The research objective is to determine the monotonic and cyclic stress-strain properties of plastically graded surfaces (PGSs) relevant to rolling contact fatigue (RCF) in high-performance ball bearings. During RCF, the surface layer experiences cyclic micro plasticity, buildup of residual stress, and contact fatigue damage, eventually leading to fatigue spallation. We propose a coordinated experimental and modeling approach that judiciously combines micro- and macro-indentation investigations with 3D elastic-plastic finite element analysis to determine the constitutive response of PGSs. By performing controlled RCF testing under realistic conditions and probing the localized plastic zones, we extract the cyclic constitutive response as a function of RCF cycles, thereby providing a means for quantitative evaluation of localized material damage evolution. The above approach is applicable to any engineering material irrespective of the nature of gradients in the plastic properties.
The challenging requirements of advanced military and space propulsion systems have led to the development of hybrid bearings with plastically graded case carburized surfaces. However, reliability of bearing systems continues to be a major concern because the current methods used in bearing fatigue life estimation are largely based on probabilistic approaches that are empirical in nature, and do not directly consider the constitutive behavior of materials under RCF loading. The proposed approach allows for development of robust life-estimation methodology by bringing together many cross-disciplinary ideas from manufacturing, materials science, tribology, fatigue, solid mechanics, and experimental mechanics. This approach will facilitate surface engineering of graded layers as well as development of a quantitative description of RCF life in rolling element bearings leading to improved performance, durability and reliability. The project will also contribute to the education and training of the manufacturing research workforce in the US and lead to increased participation by under-represented groups.
Rolling-element bearings are key precision components used in nearly all machinery. The annual revenue associated with bearings is a substantial $50.5 billion worldwide. Future demands for high performance rotor support in advanced turbine engines for commercial and military transportation, wind turbines and high-speed rails require bearings to survive hundreds of billions of revolutions under severe operating conditions. A new generation of ultra high strength case-hardened bearing steels have been designed to meet the challenges. However, the existing RCF life prediction methodology, relying on empirical Lundberg-Palmgren (LP) life prediction models dating back to the 1940’s, results in large discrepancy between observed and predicted life, for the new generation of materials with complex graded microstructures. This project with GOALI partner Pratt & Whitney has developed novel physics-based bearing steel material fatigue damage models based on fundamental understanding of relationships between microstructure and performance of case hardened steels subject to rolling contact fatigue (RCF). Results from this project will help towards design of reliable and durable aerospace bearings used in aircraft engines, high speed rails and space technology. Intellectual merit: Development of a general approach to characterize depth dependent stress-strain response of the material with yield stress gradient using experiments and reduced-order modeling to include influence of carbides. Results generated represent the very first time relevant constitutive properties have been evaluated for through and case hardened bearing steels. Broader impact lies in immediate relevance of results towards developing a deterministic life prediction approach and surface engineering of graded bearing steels.
