Axial loading fatigue tests were carried out for annealed 0.45% carbon steel at various stress ratios. When the stress level is high, the crack growth rate is expressed by the small crack growth law. dl/dN=Cσnl (dl/dN : crack growth rate, σ : nominal stress amplitude, l : crack length, C and n : constants) at each stress ratio, though the constants C and/or n in the crack growth law vary depending on the stress ratio. On the other hand, the crack growth rates are determined independently of the stress ratio by the modified crack growth law, dl/dN=C'εpn'l. where εp is the plastic strain amplitude obtained from the cyclic stress strain curve at each stress ratio, and C' and n' are constants.
