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A study has been performed relating to fatigue damage accumulation in Armco-iron with a mean ferrite grain size of 40 pm. Two constant strain amplitudes, L, = 1 x (N, = 250,000 cycles) and t, = 6 x lo-' (N,= 3050 cycles) were employed to derive base data for cumulative fatigue loading where unit blocks of low (1000 cycles) and high (10 cycles) strains were mixed until failure occurred. Observations in a scanning electron microscope permitted a periodic evaluation of surface damage to be recorded. Two regimes of damage were noted. During the early crack initiation phase, crack density increased and crack growth rate was constant. During the later phase, or crack propagation stage, new cracks did not nucleate, the smallest cracks arrested, crack density decreased, and the largest cracks grew in an exponential manner, which included linkage with smaller cracks. A damage accumulation model is presented which permits a prediction of fatigue lifetime under variable amplitude loading.

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The evolution of the surface crack distribution in single-step and two-step plastic strain amplitude controlled cycling was studied. The frequency of crack occurrence and total crack density were evaluated together with crack growth rates of the largest cracks. The crack density increases, reaches a maximum, and then decreases during fatigue life. The maximum value of crack density partitions the whole life into a "crack generation period" and a "crack propagation period". In the first period the crack growth rate is approximately constant and is determined by the applied plastic strain amplitude. The crack growth rate increases with crack length in the second period, but is basically determined by the crack growth rate in the "crack generation" period. Hence the crack growth rate in the "crack generation" period is the life determining factor.

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