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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increased relatively fast, but on the second regime the temperature stabilized to an almost constant value and the elongation increased less intensively. Temperature increases on the first regime are explained by the damping and low thermal conductivity of PA 6. However, around the glass transition temperature the mobility of molecules increases, and the heat can be transported more effectively promoting, in this case, an almost constant temperature on the second regime. Nevertheless, higher load amplitudes are responsible for larger elongations, which generate more heat and, consequently, faster temperature increments and faster failures are detected. When graphene nanoplatelets were added, temperature did not change, but the failure process was slightly longer. Finally, hybrid composites had the lowest temperatures, but at the moment of failure, temperature suddenly increases because the energy stored during crack propagation is released. This process involves a considerable release of heat. A graphene-based nanocomposite with integrated high tensile strength and toughness through poly(dopamine)- nickel (PDA) ion (Ni2+) chelate architecture that mimics byssal threads was produced by Wan et al. (2017). Therefore, based on the optimized content (95% of exfoliated graphene oxide plus 5% of poly(dopamine)), four materials with different Ni2+ contents were fabricated and conveniently characterized. Tensile fatigue tests were performed with a frequency of 1 Hz and at R=0.1. They concluded that nanocomposites incorporating Ni2+ had better fatigue performance, but higher fatigue lives were obtained when poly(dopamine) was added to nanocomposites. These benefits are explained by the synergistic interfacial interactions of covalent and ionic bonding, which effectively suppress the crack propagation in the fatigue process, resulting in a superhigh fatigue life. The effects of shape and concentration of carbon nanofillers on the cyclic-fatigue crack growth resistance of epoxy nanocomposites was investigated by Ladani et al. (2018). For this purpose, two different epoxy nanocomposites were produced containing, each one, 0.5 and 1.0 wt.% of carbon nanofibers or 0.5 and 1.0 wt.% of graphene nanoplatelets. Fatigue tests were performed under crack-opening displacement control at a constant amplitude, R=0.5 and frequency of 5 Hz. They observed higher fatigue resistance for epoxy nanocomposites, but fatigue crack growth resistance also increased with the increasing nanofillers concentration. Besides that, the epoxy nanocomposites containing carbon nanofibers or graphene nanoplatelets exhibit similar fatigue resistance in the near-threshold region. These benefits are a consequence of the toughening mechanisms induced by the carbon nanofillers under cyclic loading: debonding of the nanofillers; plastic void growth initiated by the debonded nanofillers in the process zone ahead of the crack tip; crack bridging and pull-out of the nanofillers behind the crack tip; and rupture of the nanofillers behind the crack tip. In addition to these mechanisms, graphene nanocomposites revealed the presence of debris particles along the fatigue crack, which induce a crack shielding effect and the consequent delay in fatigue crack growth. The electric field alignment of graphene nanoplatelets (GNPs) was used by Bhasin et al. (2018) to increase the fatigue resistance of epoxy nanocomposites. Different liquid epoxy nanocomposites, containing 0.5, 1.0, 1.5 and 2.0 wt.% of graphene nanoplatelets, were placed between two carbon fibre-epoxy laminates (12 plies of unidirectional T700 carbon-epoxy prepreg) to create double cantilever beam test specimens for fatigue tests. Initially randomly oriented graphene nanoparticles were aligned in the direction of the electric field (using the laminates as electrodes) before gelation and resin solidification. Mode I interlaminar fatigue tests were performed according to ASTM E-647, under cyclic displacement control, R=0.5 and 5 Hz. The results showed that the addition of randomly-orientated GNPs to the epoxy resin, up to a concentration of 1.5 wt.%, improves the fatigue strength. However, for higher values, no further improvement in fatigue crack growth resistance was observed due to the significant agglomeration of the GNPs. In fact, the addition of these nanoparticles was even very effective at retarding the rate of fatigue crack growth in the near threshold region. Finally, and regardless the concentration, aligned GNPs by the electric field (perpendicular to the crack growth direction) promoted greater improvements on the fatigue crack growth resistance than those obtained with randomly-orientated GNPs, especially near the fatigue-threshold region. These benefits were justified by multiple toughening mechanisms, like: debonding of the GNPs; microcracking leading to crack deflection and branching ahead of the main fatigue crack; pull-out, crack bridging and rupture of the GNPs; and formation of debris inside the advancing fatigue crack which promoted a crack tip shielding effect behind the crack front. However, all these mechanisms were most effective for GNPs aligned in the perpendicular direction to the crack growth. 3. Fatigue behaviour of graphene reinforced elastomers Elastomers are materials that have ability to return to their original form when exposed to a load. Due to their wide applications, graphene-enhanced elastomers offers advantages, because these nanoparticles have the ability to increase the toughness of elastomers, a property that is problematic in this type of materials (Wang et al. 2019). There are few studies on fatigue of graphene enhanced elastomers, and those available in the literature are summarized in Table 2.