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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high magnitude of the fatigue limit indicates that it can resist fatigue failures when used as an interconnect for flexible electronics. In addition, graphene on Si 3 N 4 /PEN samples exhibited a decreasing failure rate with time. In terms of composites with metallic matrices, Lin, et al. (2014) investigated the integration of single-layer graphene oxide powders in an iron matrix by laser sintering, as well as its mechanical strength and fatigue life. Fatigue tests were performed in bending mode with a stress ratio of 0.1 and a frequency of 10 Hz. It was possible to observe that the fatigue life of laser-sintered 2 wt.% of graphene increased 167% compared to laser-sintered neat iron. In fact, the surface strength was significantly improved by the presence of graphene into iron matrix, which delays the crack initiation process and promote longer fatigue lives. Simultaneously, crack pinning by graphene was also observed, which helps to improve the fatigue life during the crack propagation period. Kumar and Xavior (2016) studied the fatigue and wear behaviour of Al6061 - graphene particulate composite, where the improvement obtained with the graphene addition (0.2, 0.4, 0.6 wt.%) was compared relatively to the monolithic alloy. For this purpose, fatigue tests were performed in tensile mode, at 5 Hz and R=-1. These authors found a decrease in fatigue life compared to monolith aluminium, and the tendency is to decrease further with higher graphene contents. This is explained by the presence of pores in the composite (where higher graphene content increases the porosity level), clustering and agglomeration of the graphene in the composite. Furthermore, compaction and sintering parameters also have significant effect to decrease the fatigue life, both by crack initiation and propagation. Finally, Hwang et al. (2017) studied the benefits obtained with graphene nanolayers into a copper matrix, where copper/graphene composites with repeat layer spacing of 100 nm were tested for bending fatigue at 1.6% and 3.1% strain up to one million cycles. They observed improvements in fatigue strength 5 to 6 times greater compared to the conventional thin copper film. This is a consequence of the fatigue cracks generated within the copper layer being interrupted by the graphene interface. 5. Conclusion Literature reports several studies on the fatigue behaviour of graphene-enhanced composites, however, the existing knowledge is not yet enough to establish a complete understanding of this subject. In general, graphene acts as a block in the crack propagation path, forcing the crack to bypass graphene in the path of least resistance, delaying the crack increase. In this context, the interface strength between matrix and graphene plays an important role, because a low adhesion facilitates crack initiation and propagation along the material. Moreover, aligning graphene fillers perpendicularly to the crack growth direction promotes greater benefits on the fatigue strength than those randomly-oriented. For future studies, it is still necessary to study the fatigue behaviour of composite materials filled with functionalized graphene. It is necessary to understand the real benefit of functionalized graphene in fatigue strength, compared to neat graphene, and to understand the different damage mechanisms that can lead to this improvement. Finally, the open literature also does not report studies about the benefits of graphene fillers on the mechanical properties of composite sandwich structures, especially on fatigue strength. Acknowledgements This work was supported by the project Centro-01-0145-FEDER-000017-EMaDeS-Energy, Materials and Sustainable Development, co-financed by the Portugal 2020 Program (PT 2020), within the Regional Operational program of the Center (CENTRO 2020) and the European Union through the European Regional Development Fund (ERDF). References Anwar, Zanib, Ayesha Kausar, Irum Rafique, and Bakhtiar Muhammad. 2016. Advances in Epoxy/Graphene Nanoplatelet Composite with Enhanced Physical Properties: A Review. Polymer - Plastics Technology and Engineering 55(6): 643 – 62. Atif, Rasheed, Islam Shyha, and Fawad Inam. 2016. Mechanical, Thermal, and Electrical Properties of Graphene-Epoxy Nanocomposites-A Review. Polymers 8(8): 267 – 74. Bhasin, Mukesh, Shuying Wu, Raj B. Ladani, Anthony J. Kinloch, Chun H. Wang, and Adrian P. Mouritz. 2018. Increasing the Fatigue Resistance of Epoxy Nanocomposites by Aligning Graphene Nanoplatelets. International Journal of Fatigue 113: 88 – 97. Bortz, Daniel R., Erika Garcia Heras, and Ignacio Martin-Gullon. 2012. Impressive Fatigue Life and Fracture Toughness Improvements in