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 ratcheting behaviour and its effect on strain-controlled low cycle fatigue (LCF) behaviour 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 LCF studies were done on the annealed specimens. 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. Post-ratcheting LCF studies indicated increased stress amplitudes of the ratcheted specimens as those compared to only LCF specimens; this can be attributed to the previous cyclic hardening during ratcheting tests.