Fracture Mode Transition and Miner's Law in Extremely Low Cycle Fatigue

Low cycle fatigue tests and microcrack propagation tests were carried out under strain cycling with two-step variable plastic strain amplitude on annealed low carbon steel and commercial pure iron. The applicability of Miner's law to the extremely low cycle fatigue has been discussed in relation to the fracture mode transition and the damage accumulation mechanism in the extremely low cycle regime. It was found that the surface microcrack initiated at a low plastic strain Δ_εP1 in low to high variable amplitude tests becomes nonpropagating at a high plastic strain Δ_εP2 because of its blunting under large plastic strain, and that in such a situation, final failure occurs in a different fracture mode from the surface microcracking by the striation mechanism. The total fatigue life is controlled by the development of internal microvoid ahead of the crack in pure iron and by the initiation of an internal crack associated with the pearlite cracking in low carbon steel. In such a case, the damage accumulation does not occur linearly with strain cycling and the value of Σ n_i/N_ft becomes larger than unity.