Issue 48

C.A.C.P. Coelho et alii, Frattura ed Integrità Strutturale, 48 (2019) 411-418; DOI: 10.3221/IGF-ESIS.48.39 414 Figure 3 : Typical load-displacement curves for the different configurations. It is possible to observe a linear region in all curves shown in Fig. 3, which is longer for the non-hybrid laminates. The stiffness presents the highest value for the non-hybrid composite semi-cylindrical shells, while the lowest value occurs for hybrid composite shells involving carbon and glass fibres. According with the literature [38], the zigzag aspect of the curves represents fractures of the fibres, in compression, and some delaminations around the broken fibres are also expected. The high compressive stress concentration at the loading nose contact region associated with the low compressive strength of the fibres promotes the failure damage described previously. Laminates Maximum Load [N] Displacement at Max. Load [mm] Stiffness [N/mm] Average Std. Average Std. Average Std. 6C 873 121 4.4 1.0 354 41 2C+2K+2C 1337 203 11.3 3.1 252 62 2C+2G+2C 722 184 7.1 1.9 203 54 Table 1 : Compressive strength of the different laminate composite semi-cylindrical shells. The maximum average load obtained in the hybrid carbon and Kevlar fibres shells is 53.2% higher than the maximum load obtained in the non-hybrid shells, while this parameter for hybrid shells with carbon and glass fibres is 17.3% lower. In terms of stiffness, the highest average value occurs for shells with carbon fibres (354 N/mm) followed by the hybrid configurations that involve Kevlar fibres and glass fibres with values, respectively, 28.8% and 42.7% lower. Finally, the lowest maximum average displacement occurs for shells with carbon (about 4.4 mm) and this value increases 38% and 156.8%, respectively, for shells with carbon and glass fibres and shells with carbon and Kevlar fibres. The intrinsic mechanical properties of the fibres and the damage mechanisms justify the tendency described previously. Dong et al. [39] observed a flexural modulus decreasing with higher percentage of glass fibres and positive hybrid effects by substituting carbon fibres with glass fibres. According to Giancaspro et al. [40], carbon fibre composites fail mainly on the compression side, while glass fibre composites fail on the tension side. Therefore, adding carbon fibres on the tension side of glass fibre composites increase the flexural strength, but when they are added on the compressive side the failure mode changes from tensile to crushing. On the other hand, literature reports that there is an optimal content of glass fibre to achieve maximum flexural strength [40, 41]. According to Dong et al. [41], for carbon/glass hybrid composites, this value is around 12.5% when all glass fibres are placed on the compressive side. Impact tests were carried out and Fig. 4 shows the force-time curves for each configuration. The curves that can be observed are similar with those find in literature [23-25]. The curves contain oscillations that, according Schoeppner and Abrate [42], result from the elastic wave and are created by the vibrations of the samples. It depends on the stiffness and on the mass of the specimen and impactor being excited by the rapid variation of the cinematic magnitudes at the time of collision [43]. 0,0 0,2 0,4 0,6 0,8 1,0 1,2 1,4 0 2 4 6 8 10 12 Load [kN] Displacement (mm) 6C 2C+2K+2C 2C+2G+2C