THE SCHOOL OF MATERIALS SCIENCE AND ENGINEERING

GEORGIA INSTITUTE OF TECHNOLOGY

Under the provisions of the regulations for the degree

DOCTOR OF PHILOSOPHY

on Tuesday January 21st, 2020

2:00 PM
in MRDC 3515

will be held the

DISSERTATION PROPOSAL DEFENSE

for

Jonathan Leung

"Mechanisms of White Etching Matter Formation Under Rolling Contact Loading"

Committee Members:

Prof. Richard Neu, Advisor, MSE/ME

Prof. Arun Gokhale, MSE

Prof. David McDowell, MSE/ ME

Prof. Shreyes Melkote, ME

Prof. Jeffrey Streator, ME

Abstract:

Bearing components in wind turbines, particularly those in the gearbox, operate under demanding conditions related to large applied dynamic loads, complex loading histories, several environmental conditions, and thin lubrication films. The failure of these bearings has been attributed to rolling contact fatigue (RCF), which involves the gradual degradation of a material microstructure and crack formation and growth under repeated rolling contact loading. With the desire to increase the reliability of wind turbines for a long-term green energy source, the understanding and solutions to this problem needs to be addressed.

On inspection of failed bearings, extensive crack networks flanked by microstructural transformed regions known as white etching matter (WEM), have been found throughout the subsurface region. Due to the prevalence of WEM, it is hypothesized that bearing failure are related to WEM at subsurface cracks. Despite the extensive studies on WEM, the damage chronology and formation mechanisms for WEM is unclear, hindering the development of WEM resistant alloys. One current hypothesis for WEM formation is associated with the breakdown of the microstructure through localized rubbing and beating of preexisting defects such as inclusions and weak interfaces.

This project seeks to substantiate the rubbing and beating of subsurface interfaces hypothesis as the primary mechanism for WEM formation in bearing steels. This project will build a computational framework to systematically explore the microstructural and operating conditions to gain insights on the drivers for WEM formation. An ABAQUS based finite element (FE) framework has been developed to systematically explore the influence of friction, localized debonding and cyclic rolling contact loading conditions on the damage formation at subsurface inhomogeneities and subsurface cracks. A fretting damage parameter (FDP) will be selected and validated to quantify the magnitude of the rubbing and beating of subsurface interfaces and assess critical conditions that are most likely to form WEM.

Clarifying the mechanism for WEM formation provides insights and strategies to create RCF resistant bearing steels. By understanding how subsurface crack rubbing influence WEM formation, mitigation strategies that reduce crack formation and predict the WEM transformation rate can be used to improve bearing alloy design and extend bearing life.