Comparison of Multiaxial Fatigue Criteria to Evaluate the Life of Crankshafts

Abstract

Crankshafts are subject to multiaxial fatigue because of the complex stress distribution caused by the loads when this engine component is in service. So far there is no universally accepted approach to multiaxial fatigue; each theory is used for a determined application. Among the many high-cicle fatigue criteria, two have been largely accepted to be used with mechanical components, the one based on the critical plane and the one based on the stress invariant. This work presents the comparison of the safety factors calculated with those criteria for a conventional crankshaft. As critical plane criteria we chose those known as Matake, McDiarmid, and Dang Van, each one with a different approach. Same way we chose the Sines, Crossland, and Kakuna-kawada criterias as the stress invariant approaches. First, the work describes the basic concepts and the fatigue criteria listed. Following that the loads over the crankshaft are estimated from the loads flowing through the conrods and from the dynamic of the movement for critical points of the crankshaft. The third step was to apply the criteria to evaluate the safety when the component is working at 1700, 2100, and 3050rpm, which are the speeds where the engine is subjected to the maximum torque, maximum power, and at the maximum speed conditions. The results showed that the engine analyzed did not fail, that the critical plane criteria is more conservative, and that the safety factor is not smaller than 1.77.