**Published:** Jan 1983

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**Source: **STP801-EB

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 (*N*^{I}) were characterized in terms of cyclic stress-intensity range normalized relative to notch-tip root radius [(Δ*K*/√*ρ*)] and in terms of cyclic-stress range at the notch tip (Δ*σ*^{max}).

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 (Δ*K*/√*ρ*) versus log *N*^{I} for cyclic lives from *N*^{I} ≊ 10^{4} to 3 × 10^{6} 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 (Δ*K*/√*ρ*) ≊ 207 ± 21 MPa (30 ± 3.0 ksi) at *N*^{I} = 3 × 10^{6} cycles. No evidence of a CFCI threshold behavior was determined or apparent up to 3 × 10^{6} cycles. Compared with the respective estimated values of fatigue-crack-initiation (FCI) threshold in air [(Δ*K*/√*ρ*)^{th} ≊ 448, 552, 758, and 1138 MPa (65, 80, 110, and 165 ksi)], such values of (Δ*K*/√*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 (Δ*K*/√*ρ*) at 3 × 10^{6} cycles were also equivalent to a cyclic-stress range at the notch tip of Δ*σ*^{max} ≊ 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 [(Δ*K*/√*ρ*) or Δ*σ*^{max}]. Such results are in direct contrast with well-established fatigue behaviors in air, where the FCI threshold level [(Δ*K*/√*ρ*)^{th} or (Δ*σ*^{max})^{th}] varies directly with strength level (*σ*^{ys} or *σ*^{ts}). 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.

**Keywords:**

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

**Author Information:**

Novak, SR *Senior Research Engineer, U.S. Steel Corporation Research Laboratory, Monroeville, Pa.*

**Paper ID:** STP44805S

**Committee/Subcommittee:** E08.05

**DOI:** 10.1520/STP44805S