SEDL / STP / STP536-EB / STP49720S

Fatigue and Corrosion-Fatigue Crack Growth of 4340 Steel at Various Yield Strengths

Imhof, E. J.
Research technician, U. S. Steel Corp., Monroeville, Pa.

Barsom, J. M.
Associate research consultant, U. S. Steel Corp., Monroeville, Pa.

Pages: 24    Published: Jan 1973

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Abstract

The useful life of steel structures that contain crack-like defects and that are subjected to cyclic loads in an aggressive environment is determined primarily by the time required for the defects to grow from subcritical dimensions to the critical size at which unstable fracture occurs. For these structures, subcritical crack growth is caused either by fatigue or by corrosion fatigue. Thus, as part of a long-range program to establish the necessary relations for predicting the corrosion-fatigue behavior of structural steels, the stress-corrosion-cracking susceptibility and the corrosion fatigue-crack-growth rates at 6 cpm of 4340 Steel heat-treated to yield strengths of 130, 180, and 220 ksi (896, 1241, and 1571 MN/m²) were investigated in a 3 percent solution of sodium chloride. The results were analyzed by using linear-elastic fracture mechanics methods and were compared with fatigue-crack-propagation data obtained in a room-temperature air environment. The results showed that the threshold stress-intensity factor below which crack growth does not occur in statiscally loaded specimens decreased with increased yield strength. The fatigue-crack-growth rates per cycle in the steels investigated in an air environment and below KIscc in the room-temperature solution of sodium chloride can be represented by the equation dadN=D(t)(ΔKI)n where n was equal to 2.7. The function D(t), which is a measure of the corrosion-fatigue susceptibility of the material, increased with increased yield strength. Fatigue-crack-growth rates below KIscc were accelerated by a factor of 2 when the 130-ksi yield-strength material was tested at 6 cpm in the sodium chloride solution. Under identical test conditions, the corrosion fatigue-crack-growth rates in the 180-ksi yield-strength material were five to six times higher than the fatigue crack-growth rates in air environment. Corrosion-fatigue crack-growth data below KIscc were not obtained for the 220-ksi yield-strength material because of the high susceptibility of this material to the test environment (KIscc=10.5ksiin., or 11.5 MN/m^3/2).