Fatigue- Ratcheting Interaction Behavior of AISI 4340 Steel

Abstract

Ratcheting is known as accumulation of plastic strain during asymmetric cyclic loading of materials under non-zero mean stress. This study intends to investigate the ratcheting behavior in AISI 4340 steel which is mainly used for designing of railway wheel sets, axles, shafts, aircraft components etc. The effect of stress ratio on the ratcheting behavior in both annealed and normalised conditions, effect of previous low cycle fatigue deformation on ratcheting behavior and vice versa were investigated for investigated steel. Ratcheting tests were done at different stress ratios of -0.4, -0.6 and -0.8 and strain controlled low cycle fatigue tests were done at strain amplitudes of ±0.50 and ±0.75. The results showed that cyclic hardening takes place during ratcheting deformation while strain accumulation increases with increasing stress ratio. Prior fatigue deformation induces more ratcheting strain in the material under same magnitudes of stress ratios. Post ratcheted samples showed increase in tensile strength and hardness which increases with increasing stress ratio and these variations in tensile properties are correlated with the induced cyclic hardening. The X-ray diffraction profile analysis using the modified Williamson–Hall equation has been carried out in order to estimate the dislocation densities in the specimens subjected to both ratcheting and low cycle fatigue. Increase in strain accumulation has been explained by increase in dislocation densities in the specimens and a correlation between the strain produced by ratcheting deformation and the estimated dislocation density has been established.