Abstract
In the present study, the influences of microstructures of material on the characteristics of small-crack growth were investigated and the minimum crack length to which the small-crack growth law (dl/dN ∝ l) is applicable was evaluated using a ferrite-pearlite steel, a ferrite-martensite steel and a quenched and tempered carbon steel. The crack growth is influenced markedly by grain boundary, pearlite structure and martensite structure at the initial stage of crack growth, and the influences decrease with the increase in crack length. Even in the process in which crack growth is influenced by the microstructures, however, the mean value of crack growth rate is estimated by extrapolating the crack growth rate in the region where the crack growth is mainly controlled by the mechanical parameter: that is, the small-crack growth law is applicable in most regions of the crack growth process.
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