Low-rate fatigue crack propagation properties in sodium chloride solution were investigated for carbon steel S45C, high-strength steels SM 50B and HT 80 and stainless steels SUS 403 and SUS 304.Tests were conducted under the condition of decreasing Δ
K where maximum load was kept constant and minimum load was increased with the increase of crack length.This constant
Pmax test was confirmed to be a method free from crack closure, although the load ratio increased from 0.5 to 0.97.Low-rate fatigue crack propagation properties, obtained under closure-free condition, are summarized as follows.
1) In air, fatigue crack propagation properties including the threshold, Δ
Kth,
air, were independent of the kind of materials.The Δ
Kth,
air values for the five steels were around 2.8MPa·m
1, 2.
2) In 3% sodium chloride solution under freely corroding condition at a frequency of 30Hz, fatigue crack propagation rates were also independent of the kind of materials and coincided with those in air when Δ
K>Δ
Kth,
air, When Δ
K<Δ
Kth,
air, the crack propagated at such low region of Δ
K<1MPa·m
1/2 that the threshold, Δ
Kth, corr, did not exist in practical sense for carbon and high-strength steels.Δ
Kth, corr, which was 70% of Δ
Kth,
air, existed for stainless steels under the experimental conditions.
3) Fatigue crack propagation properties including the threshold values for all the steels coupled with zinc anode in sodium chloride solution were coincident with those in air.
4) Decreasing in frequency from 30 to 0.03Hz resulted in a sixfold acceleration of crack propagation rate regardless of materials and Δ
K levels under freely corroding condition.
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