Abstract
Fatigue-crack propagation rates in ASTM A612 Grade B steel were investigated at room temperature and −100°F (−73°C) with R ratio = −0.1 and +0.67 . The data were evaluated in terms of the crack propagation rates ( da/ dN ) as a function of the alternating stress intensition ( ΔK), according to da/ dN = e+(v − e)(− 1 n (1 − ΔK/K b)) t/k . It was found that crack growth rates were increased due to increasing R ratio. Also the dependence of crack growth rates on R ratio is strongest at the lowest crack growth rates where a ΔK fatigue threshold is established. Crack growth rates were decreased due to decreasing test temperature in the slow crack growth region. However, it was found that crack growth rates were increased due to decreasing test temperature in the fast crack growth region near the upper instability asymptote. Decreased test temperature and increased R ratio interact synergistically to increase crack growth rates for the entire range of ΔK.
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