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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Peijs and de Kok (1993), for example, studied hybrid composites with PE fibres and carbon fibres and obtained flatter S-N curves for this configuration. This means that tensile fatigue damage propagates slower and demonstrates the excellent tensile fatigue resistance of PE/carbon fibre hybrids. However, while tensile, flexural and impact properties of hybrid composites have been extensively investigated, the fatigue resistance has only been investigated by a limited number of researchers (Swolfs et al. 2014). Although the tensile and flexural properties of hybrid composites are reasonably well understood, the same is not true for other loading modes. Regarding viscoelastic behaviour, for example, this subject is not yet reported in the literature according to the authors' knowledge. Therefore, this work intends to study the effect of fibre hybridization on the viscoelastic properties of polymeric matrix composites. Experimental tests will be used to analyse the creep and stress relaxation behaviour of different hybridization configurations. 2. Materials and experimental procedure Carbon fibre woven bi-directional fabric (taffeta with 195 g/m 2 ) and glass fibre woven bi-directional fabric (taffeta with 190 g/m 2 ) with an Ebalta AH 150 resin and IP 430 hardener were used to prepare different composite laminates. Four different composite laminates were prepared by hand lay-up and with the following stacking sequences: [8C], [6C+2G], [4C+4G] and [8G]. The “numbers” represent the quantity of layers while the “letters” C and G represent the carbon fibres and glass fibres, respectively. The system was placed inside a vacuum bag and a load of 2.5 kN was applied during 48 hours in order to maintain a constant fibre volume fraction and a uniform laminate thickness. During the first 10 hours, the bag remained attached to a vacuum pump to eliminate any air bubbles existing in the composite. Finally, according to the manufacturer’s datasheet recommendations, a post-cure in an oven at 80 ºC was performed for 5 hours. Plates with overall dimensions of 330x330xt mm 3 were produced, with t = 1.7 mm for laminates [8C], t = 1.6 mm for both laminates [6C+2G] and [4C+4G] and t = 1.4 mm for laminates [8G]. Three-point bending (3PB) static tests were performed using specimens with the geometry shown in Fig. 1 and tested, according to the recommendations of the European Standard EN ISO 178:2003, with a span of 25 mm for laminates [8G] and 30 mm for all other laminates. A Shimadzu universal testing machine, model Autograph AGS-X, equipped with a 10 kN load cell was used and five specimens were tested at room temperature for each condition. These tests carried out with a displacement rate of 200, 20, 2, 0.2 and 0.02 mm/min which, according to equation (1), correspond to strain rates ( � ) of 2.84×10 0 , 2.84×10 -1 , 2.84×10 -2 , 2.84×10 -3 and 2.84×10 -4 for glass fibre laminates and 2.34×10 0 , 2.34×10 -1 , 2.34×10 -2 , 2.34×10 -3 and 2.34×10 -4 for carbon fibre laminates. � � � � 6×V T ×h L 2 (1) In equation (1) � is the peripheral fibre strain, t is the time, � is the cross-head speed, L the span length and h the thickness of the specimen. The bending strength was calculated as the nominal stress in the middle span section obtained using maximum value of the load. The nominal bending stress was calculated using: � � 3.P.L 2.b.h 2 (2) where P is the load, L the span length, b the width and h the thickness of the specimen. The stiffness modulus was calculated by the linear elastic bending beams theory relationship: E= ∆ P.L 3 48. ∆ u.I (3) where I is the moment of inertia of the cross-section and ∆ P and ∆ u are, respectively, the load range and flexural displacement range in the middle span for an interval in the linear region of the load versus displacement plot. The