## 6 Mechanical Properties of PP/GF/PP Composites
It is clear that fibers always break at their fracture limit, however the mechanism for crack propagation, and consequently the composite mechanical strength, will be quite dependent on the fiber/matrix adhesion. When adhesion is low (composites with untreated fiber) the crack propagates mainly by fiber debonding. It results in fewer and relatively longer fiber segments. Instead, when polymerized (treated) fibers are used, cracks spread radially, preventing the fiber pullout. In this case, the propagating crack finds a greater amount of bonded fiber fragments in its way; so more energy is needed to break (or evade) them and allow the crack to grow. Therefore, when a large amount of short and well-adhered fiber segments are present, a considerable barrier to crack propagation is set, essentially by increments of the fissure path tortuosity. The proposed mechanism justifies the increment in elongation at break (more than twice for polymerized fiber samples as compared to PP/GF/PP and PP/MAO-GF/PP), as is shown in Figure 20. Also, the toughness, calculated as the composite energy to break from the area under the stress-deformation curve, resulted three times greater for the treated fiber samples (see Table 7). Furthermore, the elongation at yield (related to the energy required to generate cracks) indicated that PP/COP-GF/PP needed 5% more elongation than composites with non-copolymerized fibers to initiate cracks. This evidence further validates the proposed mechanism. Other authors found similar behaviors, but attributed it to the ability of coupling agents to change the matrix nature, which could act as impediment of crack propagation. These authors did not report an increase in the strain at which the maximum stress occurs, but proposed just an increase of matrix ductility by the coupling agent use as the main cause of improvement [54, 55, 56]. In order to corroborate the proposed fracture mechanism, and the adhesion enhancement, SEM characterization of specimens after tensile tests was carried out. Figure 21 shows the fractured sample surfaces of PP/GF/PP, PP/MAO-GF/PP and PP/COP-GF/PP. It is clear from Figure 21a, b that untreated and MAO treated composites breaks mainly by fiber debonding, hence PP remains unchanged. On the other hand, in PP/COP-GF/PP samples, the PP presents a lot of holes evenly distributed throughout the sample, indicating that fibers pull the matrix before composite fracture (Figure 21c). This is consistent with the greater resistance of chemical anchorage relative to PP molecular entanglements. Moreover, the less regular overall fracture is consistent with a greater resistance to breakage. Also, PP patches remain adhered to the fiber surface after the composite fracture (see Figure 22). The observed behavior agrees with the above-proposed model. Here the interphase presents a micro-damage mode of debonding accompanied by matrix cracks and promotes a stronger adhesion between matrix and fibers. The result is better resistance to development of damage and thus higher normal tensile stress. The interface debonding can reduce the effect of the matrix crack. The breakage (instead of debonding) of several small fiber bundles triggers larger matrix cracks, causing their propagation along the longitudinal direction. Table 7 also shows the tensile modulus from non-polymerized and polymerized samples. As it is expected, no appreciable variation is observed since this is a zero strain property and neither fiber nor matrix modulus changed after treatment. From these results it is clear that the _in-situ_ polymerization technique improves the compatibility between phases allowing for more ductile composite materials without affecting their rigidity. Figure 21: _Cont._  
Figure 22: SEM micrographs of PP/COP-GF/PP at: (**a**) 2,000x and (**b**) 4,000x.