Microstructure Stress State Evaluation of Contact Fatigue in Gears

Surfaces subjected to rolling and sliding may suffer from contact fatigue such as micropitting which is a common failure mechanism in gears. In order to protect materials from surface contact fatigue and enhance their lifetime, fracture mechanics investigations have a vital role. However, when fracture mechanics is used the microstructural details are usually simplified in order to carry out computer simulations. In this study the influence of microstructure of fatigued gears on the stress distribution is investigated using the image-based Object Oriented Finite Element code (OOF2). OOF2 has the ability to simulate the realistic microstructure by using micrographs obtained by scanning electron microscopy (SEM) or maps from electron backscatter diffraction (EBSD). Fatigue tests were carried out on gears and samples taken in the cross section of the gear tooth as well samples from untested gears were prepared for microscopy and SEM images and EBSD maps were acquired. The acquired maps are used in numerical simulations to study the effect of microstructure on stress distribution. The results show that the decay of martensite strongly affects the stress state in micropitted gears.