Abstract
Crack growth behavior in a viscoelastic epoxy strip above and below the glass temperature T g is extensively discussed by focusing on (1) the threshold condition of crack growth; (2) subsequent crack growth behavior; and (3) the characteristics of fracture surface. To realize good reproducibility of crack growth behavior that depends strongly on time and temperature, a new technique for introducing an initial crack simulating the mathematical model, i.e., a single plane crack with sharp tip, has been developed. Taking experimental evidence into account, an approach to the M ID criterion, based on the displacement field around the crack tip for crack growth under monotonically increasing loading, is proposed. Two types of equations predicting crack growth rate are evaluated through the comparison with experimental results. As a measure of the critical condition of crack growth, an extended J-integral derived for linearly viscoelastic material, is discussed. The fracture surface is carefully investigated for morphological features of the submicroscopic structural change related with heterogeneous local deformation around the crack tip. Only stable crack growth of a single main crack is observed above T g , while below T g not only both stable and unstable crack growth, but sometimes branching to multi-cracks, is observed.
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