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Different amounts of graphene quantum dots (CQDs) (0, 1, 2.5, and 5 wt%) were incorporated into an epoxy matrix. The thermal conductivity, density, morphology, and dynamic mechanical thermal (DMTA) properties were reused from the study of Seibert et al.. The Pearson plot showed a high correlation between mass loading, thermal conductivity, and thermal diffusivity. A poorer correlation with density and heat capacity was observed. At lower CQD concentrations (0.1 wt%), the fracture surface showed to be more heterogeneous, while at higher amounts (2.5 and 5 wt%), a more homogeneous surface was observed. The storage modulus values did not change with the CQD amount. But the extension of the glassy plateau increased with higher CQD contents, with an increase of ~40 °C for the 5 wt% compared to the 2.5 wt% and almost twice compared to the neat epoxy. This result is attributed to the intrinsic characteristics of the filler. Additionally, lower energy dissipation and a higher glass transition temperature were observed with the CQD amount. The novelty and importance are related to the fact that for more rigid matrices (corroborated with the literature), the mechanical properties did not change, because the polymer bridging mechanism was not present, in spite of the excellent CQD dispersion as well as the filler amount. On the other hand, thermal conductivity is directly related to particle size and dispersion. [ABSTRACT FROM AUTHOR]

The delamination of a carbon/glass/epoxy hybrid composite was studied experimentally to analyze the effect of reinforcement heterogeneity on data dispersion. A statistical approach was applied to evaluate the effect of the carbon/glass/epoxy hybrid composite on the individual modes (I and II) of delamination. The analysis of variance evidenced the contribution of reinforcement type in strain energy release ratio (SERR). Curve fitting for the quasi‐static test was performed with the Weibull distribution model to predict the energy release versus crack length with different reliability levels. The 60% reliability curve presented the best fit with the average of the experimental data. For all reliability levels, the hybrid composite showed an increase in SERR. [ABSTRACT FROM AUTHOR]

In polymer composites, the porosity acts mainly as a stress concentrator, which has detrimental effects depending on the shape and position of the voids. Also, the presence of voids is detrimental to the mechanical properties, which results in the need for an accurate method for their characterization in terms of morphology, position, and volume fraction. The aim of this study was to establish an appropriate procedure for the measurement of voids in a polymer composite using the mercury porosimetry technique. Data were also collected using the Taguchi approach. Subsequently, the feasibility of applying the Hg porosimetry methodology was confirmed through a comparison with standard techniques. Statistical analysis was used to determine the best Hg porosimetry parameters and pressures between 203 and 231 MPa was found to generate reliable results for the maximum porosity measurement, with no dependence on other parameters. Since the Hg porosimetry, acid digestion, and optical microscopy methods provided porosity results with a statistically significant similarity, it can be concluded that all these methods are feasible for the analysis of voids. Finally, potential benefits of the proposed porosity analysis methodology were highlighted through the characterization of the void volume, position, and morphology. POLYM. ENG. SCI., 60:841–849, 2020. © 2020 Society of Plastics Engineers [ABSTRACT FROM AUTHOR]

In the present study, different stacking sequences on hybrid carbon/glass/epoxy composites laminate were examined in relation to thermal, dynamic mechanical and long-term behavior. A positive hybrid effect was found for both hybrid composites (interleaved-Hybrid 1 and in block-Hybrid 2) showing that in some cases hybrid composites can properly replace carbon or glass composites. The composite containing all glass fiber in the middle (Hybrid 2) presented similar thermal behavior when compared to glass fiber composite. All hybrid composites presented higher storage modulus when compared to glass composite. Dynamic mechanical analysis showed that both hybrids can satisfactorily perform the requirement in a wide temperature range. The long-term prediction was successfully applied for all composites, showing to be highly temperature-dependent. Hence, depending on the application requirement, both hybrids can be used, saving weight and cost. [ABSTRACT FROM AUTHOR]

• The mechanistic model proposed was unable to evaluate and predicts mode II fatigue crack propagation based on fracture mechanisms. • The analytical model described the energy release of the hybrid composite, evidencing greater energy available for fatigue delamination growth. • The hybrid composite exhibits higher resistance of damage propagation in the shear mode associated with rougher fractured surfaces of hybrid composites. • The proposed mechanistic model evidences each reinforcement contribution to the damage propagation, with similar results at microscopic and macroscopic levels. This work proposes a mechanistic model to evaluate and characterize the mode II delamination of carbon-glass/fiber hybrid composites under fatigue loading. To this aim, crack growth was investigated at microscopic and macroscopic levels to measure the delamination propagation based on fracture mechanisms. The energy balance principle method described the energy release of the hybrid composite, evidencing greater energy available for fatigue delamination growth. The physics-based explanation for this enhancement is associated with the rougher fractured surface (tortuous propagation) due to the change in crack direction in the two reinforcements, stiffness synergy between the two fibers, and the silane coupling agent at the glass fiber surface. The proposed mechanistic model was used to analyze the physical-based behavior of the delamination of the hybrid and non-hybrid laminates, evidencing the contribution of each reinforcement to the SERR, with similar results at microscopic and macroscopic levels. [ABSTRACT FROM AUTHOR]