smooth and frustrated micro-branches. If the energy release rate exceeds twice GIDmin, then two macroscopic smooth branches may appear. SBS fracture tests were also performed using pure PMMA specimens. In these samples, optical microscopy revealed mirror like fracture surfaces, while as expected [8, 9] the experimental fracture energy increased with crack speed. Figure 5 shows that GIDmin is similar for PMMA and RT-PMMA at the equivalent crack speed. The fracture energy of RTPMMA at a crack speed of åmb varies from 1 to 4 times GIDmin, which corresponds to smooth crack surfaces, and we sometimes observed triple branching. Hence the fracture energy might be expected to be directly correlated with the ratio SFt / (B ∆a) and the fracture energy of pure PMMA. CONCLUSION RT-PMMA represents an interesting model material to study the micro mechanisms of dynamic fracture processes. The present work shows experimentally and explains why the measured fracture surface energy has no single value at the macroscopic branching velocity, at least in materials in which the fracture energy decreases during RCP. 1E-7 1E-6 1E-5 1E-4 0.1 1 0.1 1 10 PMMA RT-PMMA °a mb ° GIc, GID (kJ/m²) a / cr Figure 5: Fracture surface energy vs. normalized crack speed for PMMA and RT-PMMA. REFERENCES 1. Yoffé, E. H., Phil. Mag., 12, (1951), p 739-750. 2. Fond, C. and Schirrer, R, Journal de Physique IV, (1997), p. C3-969-C3-974. 3. Nilsson, F, Int. J. Fract. Mech. 8, (1972), p.403-411. 4. Thesken, J. C, The Aeronautical Research Institute of Sweden, FFA-TN, 57, (1994). 5. Rittel, D., Int. J. Solids Structures, 35, 22, (1998), p. 2959-2973. 6. Ferrer, J. B., Fond, C., Arakawa, K., Takahashi, K., Béguelin, P. and Kausch., H.-H., Letters in Fract. and Micromech. 87, (1998), L77-L82. 7. Popelar, C. H. et Atkinson, C., J. Mech. Phys. Solids, 28, 14, (1980), p. 77-93. 8. Williams, J. G., Int. J. Fract. 8, (1972), p. 393-401. 9. Carlsson, J., Dahlberg, L. and Nilsson, F., Proc. Int. Conf. Dyn. Crack Propag., (1972), ed. 1973, p. 165-181. 5
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