| Abstract: | The microscopic factors relating to the energy dissipation increase in the viscoelastic dynamic crack propagation such as in PMMA [poly(methyl methacrylate)] were investigated through the observation on the microfracture process ahead of the crack tip up to the microcrack formation. It is found that the energy dissipation remains almost constant in a certain low velocity range but increases sharply in the higher velocity range for the PMMA case. A cleavagelike fracture mode works in the former, and both the cleavagelike and a ductile-fracture mode work in the latter. The increase in energy dissipation is attributed to the secondary cracks which remarkably increase in number depending on the crack velocity. The energy dissipation is proved to be independent of the crack velocity in cases where the crack is propagated by the cleavagelike fracture mode. |
