Fatigue crack initiation from notch root (local-strain damage accumulation process on crack initiation)

Abstract

Local deformation (i.e. local-strain behavior) at the notch root in a crack initiation process in annealed 0.48% carbon steel was investigated by the real-time fine-grid method. The fatigue crack initiation cycle was controlled by local-strain damage accumulation. For a quantitative expression of cumulative fatigue damage, we propose a new parameter, the “average local-strain accumulation value,” Δ \\ ̄ ge 1 , which is defined by the integration of local-strain histories until crack initiation. The relationship between average local-strain accumulation range, Δ \\ ̄ ge 1 , and crack initiation cycles, N c , showed a line whose slope was nearly −0.5 on a log-log coordinate graph. This line we term the “local-strain damage accumulation curve.” The mean stress effect in cases of R = −1, 0 on this line ( Δ \\ ̄ ge 1 vs N c ) was very small or negligible. From the results of variable-loading tests, the linear cumulative damage law based on the local-strain value was also confirmed.