Effect of prior ratcheting deformation on low cycle fatigue behaviour of AISI 4340 steel

Abstract

Fatigue damage of a material is particularly deleterious when it occurs under asymmetric cyclic loading conditions. Ratcheting is the process of fatigue damage in which plastic strain accumulates during asymmetric cyclic loading (with non-zero mean stress) of structural parts. The aim of this investigation was to study the effect of prior ratcheting deformation on hardness and tensile behavior of AISI 4340 steel in differently heat treated conditions. For this purpose, AISI 4340 steel was first given different heat treatments viz. annealing and normalizing and studied for basic microstructural characterizations and hardness. The heat treated steel rods were used to fabricate tensile and fatigue specimens according to ASTM standards. A series of stress controlled ratcheting tests were carried out on the heat treated specimens at room temperature with different stress ratios (R) of -0.4, -0.6 and -0.8. In order to assess imposed fatigue damage, post-ratcheting tensile properties and hardness of the specimens were then investigated. The results showed that accumulation of ratcheting strain increased with increasing stress ratios in both annealed and normalised conditions. Maximum accumulation of ratcheting strain (1.16% for annealed and 1.02% for normalised specimens) was observed at R = -0.4. The increase in strain accumulation with increasing stress ratio can be attributed to the increased dislocation generation and their multiplication, at higher levels of stress ratios. Also, post-ratcheting yield strength, tensile strength and hardness of investigated steel was higher as compared to unratcheted samples which can be attributed to the previous cyclic hardening during ratcheting tests.