Fatigue crack initiation processes in mild steel

Abstract

The S/N curves obtained for plain testpieces of 0.08%C mild steel subjected to axial loadings of the R = 0 and R = −1 types are compared. The curves are divided into three regimes of fatigue delineated by discontinuities when cycled at 43 Hz. The mechanisms of crack initiation at the electropolished surfaces of the steel were investigated with the optical microscope. Hardness surveys were used to establish the amount and distribution of strain hardening for fatigue cycling within the various regimes of the S/N curve. Changes in the macro-yield stress of the steel during cyclic loading were also measured. The hardness surveys, within the comparable fatigue regimes for the R = 0 and R = −1 loadings, revealed identical hardness patterns despite the large differences in the peak stress levels attained. The nominal yield stress was raised by cyclic loading up to the fatigue limit level for the R = 0 tests and lowered to the fatigue limit in the R = −1 tests, which was ∼ 40% below the macro-yield stress of the steel. The importance of premature yielding at the surface grains, and the associated cyclic strain hardening in fatigue, are discussed.