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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lower level. These benefits are explained by the increased energy absorption and different damage mechanisms observed. Regardless of the typical damage mechanisms in fibre reinforced composites (fibre/matrix debonding, formation and growth of inter-fibre fractures, delamination and fibre fractures), stiff nanoparticles cause stress concentrations in the matrix promoting nano-damages and micro-damages like nanoparticle/matrix debonding, plastic void growth and crack deflection. However, for lower fatigue loads, the dominant damage mechanism changes towards fibre/matrix debonding, like unmodified laminates. In this case, the benefits are more modest, regardless of the enormous plastic deformation in the matrix due to the higher fibre/matrix interface strength promoted by carbon nanoparticles. In addition, compared to multiwall carbon nanotubes, the few layered graphene is responsible for a higher specific surface area and an additional layer separation failure mode, which leads to higher absorbed energy. Bourchak et al. (2018) investigated the effect of adding nanoparticles on the fatigue properties of antisymmetric glass fibre reinforced polymer laminates. Composites reinforced with 0.1 wt.% of single-walled carbon nanotubes and with 0.1 wt.% of graphene nanoplatelets were compared in terms of fatigue behaviour. For this purpose, tensile fatigue tests were carried out at 10 Hz, R=0.1 and three tensile values were selected presenting 80%, 60% and 40% of the of static strength. It was possible to conclude that the highest fatigue lives were obtained with laminates reinforced by single-walled carbon nanotubes, followed by laminates reinforced with graphene nanoplatelets and, then, by laminates with neat resin. Benefits around 12 and 3 times higher than those achieved with control laminates (neat resin), respectively, were achieved. In terms of stiffness degradation, all laminates presented three regimes, which agrees with the open literature. Regime I is characterized by a rapid degradation due to initiation and growth of matrix cracks in off-axis direction, while regime II presents a linear degradation as consequence of the nearly stable transverse matrix cracks and inter fibre fractures. Finally, regime III presents a rapid degradation due to complete fibre fractures. The transition point between the first and second phase is dominated by characteristic damage state, where the number of matrix cracks in the off-axis direction reaches saturation. Consequently, the initial severe stiffness degradation is attributed to matrix cracking. However, when the nano-reinforcements were added, they helped to delay this saturated state. Finally, the hysteresis loop was used to evaluate the fatigue damage, and these authors found a dissipated energy decrease around 19% and 29%, compared to laminates with neat resin, when 0.1 wt.% of single-walled carbon nanotubes and 0.1 wt.% of graphene nanoplatelets were added, respectively. Therefore, the highest damages occurred in laminates with neat resin are responsible by shorter fatigue life observed. Considering the costs of nanoparticles and the difficulty of their insertion into matrices, Leopold et al. (2019) studied the possibility of modifying only a few layers of laminated composites without compromising the mechanical properties required for very specific applications. For this purpose, only transverse and parallel layers to the loading direction were nano-enhanced with 0.3 wt.% of few-layer graphene particles. In addition to the static characterization, in tensile and bending mode, these authors also performed fatigue tests in tension-compression regime (R=-1), at 6 Hz and two load levels were considerate (23% and 21.6% of the mean tensile strength). These authors concluded that incorporating graphene nanoparticles into matrix reduces the fatigue life, but this decrease was more pronounced for 90°-modified specimens. The final failure is initiated by large delaminations between the 90°-layer and the 0°-layer but, after their initiation, the failure occurs by compression in form of buckling of the 0°-layer. However, the delamination growth is more pronounced for nano-enhanced laminates, with consequent shorter fatigue life. In fact, according with the open literature, the size of graphene-based nanoparticles presents a significant influence on delamination crack propagation. Therefore, particles with larger lateral dimensions, like those used in this study, tend to align with the carbon fibres in the interlayer and along the crack growth direction, generating, in this case, numerous weak links. On the other hand, particles with smaller lateral dimensions can also orientate transversely to the intercalary plane, becoming an obstacle in the growth of interlaminar cracks. Finally, literature also reports the benefits achieved with graphene nanoparticles in other type of composites. Kim et al. (2012), for example, showed that an inelastic aerogel made of single-walled carbon nanotubes can be transformed into a superelastic material by coating it with graphene nanoplates. They concluded that the graphene-coated aerogel exhibits no change in mechanical properties after more than 1x10 6 compressive cycles, and its original shape can be recovered quickly after compression release. The excellent fatigue strength was attributed to the graphene coating, which strengthens the existing crosslinking points. Zhang et al. (2019) developed a superelastic and fatigue resistant graphene aerogel, which maintains the shape integrity after 10 6 compressive cycles at 70% strain and 5 Hz. These results are superior to those obtained for carbon monoliths and this superior performance is consequence of the highly ordered and closely packed pore structures. Fatigue behaviour of graphene on Si 3 N 4 /PEN and ITO on Si 3 N 4 /PEN was studied by Paradee et al. (2015) and, while the ITO showed a fatigue limit around 400 MPa, graphene showed a well-defined fatigue limit of 80 GPa. The