Correlations among growth law of small cracks, low-cycle fatigue law and applicability of miner's rule

The initiation and propagation process of a small crack in low-cycle fatigue of a medium carbon steel is observed in detail by the plastic replica method. It is found that the Manson-Coffin relation is virtually the same as the crack growth law of a small crack; more than 90% (almost 100%) of the fatigue life of plain specimens is occupied by the crack propagation process. From this viewpoint, the ambiguity in the definition of the initiation of "an engineering-size crack" can be removed. The effects of prior fatigue history on the growth rate of a small crack are investigated systematically using special specimens containing an artificial small hole with diameters of 40, 100 and 200 μ m. Prior fatigue history which may be accumulated at the region where a crack will later propagate hardly influences the crack growth rate, i.e. prior fatigue history itself should not be regarded as fatigue damage. This is an essential condition for Miner's rule to apply.