13th International Conference on Fracture June 16–21, 2013, Beijing, China -4- 3. Results 3.1. Fractography In Table 1, the degree of crystallinity χ is zero for %F ranging from 0 to 50. Additionally, Tg values were “stabilized” for %F ≥ 50. As already mentioned, the blends were considered as semi-crystalline when their PVDF content was more than 50 wt%. By examining fracture surfaces of broken SENB specimens (fig.1), it was demonstrated that typical semi-crystalline features were identified on blends corresponding to 50 ≤ %F ≤ 80. Accordingly, investigations focused on blends for which %F ≥ 50, that is 14% ≤ χ ≤ 55%. Indeed, to better assess the transition between brittle (low fracture energy) and ductile (high fracture energy) fractures, the failure mechanisms should be ensured to be the same. Moreover, since Tg does not vary so much for semi-crystalline blends and since all tests were carried out the room temperature, the degree of crystallinity χ was selected as the relevant parameter allowing for the characterization of this transition. 3.2. Mechanical parameters 3.2.1. Net stress versus δ curves Figure 2. Typical net stress versus opening displacement curve for %F = 50. The main mechanical parameters were derived from the curve of net stress versus the opening displacement δ as illustrated in fig.2. The definitions of σnet max, δ(σnet max) and δ(σnet = 0) were clearly indicated. Moreover, two specific steps could be distinguished in this plot: i) the loading stage (up to maximum net stress), where the crack is supposed to be stationary at the initial value of a/W = 0.42. This part is concerned with “crack initiation” process and therefore to Ei; ii) the crack 0 0.5 1 1.5 2 2.5 0 0.1 0.2 0.3 0.4 δ (mm) σnet (MPa) δ(σnet = 0) δ(σnet max) Ei Ep σnet max
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