遅れ破壊き裂の枝分かれにおよぼす旧オーステナイト結晶粒径の影響

抄録

The macroscopic crack branching behavior in delayed failure was investigated on the high strength Ni-Cr-Mo steel with various pre-austenite grain sizes. There appear two regions in the relation between crack propagation velocity da⁄dt and stress intensity factor K, i.e, the region I, where da⁄dt increases with increase in K, and the region II, where da⁄dt is constant independent of K and macroscopic crack branching is observed. The crack propagation velocity in the two regions increases with increase in grain size and testing temperature, and stress intensity factor dependency of da⁄dt in region I increases with increase in grain size. The stress intensity factor at crack branching, K_IB, increases with increase in grain size and temperature. However, the crack branching does not occur in a larger grain size and at a higher temperature. The grain size and temperature dependency of K_IB can be well explained by using the assumption that the crack branching occurs when the trapping of hydrogen atoms by dislocations and microvoids causes the broadening of the diffusion path of hydrogen atoms at a crack tip.