Mechanism of the fatigue life increment due to dynamic strain ageing during hold time

The effect of hold time on the low-cycle fatigue behaviour of AlSl 4140 steel at 303 to 473 K was investigated in air and argon gas. All tests were conducted under a constant total strain amplitude of 0.5, 0.65, 1.0, 1.3 and 1.6×10^–2 with zero mean level. Experimental results show that dynamic strain ageing is occurring during the hold time period and that piled-up dislocations near the carbides are locked by the Cottrell atmosphere. Formation of the Cottrell atmosphere around dislocations near the carbides induces the redistribution of carbon or carbides in the matrix. These randomly dispersed grown up carbides behave like the increased number of dislocation pile-up sites from which fatigue cracks may initiate. Therefore, as the hold time is applied in fatigue cycles under constant strain amplitude the dislocation density at each site will be lowered to retard the fatigue damage or improve the fatigue life.