## 6 Mechanical Properties of PP/GF/PP Composites
\begin{table}
\begin{tabular}{c c c} \hline
**Property** & **PP** & **GF** \\ \hline E [MPa] & \(685\pm 69.6\) & \(47,600\pm 2,015\) \\ \(\sigma_{\text{u}}\) [MPa] & \(25.7\pm 1.7\) & \(234\pm 31.3\) \\ \(\varepsilon_{\text{y}}\) [\%] & \(10.0\pm 1.8\) & \(\longrightarrow\) \\ \(\varepsilon_{\text{b}}\) [\%] & \(412\pm 92\) & \(2.6\pm 0.9\) \\ \hline \end{tabular}
\end{table} 
Table 5: Mechanical properties of PP and GFs. The fracture shape, as well as the morphological features of fiber surface and microcomposites subjected to mechanical tests were studied by SEM. Figure 19 shows tensile stress-strain curves for all the composites tested, PP/GF/PP, PP/MAO-GF/PP and PP/COP-GF/PP. The corresponding properties are summarized in Table 7.  
The possible occurrence of fiber damage caused by the MAO attack was studied in a previous study [50], in which tensile stress-strain tests on both untreated and MAO treated single GFs were done. The strength data, statistically treated according to the Weibull distribution, showed that MAO induced only a slight decrease in the fiber tensile strength, indicating that a very small fiber damage could occur in the presence of MAO. This trend is also confirmed for composite samples, since only a small variation in strength and elongation properties was observed for composites having either untreated and MAO treated fibers (Figure 19). From the strain-dependent tensile behavior, it is observed that the mechanical strength of PP/GF/PP and PP/MAO-GF/PP composites is reached at 1 to 3% deformation; while for PP/COP-GF/PP, the deformation is close to 7%, _i.e._, three times larger. This is related to the composite fracture mechanism. \begin{table}
\begin{tabular}{c c c c} \hline
**Property** & **PP/GF/PP** & **PP/MAO-GF/PP** & **PP/COP-GF/PP** \\ \hline E [MPa] & 8,125 \(\pm\) 1,060 & 8,491\(\pm\)1,029 & 8,852 \(\pm\) 1,017 \\ \(\sigma_{u}\) [MPa] & 101.6 \(\pm\) 14.1 & 103.4 \(\pm\) 12.3 & 104.7 \(\pm\) 11.1 \\ \(\varepsilon_{y}\) [\%] & 2.5 \(\pm\) 0.8 & 1.6 \(\pm\) 0.4 & 6.8 \(\pm\) 0.9 \\ \(\varepsilon_{b}\) [\%] & 14.1 \(\pm\) 8.3 & 16.56 \(\pm\) 6.4 & 35 \(\pm\) 7.2 \\ Toughness [J] & 12,654 \(\pm\) 1,100 & 12,054 \(\pm\) 512 & 38,579 \(\pm\) 1,600 \\ \hline \end{tabular}
\end{table} 
Table 7: Mechanical properties for the composites PP/GF/PP. Figure 19: Stress-strain curves of PP/GF/PP composites. In the first two, the whole composite fracture occurs by fiber breakage, at \(\epsilon\) < 3%, without any matrix contribution to resistance due to the low matrix-fiber adhesion. On the other hand, for copolymerized fiber composites, the fibers start breaking at about 2% strain (Figure 19), according to the main break strain of GF. However, the strain (at which strength is achieved) increases up to 7%, and can be attributed to a greater matrix/fiber adhesion. As was demonstrated above, the copolymerized fiber breaks into much smaller segments than samples containing untreated fibers. Then, although the first fracture occurs at the same strain for both treated and untreated fibers, the treated ones keep breaking as the deformation increases over this strain. This is due to the more effective matrix/fiber load transfer that requires greater energy consumption to fracture. In composites with polymerized fibers, GF/PP interface essentially resists by chemical anchorage. Once the fibers fracture, composites do not break immediately. This fact can be assigned to the greater stability of the glass-PP interface as compared to PP-PP entanglement resistance. The composite initially contains continuous and aligned fibers, but once the first fiber fracture occurs, it gradually converts into a short fiber composite. Here the fiber fragments remain aligned with the load direction and well attached to the matrix. As the strain increases, each fiber fragment bears a growing stress up to its rupture limit. Significantly, the rate at which the load is transferred to the fibers decreases as voids begin to form and grow at the fiber tips. Figure 20 outlines this mechanism.