PSI - Issue 25

J.M. Parente et al. / Procedia Structural Integrity 25 (2020) 282–293 J.M. Parente/ Structural Integrity Procedia 00 (2019) 000 – 000

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the other hand, the tilting and twisting of the crack front also forces the crack to grow locally in mixed mode conditions, which requires a higher driving force than in Mode I. Bortz et al. (2012) achieved significant improvements in fatigue life when 0.5% of graphene oxide sheets were added to a thermosetting epoxy system. Tensile fatigue tests were performed at R=0.1 and 5 Hz. For all stress amplitudes, fatigue life was higher for graphene nanocomposites, reaching values around 420% and 1580% higher than those obtained with neat resin for stress amplitudes of 40 MPa and 25 MPa, respectively. According to the authors, this is consequence of the large surface area, strong interfacial bonding and higher tensile properties that prevent the fatigue crack growth. Epoxy based nanocomposites enhanced by graphene oxide (GO) sheets and multi-walled carbon nanotubes (CNTs) were studied by Li et al. (2013), and a remarkable synergetic effect was found with consequent benefits on mechanical properties. The optimal mechanical properties were achieved with 0.2 wt.% GO and 0.04 wt.% CNTs. In terms of fatigue life, for example, an increase around 950% was achieved by the addition of GO dispersed CNTs. The fracture surfaces were observed by SEM, and a significant increase on surface roughness was found, evidencing that pull out and crack deflection may both play a significant role and, consequently, higher fatigue lives compared to neat resin. The effect of graphene nanoplatelets on the bending properties of an epoxy nanocomposite was studied by Shokrieh et al. (2014). For this purpose, fatigue tests were carried out under displacement-controlled bending loading, at different displacement amplitudes, at room temperature and 5 Hz. Authors obtained significant improvements with 0.25 wt.% of graphene nanoplatelets, where, considering a stress ratio of 0.43, the fatigue life is about 27.4 times higher. The dominant failure mechanism was pull-out, which, combined with a uniform dispersion, justifies the improvements achieved. In a similar work (Shokrieh et al. 2014, these authors studied the same nanocomposite but with different weight percentages (0.1, 0.25 and 0.5) of graphene nanoplatelets. The experimental results showed that nanocomposites with 0.25 wt.% of graphene nanoplatelets were more efficacious on the fatigue strength than those with 0.1 or 0.5 wt.%. However, all presented better fatigue performance than the neat resin. For instance, at the bending strength ratio equal to 0.43 and adding 0.1, 0.25 and 0.5 wt. % of graphene, authors found improvements around 22.4, 27.4 and 17 times higher compared to the neat epoxy resin, respectively (Shokrieh et al. 2014). Finally, the same authors studied the effects of adding a combination of synthesized graphene nanosheets with carbon nanofibers (CNFs) on the flexural fatigue behaviour of epoxy polymer (Shokrieh et al. (2014)). Fatigue tests were carried out at different displacement amplitudes fatigue loadings, at room temperature and 5 Hz. Three different nanocomposites were produced by adding 0.25 wt.% of graphene nanoplatelets into the epoxy resin; adding 0.25 wt.% of carbon nanofibers into the epoxy resin; and adding 0.25 wt.% of graphene and 0.25 wt.% of carbon nanofibers (0.5 wt.% of nanoparticles) into the epoxy resin. While fatigue life was 27.4 times higher with graphene nanoplatelets and 24 times higher with carbon nanofibers, comparatively to the neat resin, the highest improvement, around 37.3 times, was achieved with hybrid nanoparticles (0.5 wt.% of nanoparticles). These values were obtained with a strength ratio equal to 43%. The benefits of the hybrid configuration (0.25 wt.% of graphene and 0.25 wt.% of carbon nanofibers) were explained through the fracture surface of the specimens. While the pull-out of the carbon nanofibers increases the strength and the graphene nanoplatelets increase the stiffness, the hybridization of these nanoparticles promotes the toughening and influences the crack propagation. Mészáros and Szakács (2016) produced hybrid composites with graphene and basalt fibre content, and their characterization was performed in terms of static and fatigue properties. Different composites based in polyamide 6 (PA 6) and reinforced with 30 wt.% of Basalt fibres (BF) and the same architecture with 0.25, 0.5, 0.75 and 1 wt.% of graphene nanoplatelets were produced by injection moulding. The load-controlled fatigue tests were performed in tensile mode with a frequency of 2 Hz and at R = 0.1. During the tests, the surface temperature of the samples was measured with an infrared camera. From the static characterization, they observed a moderate increase in tensile strength for lower nanoparticle contents, but the opposite tendency for higher contents. Similar tendency was found for stiffness and the elongation decreased with the increasing filler content. These results are consequence of an inappropriate dispersion of the graphene in the matrix. In terms of fatigue performance, graphene nanocomposites show significant decreases in fatigue life due to the high number of aggregates observed. However, a favourable effect was observed for hybrid composites, especially when 0.25 wt.% of graphene was added, where remarkable increase in fatigue life was obtained at all applied load levels. The instantaneous elongation and the maximum temperature on the neat PA 6 were recorded in each cycle, and two regimes were observed. In the first, temperature and elongation