the preparation for debulking. A valve, which was subsequently connected to a vacuum pump by means of a tube, was inserted in this last film to create the vacuum. After the compaction phase, in which the component remained for a period equal to about 4 hours, there was the polymerization phase. For the cure process, the part was heated in the autoclave for 5 hours, at a temperature of 130 °C. A total of 5 structures were produced for the experimental tests. Analysis of the structure quality At the end of the curing process, some experimental tests were made on the produced parts. These tests were carried out to analyse the physical and the structural strength characteristics of the composite in an experimental manner. In particular, after a preliminary visual inspection, a chemical test and a mechanical one were carried out: the former was the calcination test and the latter was the interlaminar shear strength test. The calcination test is suitable to determine the resin content in the part after polymerization, to understand how it is distributed and so to have an index on the areas with greater compaction. The specimens were extracted from those areas that appeared most interesting for this test, both in the intersection point and on a single rib. As visible in Fig. 7, six zones were highlighted from which the six specimens were respectively named: A1, A2, A3, A4, A5 and A6. Once the six specimens were extracted from the structure, the test consisted of a first phase in which the dry specimens were weighed, then they were weighted in distilled water, using a high precision balance for both operations. After being weighed, the specimens were placed in the oven at 580 °C for 2 hours, in this way the resin was pyrolysed. The hot specimens were then placed in a dryer to further promote the resin elimination, leaving the fibres unaltered. The dryer presented dehumidification salts at its base and a valve for the vapours emission in the upper end. Therefore, the so obtained specimens made of only glass fibres were weighed again. From the obtained data it was possible to calculate the resin and fibre content in terms of weight and volume percentage as well as the density and percentage of voids present in the specimens. Figure 7 : Specimens selection for the calcination test. One of the better way to determine the quality of the gluing between the composite layers is the interlaminar shear strength test, carried out according to ASTM D2344. It was carried out by performing a bending test with a ratio between the supports span length and the specimen thickness equal to 4. As observable in Fig. 8, the specimens were taken from the most linear areas. The test specifications state that the specimen length must be six times the specimen thickness.