The Effects of Overload on the Fatigue Life

Automotive vehicles undergo various ranges of road loads according to the driving conditions. Sometimes it experiences unusually large overload such as pot-hole impact or curb strike whose forces are several times of the vehicle weight. Those overloads may induce plastic deformations at some components and these plastic deformations reduce the fatigue life of the components. In some cases, the fatigue crack initiation points may be changed due to the residual stresses which were generated by the overloads. Predicting the fatigue life by general fatigue analysis methodology, which uses linear stress analysis results and linear damage accumulation rule, is very difficult if any component contains residual stresses. This study was performed to assess the effects of overload on the fatigue behavior of automotive suspension components and to develop a fatigue analysis methodology predicting the fatigue life under overload. Fatigue tests were performed for aluminum knuckle with the application of single overload whose magnitude is large enough to generate plastic deformations on the knuckle. The fatigue life of knuckle was reduced and crack initiation points were changed after applying single overload. Those phenomena could not be predicted by adopting linear stress analysis and Miner’s linear damage accumulation rule. By using non-linear stress analysis results and considering residual stress, it was satisfactory to predict the reduction of fatigue life and change of crack initiation points.