Mechanical Conditions Dominating Cyclic Stress Corrosion Cracks of High-Strength Steel under Combined Cyclic/Static Multiaxial Loads

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Cyclic stress corrosion cracking (SCC) tests of smooth specimens under combined cyclic/static tensile and torsional loads were conducted on a 4135 high-strength steel in 3.5% NaCl solution at 298 K. The stress waveform was triangular at a stress cycle frequency of 0.1 Hz. Two series of loading patterns were employed; multiaxial cyclic load tests under in-phase cyclic tension and torsion (tensile stress ratio, Rσ, of zero, and torsional stress ratio, Rτ, of zero), and multiaxial cyclic-static tests in which one of the two applied loads was cyclic and the other static, i.e., (Rσ=0, Rτ=1) or (Rτ=1, Rτ=1). The combined stress ratios of τmax/σ<max> were selected as 0, 1/2, 1, 2, and infinity. In the case of multiaxial cyclic load tests, a cyclic SC crack normal to the maximum principal stress was initiated at the bottom of the corrosion pit. In this case, the life of cyclic SCC was dominated by the maximum principal stress, σ1, max, which equaled the maximum principal stress range, 1, max. In the case of multiaxial cyclic-static loads a cyclic SC crack normal to the maximum tensile stress range was initiated at the bottom of the corrosion pit, provided that the static tensile load in the plane of the maximum tensile stress range was relatively small. IN this case, the life of cyclic SCC was dominated by the maximum tensile stress range, n, max. In either case, the mechanical condition dominating crack initiation at the bottom of the corrosion pit was determined by the stress intensity factor range, K1FSCC, calculated on the assumption that the spherical corrosion pit was a sharp surface crack.