PSI - Issue 26

Costanzo Bellini et al. / Procedia Structural Integrity 26 (2020) 120–128 Bellini et al. / Structural Integrity Procedia 00 (2019) 000 – 000

123

4

As concerns the manufacturing process of the laminates, the standard prepreg vacuum bag procedure was adopted, that is represented in Fig. 2. First of all, all the required aluminium sheets, plies of carbon fibre prepreg and adhesive patches were prepared for the stacking operation by cutting them in the right dimensions. Once the raw material had been prepared, all the different materials were stacked on the mould according to the four different sequences. After the completion of the stacking sequence, the arranging of the vacuum bag was needed for the cure in the oven. Therefore, the laid laminate was covered with a release film and a breather cloth, and the whole stack was put in a vacuum bag, that was sealed with a butylic tape. The mould closed in the vacuum bag was connected to the vacuum pump and, after drawing the air present in the bag, the mould was put in the oven for the cure cycle. For the curing of the CARALL laminate, a temperature cycle suitable for both the prepreg material and the adhesive was chosen. It consisted of a heating ramp of 2 °C/min, a dwe ll at a constant temperature of 127 °C and a cooling stage.

Fig. 2. The manufacturing process for the FML.

At the end of the manufacturing process, the obtained laminates were extracted from the mould and cut with a diamond disk saw, in order to obtain the specimens to be tested. In this work, the three-point bending test was chosen as the experimental procedure. This type of mechanical test is characterized by reliability and simplicity; in fact, both long and short beam can be tested by varying the distance between the supports, the so-called span. Both short and long beams were obtained from the same laminate, as visible in the Fig. 3, in which the dimensions of the produced laminate are in red and those of the specimens in black colour. As prescribed by the ASTM D790 standard, that is the guideline adopted for flexural strength of the long beam, the dimension of each specimen depended on the thickness of the laminate, so each specimen had a length of 160 mm and a width of 20 mm. As concerns the parameters of the three-point bending test, a loading nose speed of 6 mm/min was adopted, while the span length was 136 mm. Instead, the specimen dimensions for the ILSS were determined according to the ASTM D2344 and were 25 mm of length and 10 mm of width. In this case, the parameters of the three-point bending test provided a loading nose speed of 1 mm/min and a span length of 20 mm. The present work was enriched by a post-mortem micrographic analysis, that was performed in order to determine the fracture modes. The samples were carefully extracted from the broken specimens, with the aim of avoiding further damage in the material, and then prepared for the inspection. The extraction operation was carried out through an abrasive disk saw, cooled with water. Then, the samples were mounted in resin panels: a resin curing at room temperature was chosen in order to avoid thermal damage of the material. Finally, the samples were polished using a grinder with different silicon carbide disks, with increasing mesh number, and a felt disk on which an alumina aqueous suspension was spread.