## Dissertations

2021

Dissertation

#### Degree Name

Doctor of Philosophy (PhD)

#### Department

Mechanical and Aerospace Engineering

Kavan Hazeli

Jason Mayeur

George Nelson

Mark Lin

Garrett Pataky

Cyril Williams

#### Committee Member

Evolution of mechanical properties due to fatigue loading is often overlooked in the design process. Material properties in the as-received condition from a vendor are characterized and then these properties become the basis of design considerations. The effect of expected fatigue loading during operation and its potential to degrade mechanical performance is frequently accounted for with appropriate safety factors. Most notably, the aerospace industry does not have the luxury of large safety factors due to the detrimental effect of weight on cost and performance. Therefore, it becomes necessary to quantify not only the change in mechanical properties as a function of its fatigue life, but also understand what microstructural mechanisms are responsible for any property changes in order to optimize component design. Research into material evolution at both the macro and micro length scale has become even more relevant in the past few years with the advent of reusable launch systems. This research investigates the microstructural evolution during elastic fatigue loading and the mechanisms driving subsequent mechanical property change. Two different material systems are analyzed, pure $\alpha$-iron and 7075-T6 aluminum alloy. A unique experimental approach was implemented in which multiple sub-tensile specimens are obtained from the interior of a fatigue specimen. Pre-fatigued sub-tensile specimens are tested under quasi-static and dynamic strain rates. Subsurface material for microscopy characterization is also obtained from the fatigue specimens. It was found that the evolution of mechanical properties is dependent on both the subsequent loading strain rate and the amount of dislocation reversibility (controlled by the fatigue stress ratio) in each fatigue loading cycle. In-depth microscopy characterization revealed that microscopic fatigue-induced voids are the primary factor in subsequent strength degradation. Results from the research advance our current understanding of microstructural defect accumulation during fatigue loading and their effect on subsequent mechanical properties. Additionally, the dependence of post-fatigue material behavior on both strain rate and dislocation reversibility highlight the need to investigate additional materials and different fatigue conditions. Lastly, several analysis techniques utilizing statistical analysis and machine learning have been applied to microscopy datasets in order to identify underlying microscopic mechanisms that contribute to changes in subsequent mechanical response.