Fatigue behaviour of gear teeth made of case hardened steel: from competing mechanisms to lifetime variability

Abstract

Jet engines gears are generally made of steel and are case-strengthened via thermochemical treatments such as carburizing or nitriding causing microstructural modifications, superficial hardening and compressive residual stresses in the surface layers. These treatments increase the resistance to cyclic loads caused by contact between teeth and by the bending loads applied to the teeth. Both of these create high stress gradients at the surface. A large number of studies concerning the effect of thermochemical treatments on fatigue resistance have been carried out for uniaxial loads (rotating bending, tension or plane bending). Most of them were undertaken using smooth specimens, which do not correctly reproduce the stress gradient at the root of the gear teeth. As a consequence, a strong dependence between the loading mode and the position of the crack initiation site is observed. The present work aims at experimentally investigating the fatigue behavior of case hardened steel with a special focus on crack initiation and growth mechanisms. A vast experimental campaign composed of two parts is undertaken. Firstly, a Single Tooth Bending Fatigue (STBF) test is carried out on gears made of 16NiCrMo13 carburized steel. The resulting Wöhler diagram shows high scatter at certain stress levels which suggests a bi-modal behavior, characterized by very different crack initiation phases. Secondly, a fatigue tests conducted on plane bending notched specimens, designed to accurately reproduce the stress gradient observed at the gear tooth root are carried out to confirm this bimodal behavior and to characterize the failure mechanisms.