クラック不安定発生の力学

残留ひずみの影響

抄録

A model of unstable crack initiation from an original crack tip, at which a plastic zone has been formed, is presented. The model provides an answer to the question how and when an increasing load leads to unstable crack initiation instead of further growth of the plastic zone. It is shown that some energy must be supplied to initiate cracking because of the residual strain in the plastic zone, which implies that the crack initiation necessarily accompanies an instability. For brittle materials, the required energy is small and the Griffith criterion turns out to be valid even with the plastic zone at the original crack tip. With increasing ductility of the material, the energy required for the crack initiation increases dramatically, which may lead to the transition from brittle (unstable) crack growth to ductile (stable) one. The present study seems to provide a clue to the problem of the transition from stable, ductile crack growth to the following unstable crack growth. The emphasis is placed on the effect of the residual strain which seems to play an important role in all modes of crack growth. The results of this study suggest a way to estimate the increase in the material resistance to fracture induced by generating intensive plastic deformation at the crack tip prior to its extension.