13th International Conference on Fracture June 16–21, 2013, Beijing, China -2- corresponding to either fracture toughness or impact strength, the value of this critical degree of crystallinity was changed. 2. Experimental procedure 2.1. Materials - Specimen The PVDF/PMMA blends were prepared from the homopolymers PVDF Kynar® 721 and PMMA Oroglas® V825, both provided by the company Arkema France. Blending was achieved by co-precipitation from a homogeneous solution [1]. Depending on the fraction of PVDF, either amorphous or semi-crystalline blends were produced. Typically, the blends remain amorphous when their PVDF content is less than about 50 wt%. In the following, blend identification was based on weight composition. For instance, %F = 20 or F/M 20:80 deals with a blend containing 20 wt. per cent of PVDF and 80 wt. per cent of PMMA. Of course, blend composition influenced the values of the glass transition temperature. Moreover, the presence of amorphous PVDF segments constrained by the crystallites was evidenced in the crystalline blends [1]. Table 1. Blends characteristics Sample %F Tg (°C) χ (%) M 0 110 0 F/M 20:80 20 85 0 F/M 30:70 30 77 0 F/M 40:60 40 58 0 F/M 50:50 50 66 14 F/M 60:40 60 54 19 F/M 70:30 70 45 27 F/M 80:20 80 54 36 F/M 90:10 90 53 45 F 100 -40 55 Table 1 recalls the main characteristics of the blends under study, namely overall wt percentage of PVDF, %F, glass transition temperature, Tg and the degree of crystallinity, χ. The blends were compression-molded in the form of 5 mm-thick sheets. Then, single edge notch bending (SENB) specimens were machined out of the sheets. Their characteristic dimensions were kept constant: thickness T = 5 mm, width W = 12 mm, crack depth a = 5 mm (thus a/W = 0.42), length L = 70 mm, span S = 50 mm. The pre-crack was machined with a notch root radius of approximately 250 µm. 2.2. Fracture energy measurements Fracture tests on SENB specimen were carried out at room temperature on an Inströn testing machine with a constant crosshead speed of 0.5 mm.s-1 for all tests. For the sake of reproducibility, all tests were repeated twice. The load F, the notch opening displacement δ and the crosshead displacement were recorded as a function of the running time. The net stress (σnet) is defined as the
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