低炭素鋼の延性脆性破壊遷移挙動を支配する安定き裂伝播抵抗と脆性破壊起点

抄録

The fracture process of low carbon steels have been investigated by means of a fracture mechanics test and microscopic observations. Three steels with characteristic differences in the ductile-brittle transition behaviors such as the transition temperature, R-curve and brittle fracture initiation stage are employed. A new microstructural parameter, the constraint factor, is defined to take into account the constraint of plastic deformation by grain boundary carbides. The constraint factor is successfully correlated with the resistance against the stable crack growth. The crack tip field is analyzed in terms of the elastic-plastic analyses in terms of J/σ_y and COD, and the elastic-plastic analyses are found to be applicable at the ductile-brittle transition region. The brittle fracture initiation is controlled either by the local stress or by the local strain for respective steels. The constraint factor is also correlated with the location of the brittle fracture initiation sites. The estimated local stress at the brittle fracture initiation site shows a unique temperature dependence irrespective of the microstructure of steels. It is suggested that the nucleation of an incipient crack at the brittle initiation site is microstructure dependent through a local deformation behaviors.