PSI - Issue 25

P. Santos et al. / Procedia Structural Integrity 25 (2020) 370–377 P. Santos../ Structural Integrity Procedia 00 (2019) 000–000

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Three different composite laminates were prepared by hand lay-up and with the following stacking sequences: [8FF], [2GF+8FF+2GF] and [2GF+4FF+2GF+4FF+2GF]. The “numbers” represent the quantity of layers while FF and GF are flax fibres and glass fibres, respectively. The system was placed inside a vacuum bag and a load of 2.5 kN was applied during 24 hours in order to maintain a constant fibre volume fraction and uniform laminate thickness. During the first 4 hours the bag remained attached to a vacuum pump to eliminate any air bubbles existing in the composite. The post-cure was followed according to manufacturer datasheet in an oven at 40 ºC during for 24 hours. The overall dimensions of the plates were 330x330xt mm 3 , with t = 3.8 ± 0.1 mm for laminates with eight layers flax fibres, t = 4.2 ± 0.1 mm for laminate with two layers of glass fibres and eight layers of flax fibres and t = 4.8 ± 0.1 mm for laminate with four layers of glass fibres and eight layers of flax fibres, according the stacking sequence presented previously. The future denomination of these laminates used throughout the text will be: flax for [8FF], 1xhybrid for [2GF+8FF+2GF] and 2xhybrid for [2GF+4FF+2GF+4FF+2GF]. Three-point bending (3PB) static tests were performed using specimens with the geometry shown in Fig. 1 and tested with a span of 80 mm according to the recommendations of the ASTM D790-03 (ASTM 2003). A Shimadzu universal testing machine, model Autograph AGS-X, equipped with a 10 kN load cell was used and for each condition, at least five specimens were tested at room temperature and at a displacement rate of 2 mm/min.

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b) Fig. 1. (a) Specimens geometry (dimensions in mm); (b) Schematic view of the three-point bending apparatus.

The bending strength was calculated as the nominal stress at middle span section obtained using maximum value of the load. The nominal bending stress was calculated using the equation (1):  � 2 3 ℎ � (1) where P is the load, L the span length, b the width and, h the thickness of the specimen. Stress relaxation (SR) tests were also performed in the same equipment (Shimadzu AGS-X), at room temperature, and with geometry similar to those used on the bending tests. All experimental procedure was supported by ASTM E328-02 (ASTM 2008), where a fixed strain was applied (correspondent to around 50 MPa for all configurations) and the stress recorded during the loading time of 10800 s. This value corresponds to 39%, 23%, and 21% of the maximum bending stress for Flax, 1xhybrid and 2xhybrid laminates, respectively, which was selected to guarantee that all SR tests were carried out in the elastic regime of all conditions studied. Finally, the effect of the creep tests was also performed in the same machine, where a fixed bending stress was applied (with similar values to those previously reported, 50 MPa) and the displacement recorded during the loading time. 3. Results and discussion Flexural static tests were performed to obtain the hybridization effect on the flexural properties and, according to the maximum bending stress, to select the values for the stress relaxation tests. In this context, Fig. 2 presents the bending stress-strain curves obtained for each condition and they are representative of the other ones and the bending strength for all laminates. From Fig. 2a) it is possible to observe a linear increase of the bending stress with the strain, followed by a non linear behaviour in which the maximum bending stress is reached. With the hybridization, the linear regime increases. On the other hand, from Fig. 2b) it is possible to observe that higher hybridization promotes higher bending stresses.