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 \ ̄ge1, which is defined by the integration of local-strain histories until crack initiation. The relationship between average local-strain accumulation range, Δ \ ̄ge1, and crack initiation cycles, Nc, 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 \ ̄ge1 vs Nc) 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.
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