This study was conducted to investigate the effect of cyclic frequency on the corrosion-fatigue crack-initiation behavior in regions of stress concentration in ASTM A517 Grade F steel. The tests were conducted on notched compact tension specimens at a stress ratio of 0.1 in a room-temperature 3.5 percent solution of sodium chloride. The test results showed that the corrosion-fatigue crack-initiation life under full immersion conditions was significantly less than the fatigue-crack-initiation life in air. Moreover, the test results showed no effect of cyclic frequency in the range of 12 to 300 cpm on the corrosion-fatigue life. The data indicate the possible existence of a corrosion-fatigue crack-initiation limit, below which cracks did not initiate, at a ΔK/√ρ of about 172 MPa (25 ksi). This value of ΔK/√ρ corresponds to a maximum stress range, Δσ_max, of about 207 MPa (30 ksi) and is one-fourth the value for the fatigue-crack-initiation limit in air.

Fatigue-crack-initiation tests results on precorroded notched specimens showed a 25 percent reduction in the fatigue-crack-initiation limit. This decrease was attributed to an increase in the stress concentration caused by corrosion pits on the surface of the notch radius.

Metallographic investigations showed that corrosion-fatigue cracks initiate at corrosion pits on the surface of the notch tip. These cracks initiate as microcracks that form by a sharpening of the corrosion-pit tip under the combined influence of the environment and cyclic loads. No relationship was found between microstructure and corrosion-pit sites.