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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styrene rubber filled by graphene. They found that natural rubber and solution-polymerized butadiene styrene rubber had much higher crack growth rates than natural solution-polymerized butadiene styrene rubber, however, when graphene was added to natural solution-polymerized butadiene styrene rubber, these nanocomposites had the lowest crack growth rates. Moreover, with the incorporation of graphene, the crack growth rate presented less dependence on tear energy. Wei et al. (2018) incorporated hybrid fillers of graphene oxide/carbon nanotubes and carbon black into natural rubber matrix and compared the mechanical properties of nanocomposites with two concentrations of graphene oxide/carbon nanotubes (1:1 and 1:3) and rubber with and without carbon black. Fatigue tests were carried out with edge-notched prismatic specimens (initial crack length around 5 mm), with displacement-controlled mode, at room temperature, frequency of 3 Hz, maximum strain of 0.2 and R=0. These authors concluded that, independently of the hybrid fillers (graphene oxide/carbon nanotubes) with a ratio of 1:1 improve the crack resistance, the best fatigue crack growth resistance is obtained when the carbon black filler is added. Simultaneously, the hybrid fillers reduced the heat build-up of natural rubber composites. The synergistic reinforcement obtained by incorporating hybrid fillers and carbon black into natural rubber matrix explains these benefits. In this case, a more robust and efficient network was achieved because the addition of hybrid fillers improved the carbon black dispersion. On the other hand, the hybrid fillers (graphene oxide/carbon nanotubes) with a ratio of 1:3, in all configurations, were responsible for the worst results, including those obtained with natural rubber. In this case the fillers cannot dissipate energy efficiently and, simultaneously, cause stress concentration. Finally, Xu et al. (2020) studied the effect of graphene oxide and carbon nanotubes on the fatigue properties of silica/styrene-butadiene rubber composites under uniaxial and multiaxial cyclic loading. According to previous studies developed by the authors, the content of graphene oxide or carbon nanotubes in the composites was 3 phr. The uniaxial crack growth rate was carried out at 25 °C and at a frequency of 20 Hz, while the multiaxial fatigue tests were performed between a minimum and maximum stress of 0 and 9 MPa, respectively. It was possible to concluded that graphene oxide enhanced silica/styrene-butadiene rubber composites and carbon nanotubes enhanced silica/styrene butadiene rubber composites present lower crack growth rates than those of silica/styrene-butadiene rubber composites. However, when the effect of nanoparticles is compared, lamellar graphene oxide is superior in resisting crack growth than tubular carbon nanotubes. This is explained by the filler networks that have a significant influence on the fatigue properties of rubber composites. While the cracks bypass the SiO2 particles on silica/styrene-butadiene rubber composites due to the poor interface between the SiO2 aggregates and the ne rubber matrix, the synergistic effect between graphene oxide and SiO2 or carbon nanotubes and SiO2 inhibits the path of crack propagation and reduces the rate of crack growth. In this case, the breakage of the complex filler networks dissipate energy under cyclic loadings, which improves the fatigue properties of rubber. Moreover, graphene oxide sheets have a larger surface area, therefore, the filler network created between graphene oxide and SiO2 has higher strength. From the multiaxial fatigue tests, it was possible to conclude that the fatigue life of silica/styrene-butadiene rubber composites decreased for higher values of maximum stress and, under the same maximum stress, the longest fatigue lives were obtained in composites containing graphene oxide due to the synergistic effect previously reported. The desire and necessity to improve the efficiency of composites for engineering applications has led to use of several nano-sized fillers. Of all nanoparticles studied to improve the mechanical performance of composite materials, graphene is the most promising due to its unique mechanical properties combined with its good electrical performance. In this context, Table 3 summarizes the existing studies on this subject. A fatigue characterization of fiberglass/epoxy composites with various weight fractions of graphene platelets (0.05, 0.1 and 0.2 wt.%) was developed by Yavari et al. (2010). These tests were performed in bending mode with a frequency of 5 Hz and R=0.1. When graphene platelet content increased from 0.05 to 0.2%, longer fatigue lives were observed. At a maximum stress of 500 MPa, for example, fatigue lives around 1200 times higher were observed when 0.2 wt.% of graphene platelets were added. On the other hand, these values achieved two orders of magnitude for the 4. Fatigue behaviour of graphene reinforced composites