13th International Conference on Fracture June 16–21, 2013, Beijing, China -5- growth stage, characterized by the decrease of net stress, related then to Ep. 3.2.2. Maximum net stress Figure 3. σnet max as a function of the degree of crystallinity χ (%) The maximum values of the net stress (σnet max) were plotted as a function of χ (%) in fig.3. A continuous increase in σnet max is observed, slightly between 14% and 35% then significantly for the last values of the degree of crystallinity. Moreover, excellent reproducibility of (σnet max) values could be noticed. 3.2.3. Opening displacements: δ(σnet max ) and δ(σnet = 0 ) Fig.4 displays the crack opening displacements δ corresponding to respectively, σnet = 0 at the complete failure of the specimens (first Y-axis) and σnet max (second Y-axis). The same trend as in fig.3 is observed for δ(σnet max). This similarity indicates that during the loading step, the net stress versus δ curve was quasi-linear. Instead, the evolution of δ(σnet = 0) versus χ (first Y-axis) exhibited a jump between 20% and 30%. Actually, this jump was essentially due to the propagation stage where additional deformation of the crack tip (blunting) was observed. For χ ≥ 35% a saturation of δ(σnet = 0) value was observed. 0 2 4 6 8 10 10 20 30 40 50 60 χ (%) σnet max (MPa)
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