Volume 6, Issue 1 (January 1978)
Threshold Corrosion Fatigue Crack Growth in Steels
The threshold stress intensity factor range ΔKth of three steels was studied in air and steam. Constant load and constant crack opening displacement (COD) tests were conducted by using tension-tension sinusoidal wave function loading. The COD tests were controlled by a high frequency clip-on strain gage. Crack growth rate was measured by both optical and COD compliance techniques and verified by metallographic examination of the broken specimens. The ΔKth value was determined as the asymptote to the best-fit data below a crack growth rate of 2.5 × 10−8 cm/cycle. The ΔKth values were higher in steam than in air. Crack growth rates in air and steam generally crossover or merge in the range of 2.5 × 10−6 to 2.5 × 10−7 cm/cycle; below this range the growth rate is lower in steam than in air. The higher values in steam tests were explained by environmentally assisted crack branching and corrosion products formed at the crack tip after long-time testing at growth rates of less than one lattice spacing per cycle of loading.