13th International Conference on Fracture June 16–21, 2013, Beijing, China -7- coincided to that of δ(σnet = 0). Furthermore, fig.5 showed that at low values of fracture energy (χ ≤ 20%), Et ≈ 2Ei. This was due to that Ei and Ep had approximately the same value. In the upper shelf of the fracture energy (χ > 40) Ep was of higher order of magnitude than Ei. 4. Discussion The effects of the PVDF content on the fracture energies were analyzed for %F ≥ 50 corresponding to χ ≥ 14%. Depending on the selected fracture parameter, the observed gain in fracture energy may significantly differ in their absolute value (factor 10). Indeed, a jump in the total fracture energy Et was clearly established whereas a smoother increase in Ei was evidenced at the same time. It should be recalled that Et is representative of the Charpy impact strength. The “gain in toughness” observed for both parameters was shown to be of the same order of magnitude. In practice, the shape of Et suggests that the degree of crystallinity corresponding to the brittle to ductile transition was estimated at about χ = 23%. Around this transition, Et value drastically increased from 0.13 kJ/m2 to 5.3kJ/m2 (factor 40). It is to be noted that for higher values of χ, Et was still growing from 5.3 kJ/m2 to 29 kJ/m2. For the fracture toughness parameter Ei, a slight and progressive transition was observed at χ values ranging from 30% to 45%. In this χ range, Ei increased from 0.08 kJ/m2 to 1.2 kJ/m2 (factor 10). 5. Conclusion Consideration of the blends of PVDF and PMMA all over the composition range proved to be a powerful tool to assess the characteristics that govern the fracture behavior of polymers. The amorphous blends (PMMA-rich materials) corresponding to PVDF content lower than 50% exhibited zero degree of crystallinity, low values of fracture energy and no particular pattern on their fracture surfaces. They were not taken under consideration in this work. The PVDF-richer blends exhibited higher degrees of crystallinity χ ≥ 14%. The fracture surfaces issued from tested SENB samples showed dimples characteristics of semi-crystalline polymers. Transitions from brittle to ductile fracture behavior of these semi-crystalline blends were identified, in terms of critical degree of crystallinity χ. The value of such a transition was found to depend on the fracture mechanics parameter: either the fracture toughness (fracture energy to crack initiation Ei) or the equivalent of the Charpy impact strength in quasi-static case (total fracture energy Et). The transition for Ei and Et was estimated at χ ~ 38% and χ = 23%, respectively. At these χ-transitions, Ei was found to increase from 0.08 kJ/m2 to 1.2 kJ/m2 (factor 10) whereas Et jumped from 0.13 kJ/m2 to 5.3kJ/m2 (factor 40). Acknowledgements The authors would like to acknowledge Julie Heurtel (Mines ParisTech) for technical support.
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