Low Cycle Fatigue Damage Accumulation in Armco‐iron

A study has been performed relating to fatigue damage accumulation in Armco‐iron with a mean ferrite grain size of 40 μ. Two constant strain amplitudes, εa= 1 × 10−3 (Nf= 250,000 cycles) and εa= 6 × 10−3 (Nf= 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.