Influence of the transient material behaviour in the fatigue life estimation under random loading

The estimation of the fatigue life under variable amplitude loading is a fundamental step in the design phase. To consider variable amplitudes in the fatigue design the linear damage accumulation is applied, thus disregarding non linearity. When materials show cyclic hardening/softening, the hypotheses from Palmgren and Miner are not valid and this causes appreciable errors in the fatigue life estimation. Fatigue tests were carried out on the steel HC340LA. The influence of strain softening and mean stress relaxation is shown. A simplified method for considering this transient phase is presented. More accurate results can be achieved by applying this method.