Effect of relative humidity on the fatigue characteristics of mild steel in reversed torsion

Reversed torsion fatigue tests were conducted on both notched and unnotched specimens of SAE 1018 steel. The tests were made on a Sonntag Universal Fatigue Testing Machine at the rate of 1800 stress cycles per minute. The notched specimens were fatigued in air of zero-five percent relative humidity and in air of 90-95 percent relative humidity. The unnotched specimens were tested in air at three relative humidity levels: 0-5 percent, 20-25 percent, and 90-95 percent.

High humidity had a detrimental effect only on the fatigue life of the unnotched specimens indicating that the humidity effect is confined to the crack initiation stage of the fatigue process. The tests conducted in air of 20-25 percent relative humidity indicated that the effect of 20-25 percent humidity is just as great as the effect of 90-95 percent humidity.