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This study was undertaken to investigate the corrosion-fatigue crack initiation (CFCI) behavior of steels with nominal yield strengths from 248 to 1034 MPa (36 to 150 ksi). Notched specimens were exposed to a 3.5% NaCl solution under constant-load-amplitude conditions at a stress ratio R of 0.10 and at a cyclic frequency of 12 cycles per minute (0.2 Hz). Results of crack-initiation life (NI) were characterized in terms of cyclic stress-intensity range normalized relative to notch-tip root radius [(ΔK/√ρ)] and in terms of cyclic-stress range at the notch tip (Δσmax).
Results showed that the CFCI behaviors for the A36, A588-A, A517-F, and V-150 steels investigated were virtually identical. Each steel exhibited a linear relationship of (ΔK/√ρ) versus log NI for cyclic lives from NI ≊ 104 to 3 × 106 cycles, where the latter value reflects a continuous testing period of about 180 days (6 months). All four steels exhibited about the same cyclic-stress range of (ΔK/√ρ) ≊ 207 ± 21 MPa (30 ± 3.0 ksi) at NI = 3 × 106 cycles. No evidence of a CFCI threshold behavior was determined or apparent up to 3 × 106 cycles. Compared with the respective estimated values of fatigue-crack-initiation (FCI) threshold in air [(ΔK/√ρ)th ≊ 448, 552, 758, and 1138 MPa (65, 80, 110, and 165 ksi)], such values of (ΔK/√ρ) for CFCI behavior correspond to degradations of about 54, 62, 72, and 82% for the A36, A588-A, A517-F, and V-150 steels, respectively. The cited values of (ΔK/√ρ) at 3 × 106 cycles were also equivalent to a cyclic-stress range at the notch tip of Δσmax ≊ 234 ± 24 MPa (34 ± 3.4 ksi).
The results show that the CFCI behavior of A36, A588-A, A517-F, and V-150 steels is finite for all current test conditions and is determined by the absolute level of the cyclic-stress range [(ΔK/√ρ) or Δσmax]. Such results are in direct contrast with well-established fatigue behaviors in air, where the FCI threshold level [(ΔK/√ρ)th or (Δσmax)th] varies directly with strength level (σys or σts). Thus, the present results show that, whereas FCI behavior varies directly with strength level, the CFCI behaviors for all four steels studied were virtually identical and occurred independently of strength level.
corrosion fatigue, corrosion-fatigue crack initiation, corrosion-fatigue endurance, corrosion-fatigue strength, corrosion-fatigue threshold, crack initiation, cracks from notches, cyclic loading, environmental behavior, environmental cracking, environmental evaluation, environmental fatigue, fatigue testing methods, ferrite-pearlite steel, fracture mechanics, high-strength steels, linear elastic fracture mechanics, long-life behavior, long-term testing, material behavior, martensitic steel, mechanical behavior, notched-specimen behavior, notched-specimen fatigue, notched-specimen cracking, Stress range, Stress-intensity (factor) range, Structural behavior, structural integrity, structural steels, salt-water cracking, sodium chloride solution, stress-concentration effects, strength-level effects
Senior Research Engineer, U.S. Steel Corporation Research Laboratory, Monroeville, Pa.