In this study, strain level and temperature-dependent stress relaxation behavior of neat epoxy and graphene-epoxy nanocomposites are experimentally investigated. Tests are performed for neat epoxy, graphene-epoxy nanocomposite with content of 0.1 wt% and 0.5 wt% graphene. To determine the effect of strain levels on relaxation behavior, three constant strain levels are chosen: 3.16%, 7.15%, and 35.5%. Those strain levels are chosen to correspond to different zones on the stress–strain curves as: viscoelastic region, yield region, and visco-plastic region. Results have shown that with the addition of 0.1 wt% graphene, relaxation modulus improved 41% at 3.16% constant strain. The effect of graphene is less effective at high strain values: for 7.15% strain, the change in relaxation modulus is 32%, and at 35.5 %strain, the change is 21.25%. In addition, tests are performed for three temperature values: 23, 65, and 120°C to determine the effect of temperature on the relaxation behavior. As expected, nano composition has an improved effect on the stress relaxation behavior. An increasing temperature increases the stress relaxation values. At room temperature (for 7.15% constant strain level) the drop in relaxation modulus for neat epoxy is 32%, this value drops to 22% with the addition of 0.1 wt% graphene. At 65°C, the rate of decrease in modulus with the addition of graphene is almost unchanged. At 120°C, the relaxation performance of the material slightly decreases. Therefore, as a novel result of the research, it is determined through the conducted experiments that increasing temperature lessens the improvement effect of graphene composition on the stress relaxation behavior.

中文翻译：

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石墨烯-环氧树脂纳米复合材料的应力松弛行为：石墨烯分数、应变水平和温度的影响

在这项研究中，对纯环氧树脂和石墨烯-环氧树脂纳米复合材料的应变水平和温度依赖性应力松弛行为进行了实验研究。对含量分别为 0.1 wt% 和 0.5 wt% 石墨烯的纯环氧树脂、石墨烯-环氧树脂纳米复合材料进行了测试。为了确定应变水平对松弛行为的影响，选择了三个恒定应变水平：3.16%、7.15% 和 35.5%。这些应变水平被选择为对应于应力-应变曲线上的不同区域，例如：粘弹性区域、屈服区域和粘塑性区域。结果表明，添加 0.1 wt% 石墨烯后，在 3.16% 恒定应变下，松弛模量提高了 41%。石墨烯的效果在高应变值下效果较差：对于 7.15% 应变，松弛模量变化为 32%，而在 35.5% 应变时，变化为 21.25%。此外，还对三个温度值进行测试：23、65 和 120°C，以确定温度对松弛行为的影响。正如预期的那样，纳米组合物对应力松弛行为具有改善的效果。温度升高会增加应力松弛值。在室温下（7.15% 恒定应变水平），纯环氧树脂的松弛模量下降了 32%，添加 0.1 wt% 石墨烯后，该值下降至 22%。在 65°C 时，添加石墨烯后模量的下降率几乎没有变化。在120℃时，材料的松弛性能略有下降。因此，作为该研究的一个新颖结果，通过所进行的实验确定，升高温度会减弱石墨烯组合物对应力松弛行为的改善效果。