PSI - Issue 28

P. Santos et al. / Procedia Structural Integrity 28 (2020) 1816–1826 P. Santos/ Structural Integrity Procedia 00 (2019) 000–000

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stiffness modulus was obtained by linear regression of the load-displacement curves considering the interval in the linear segment with a correlation factor greater than 95%. Finally, the flexural strain was calculated according to the European Standard EN ISO 178:2003 by the following equation: � � 6.S. h L 2 (4) where S is the deflexion, L the span length and h the thickness of the specimen.

Fig. 1. (a) Stacking sequence; (b) Specimens geometry; (c) Schematic view of the three-point bending apparatus.

The same machine (Shimadzu, model Autograph AG-X) was used to carried out stress relaxation and creep tests, at room temperature and with similar samples to those shown in Fig. 1. For the first tests a fixed displacement was applied, and the stress recorded during the loading time, while in the creep tests a fixed stress was applied and the displacement recorded during the loading time. For both tests, 50% of the maximum stress obtained in the bending tests was considered, ensuring that all values are in the elastic region of the bending stress-strain curve. 3. Results and discussion Flexural static tests were performed in order to obtain the hybridization effect on the bending properties. Fig. 2 presents the flexural stress-strain curves obtained for each condition, which are representative of the other ones. All curves present a nearly fragile behaviour with a linear region up to the maximum load, followed by a significant drop after this value. In some cases, the zigzag appearance of the curves results from the fibres breakage. The subsequent load drop is a consequence of the propagation of delaminations initiated in regions with broken fibres. As shown in Fig. 3, fibres breakage in compression side are the main damage mechanism observed, which is in good agreement with the open literature (Giancaspro et al. 2010; Amaro et al. 2011; Reis et al. 2019). In addition to the low compressive strength of the carbon fibres (Giancaspro et al. 2010), the high compressive stress concentration in the pin load contact region explains the damage mechanism observed (Reis et al. 2007). Table 1 presents the bending properties in terms of average values and respective standard deviation. It is possible to observe that the maximum bending stress is obtained for full carbon composites and the lowest value for full glass fibre laminates (around 24.9% lower). For hybrid laminates, it is possible to observe that the presence of glass fibres decreases the flexural strength and this decrease increases with the number of glass fibre layers (22.6% and 24.4% for two and four layers of glass fibres, respectively). A similar behaviour is noticed for the bending stiffness, where the full glass fibre composites present a bending stiffness around 53.3% lower than the value observed for full carbon laminates. On the other hand, the hybrid laminates present an intermediate value (around 34.1% less compared to the carbon laminates) and the number of layers considered in this study do not present any influence on the bending