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Double Torsion has been adopted to characterize fracture toughness of two polystyrenes having different
average molecular mass. Stable crack propagation has been obtained for all temperatures and
displacement rates considered. The time-temperature equivalence principle has been applied to derive
"master" curves of fracture toughness, GIc, vs. crack speed. Results show that the toughness of the
low-molecular mass material is substantially lower for lower crack speeds, but the difference is reduced
with increasing crack speed. The observed behaviour has been interpreted in terms of fracture
micromechanisms, using electron microscopy.