Influence of microstructure in rolling contact fatigue of bearing steels with inclusions
Alley, Erick Shaw
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The use of bearings can be found in virtually all aspects of mechanical systems today. Reliability of these critical components is an important issue. Fatigue performance of bearings is a function of many factors, including service conditions, loading, material properties, environmental factors, and manufacturing processes. Crack nucleation, first spall generation and spall growth in rolling contact fatigue are known to be highly sensitive to the heterogeneity of the microstructure. Yet the current state-of-the-art in the design of high performance bearing materials and microstructures is highly empirical requiring substantial lengthy experimental testing to validate the reliability and performance of these new materials and processes. The approach presented here is designed to determine relative rolling contact fatigue performance as a function of microstructural attributes. A fully three-dimensional finite element modeling allows for end effects to be captured that were not previously possible with two-dimensional plane-strain models, providing for a more realistic assessment of inclusion morphology and arbitrary orientations. The scaling of the finite element models has been optimized to capture the cyclic microplasticity around a modeled inclusion accurately and efficiently. To achieve this, two scales of geometric models were developed to incorporate different sized microstructural phenomena, with both models using traction boundary conditions derived from Hertzian contact stresses. A microstructure-sensitive material model adds additional capability. A hybrid model that includes both martensite and austenite phases with additional internal state variable to track the volume fraction of retained austenite due to stress-assisted transformation were developed. This represents an advance over previous models where transform plasticity and crystal plasticity were not simultaneously accounted for in a homogenized element containing both phases. Important links between microstructural features and fatigue indicator parameters (and relative fatigue performance) were determined. Demonstration cases show the relationship between inclusion orientation and relative fatigue performance, allowing for the identification of critical angles which maximize fatigue and reduce performance. An additional case study showed that increasing initial volume fraction of retained austenite reduces relative fatigue life. The tools developed allow for investigations of the influence of many microstructural aspects on relative fatigue performance with a numerical model that were not previously possible.