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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1. Introduction The application of polymer matrix composites (PMC) has increased over the years in the most diverse industrial sectors, mainly in applications that are more sensitive to weight. However, the high stiffness and strength of these composites contrast with limited toughness. To overcome this disadvantage, many different strategies have been proposed to make these materials more damage resistant and less brittle. According to the literature, the failure strain and toughness can be increased if the brittle fibres are replaced by less brittle ones. Therefore, it is in this context that the term "hybrid composites" appears, which is generally used to describe a matrix containing at least two types of reinforcements (Swolfs et al. 2014). However, when these fibres are combined, the mechanical properties of the fibre-hybrid composite often end up lying in between the properties of the constituent composites (Swolfs et al. 2019). In terms of tensile properties, for example, it is possible to obtain a pseudo-ductile behaviour, because after breaking the low elongation fibres (LE), the high elongation fibres (HE) can sustain the additional load applied due to the redistribution of stresses. Nevertheless, depending on the fraction of HE fibres, two possibilities arise after the LE fibre failure: for high fractions the stress is able to reach levels higher than the stress at the failure strain of the LE fibre (the strength is dominated by the stress contribution of HE fibres at their failure strain); while for low fractions the stress at HE failure does not exceed the stress at the failure strain of the LE fibres (Kretsis 1987). On the other hand, the tensile strength for interlayered hybrids is slightly higher than for interlayered hybrids, demonstrating that increased dispersion leads to better mechanical performance in hybrid composites (You et al. 2007). For example, You et al. (2007) reported a hybrid effect of 9% to 33% in unidirectional carbon/glass hybrids, while Zhang et al. (2012) found improvements in failure strain ranging between 10% and 31% for woven composites. In terms of longitudinal tensile modulus, several researchers report that this property obeys a linear rule of mixtures (Kretsis 1987; Zhang et al. 2013). However, Ren et al. (2010) reported a higher modulus for intralayer than for interlayer unidirectional carbon/carbon hybrids. Regarding the flexural properties, they are highly dependent on the layup. Dong et al. (2012) obtained flexural strengths for carbon/glass intralayer hybrids 40% and 9% higher than full carbon and full glass composites, respectively. This can be explained by the studies developed by Giancaspro et al. (2010), where they noticed that glass fibre composites fail on the tension side, while carbon fibre composites fail mainly on the compression side. Therefore, adding carbon fibres to the tensile side of glass fibre composites increases the flexural strength, while this is not the case when they are added to the compressive side. In this context, Dong and Davies (2012) found that the highest flexural strength in carbon/glass hybrids is achieved for a relative content of 12.5% of glass fibres, all placed on the compressive side. Interlaminar fracture toughness and translaminar fracture toughness (resistance to crack propagation between layers and resistance to crack propagation perpendicular to fibres/layers, respectively) are subjects that have received very little attention for hybrid composites (Swolfs et al. 2019). In terms of interlaminar fracture toughness, for example, Hwang and Shen (1999) found that a carbon/glass ply interface has a higher mode I interlaminar fracture toughness than a carbon/carbon ply interface, however, it was not possible to associate this improvement to the synergistic effect, because the fracture toughness of the glass/glass layer interface was not measured. Regarding the translaminar fracture toughness, Donadon et al. (2007) observed, for carbon/glass woven fibre-hybrids, that the architecture of the intralayer woven fibre-hybrid fabric caused a more tortuous crack path. In this case, a component of mode II was added to a test that is supposed to be mode I-dominated. Ortega et al. (2017) analysed different combinations of woven glass, woven carbon and unidirectional carbon plies, and they found minimal changes to the translaminar fracture toughness. Concerning the impact strength, Swolfs et al. (2014) developed a review study and concluded that for symmetric layups the penetration impact strength can be improved by placing LE fibres in the middle of symmetric layups. On the other hand, for asymmetric hybrid composites, no clear conclusion can be found in the open literature about the influence of positioning of the layers in the penetration impact strength. In this case, when the carbon layers are on the impacted side, literature reports benefits in terms of penetration impact strength for carbon/glass composites (Park and Jang 2001; Sayer et al. 2010), while the opposite tendency occurred for similar composites was found by Onal and Adanur (2002). Finally, the HE fibres in a hybrid composite can act as crack stoppers for the broken LE fibres (Swolfs et al. 2014). This phenomenon is expected to increase the fatigue life of hybrid composites compared to non-hybrid composites.