Abstract
Gears are mission critical machine elements used for primary and accessory power transmission in many industrial, automotive and aerospace mechanical systems. In normal operation gear teeth experience torque dependent cyclic loading which can initiate and propagate fatigue cracks in the roots of their teeth, a phenomenon that is commonly called tooth bending fatigue. Most of gear applications involve repeated loads that at every cycle do not exceed the material elastic limit, achieving failure after millions of cycles (i.e., high-cycle fatigue). Consequently, most of the available literature focuses on tooth bending in high-cycle fatigue conditions. Because it represents a less common type of loading in gear applications, very little has been researched regarding tooth bending under low-cycle fatigue (i.e., repeated loading with plastic deformation at every cycle, and failure after few thousands or even hundreds of cycles). Even common ISO and ANSI standards for gear design do not cover such area. Yet, it is a phenomenon that can be encountered in high-demand industrial sectors, such as aerospace, aviation, and additionally, unanticipated overloads can arise in various other engineering contexts. As a result, the current scientific literature lacks a framework capable of describing low-cycle tooth bending fatigue in gears, especially case-carburized ones. The objective of this Ph.D. thesis is to fill this gap. A full literature review was performed to identify all available models for the description of multiaxial low-cycle fatigue. The most suitable ones for the description of low-cycle tooth bending fatigue (namely, critical plane criteria) were selected and implemented in a numerical routine which was coupled with a Finite-Element-Method simulation framework capable of replicating low-cycle tooth bending fatigue in gear teeth. The framework was enriched so that residual stresses due to case-carburizing could be taken into account, and validated through single tooth bending fatigue tests in the low-cycle fatigue regime.