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    Experimental investigation of oligo cyclic compression behavior of pure epoxy and graphene-epoxy nanocomposites
    (Springer, 2021) Colak, Ozgen U.; Uzunsoy, Deniz; Bahlouli, Nadia; Francart, Charles
    The loading-unloading compression behavior and the oligo cyclic behavior of pure epoxy and graphene-epoxy nanocomposites are investigated since the systematic evaluation of the mechanical behavior under cyclic loading is of great importance in the development of damage characterization and fatigue models for polymer composites. High purity graphene nanoflakes (GNF) are synthesized by electric arc discharge method, and the manufacturing of graphene epoxy nanocomposites is done using solution blending. The structural characterizations of produced GNF are performed using several techniques such as transmission electron microscopy (TEM), Raman spectroscopy and Brunauer-Emmett-Teller (BET). Oligo quasi-static strain-controlled cyclic tests are performed at the elastic (or viscoelastic) region, around yield and after softening at the viscoplastic region. Comparing the behavior under compression, loading-unloading and oligo (repeated) cycled reveals that prehistory does not have much effect on the subsequent behavior. The change in the elasticity modulus during repeated cyclic compression is determined. It is observed that elasticity modulus decreases initially, and then, it progressively increases with the increase in applied maximum strain. Compared to epoxy, the yield stresses of graphene-epoxy decrease in both strain rates and a small increase in the elasticity modulus of graphene-epoxy is observed at low strain rate (1.E-4 /s).
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    Functionalized graphene-epoxy nanocomposites: experimental investigation of viscoelastic and viscoplastic behaviors
    (Springer, 2023) Colak, Ozgen U.; Birkan, Besim; Bakbak, Okan; Acar, Alperen; Uzunsoy, Deniz
    In this work, graphene-epoxy nanocomposites are produced for two different graphene fractions (0.1 and 0.5 wt%). Three-roll milling is used as the main strategy to achieve a homogeneous dispersion and prevent agglomeration. To improve the interfacial bonding between graphene nanoflakes (GNF) and epoxy matrix, GNFs are functionalized using Triton X-100 as a surfactant. The effectiveness of this functionalization is investigated using Raman spectroscopy and Fourier transform infrared spectroscopy (FT-IR). These spectroscopy results show that the Triton X-100 molecules are successfully adsorbed on the surface of GNFs. To investigate the total viscoelastic-viscoplastic behavior of the nanocomposites, compression tests at three different quasistatic strain rates (1.E-1, 1.E-2, 1.E-3 /s), creep tests at two different stress levels and relaxation tests at two different strain levels are performed. The total time-dependent mechanical behavior of the produced nanocomposites is therefore characterized comprehensively. Elasticity-modulus values obtained from compression tests increased up to 29% and yield stress increased up to 18%. In creep tests, it is observed that the creep strain decreased 32% and 65% at 50 and 100 MPa stress levels, respectively, at 0.1 wt% functionalized graphene flakes (f-GNF)-epoxy nanocomposite. At the same time, with the addition of 0.1 wt% f-GNF to epoxy, during relaxation tests, the stress drop decreased up to 47% compared to pure epoxy at a 3.16% constant strain level. Both creep and relaxation resistance improved when compared to pure epoxy. This total improvement in the mechanical behaviors is explained with the effective dispersion of the GNFs and also a strong interface between the GNFs and the epoxy matrix.
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    High strain rate behavior of graphene-epoxy nanocomposites
    (Elsevier Sci Ltd, 2020) Colak, Ozgen U.; Bahlouli, Nadia; Uzunsoy, Deniz; Francart, Charles
    This work consists of the synthesis of high purity graphene nanoflakes (GNF), the manufacturing of GNF-epoxy nanocomposites and the mechanical characterization of the nanocomposite at high and quasi static strain rates, (2750/s - 1.E 5/s). GNF were synthesized by using the electric arc discharge technique. Thermogravimetry/ Differential Thermal Analysis (TG/DTA) of synthesized graphene reveals high purity and high crystallinity. Raman spectra and the broad Brunauer-Emmet-Teller (BET) specific surface area indicate that the synthesized graphene has several layers. Following the solution mixing manufacturing process of GNF-epoxy nanocomposites, the influences of strain rate on the mechanical behaviors are investigated under quasi static and dynamic loadings. High strain rate uniaxial compression tests (1270-2750/s) using Split Hopkinson Pressure Bar (SHPB) and quasi static compression tests (1.E-3 and 1.E-5/s) of GNF-epoxy with two graphene contents (0.1 and 0.5 wt %) are performed at room temperature. The maximum elasticity modulus achieved by the GNF-epoxy with 0.5 wt% at the strain rate of 2350/s corresponds to a 68% increase compared to the neat epoxy. The yield strength of the material is doubled under dynamic loading conditions compared to the quasi static loading.