# Mechanical Properties of Green Synthesized Graphene Nano-Composite Samples
## 2 Materials and Methods
### Flexural Properties  
Figure 10 shows the load-displacement curve for the different prepared samples. The slope of the tangent to the initial straight line is the m coefficient in Equation (3). Figure 11a,b shows the modulus of elasticity (flexural modulus), flexural strength and flexural strain (strain at failure point) for different weight percentages of the graphene quantum dots. The results showed that adding 0.25% wt. of the graphene quantum dots increased the strength from 80 MPa to about 112 MPa (40% increase in strength). Adding a greater percentage of graphene quantum dots would not increase the flexural strength meaningfully. The flexural modulus decreased from 2.70 GPa to 2.39, 2.38 and 2.06 GPa by adding 0.25, 0.5 and 1% wt. graphene content, respectively. The flexural modulus decreased up to 23% for the 1% wt. graphene. Figure 11c shows the flexural strain (strain at failure point) for the different tested samples. The flexural strain increased considerably. The neat polyester deformed to 3.11% strain, which shows a brittle fracture in the materials. By adding the 0.25, 0.5 and 1% wt. graphene, the strain increased to 7.81, 8.35 and 14.20%, respectively. The stress-strain behavior of the three-point bending test shows that adding green synthesized graphene quantum dots increased the strength and ductility of the nanocomposites, and the fracture behavior changed from brittle to ductile. By increasing the flexural strength and flexural strain and decreasing the modulus of elasticity, the composite samples could absorb significantly higher energy before failure (ductility). Cracks propagated with a lower growth rate. This was mainly because the nanoparticles decreased the crack growth rate. Crack deflection, particle debonding and plastic void \begin{table}
\begin{tabular}{c c} \hline \hline
**Sample** & **Limiting Oxygen Index (LOI)** \\ \hline Pure polyester resin & 17 \\ Polyester resin, 0.25\% wt. of graphene & 18 \\ Polyester resin, 0.5\% wt. of graphene & 20 \\ Polyester resin, 1\% wt. of graphene & 22 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: The limiting oxygen index (LOI) for different graphene weight percentages. Figure 9: Schematic of the UL-94 test of the polymeric nanocomposites. growth are three main mechanisms of failure in nanocomposites [35]. The crack growth direction changes when it receives the nanoparticles, and the micro-void coalescence is retarded. The error bar of the flexural modulus, flexural strength and flexural strain was considerably low. Several pieces of research have been published about the improvement of the mechanical properties of composites by adding nanoparticles. As a special case, [PERSON] et al. [36; 37] improved the flexural strength of polyester nanocomposites by about 18% using multi-wall carbon nanotubes (CNT). Additionally, various carbon-based nano-additives were applied in the polymer nanocomposites, but to the best knowledge of the present authors, this article is the first attempt at using green synthesized graphene quantum dots to enhance the mechanical properties of nanocomposites. As the results show, considerable enhancement in the flexural behavior and strain at failure was obtained. The uniformity of dispersion of graphene quantum dots is important in composite fabrication. The agglomeration of graphene quantum dots decreases the strength of the composite by the formation of secondary bonding interactions between the graphene quantum dots rather than the primary bonding with the matrix [27]. Production of graphene quantum dots by green synthesis is a very cheap method compared with other synthesis processes. This article focuses on the feasibility of using green synthesis for the production of composite parts and studying the effect of adding graphene quantum dots on the mechanical properties of the fabricated parts.