Shot peening is a dynamic cold working process involving the impingement of peening media onto a substrate surface. Shot peening is commonly employed as a surface treatment technique within the aerospace industry during manufacturing, in order to improve fatigue performance of structural components. The compressive residual stress induced during shot peening is understood to result in fatigue crack growth retardation, improving the performance of shot peened components. However, shot peening is a compromise between the benefit of inducing a compressive residual stress and causing detrimental surface damage. Due to the relatively soft nature of AA7050-T7451, shot peening can result in cracking of the constituent particles, the effect of which is recognized as a ‘critically detrimental influence upon the component’s fatigue performance.’ The intention of this thesis is to understand the balance and fundamentals of these competing phenomena, through analysis involving a comparative study throughout the fatigue life cycle of ‘as manufactured’ versus shot peened AA7050-T7451. A series of dog bone samples were manufactured, with a subset undergoing shot peening surface treatment. Microstructural grain characterization and comparison of ‘as manufactured’ and shot peened AA7050-T7451 has been carried out using scanning electron microscope (SEM), electron back scatter diffraction (EBSD), and energy dispersive spectroscopy (EDS) techniques. A residual stress analysis through interrupted fatigue of ‘as manufactured’ and shot peened AA7050-T7451 was completed utilizing a combination of x-ray diffraction (XRD) and nano-indentation. The fatigue life cycle performance of the ‘as manufactured’ versus shot peened material has been evaluated, including qualitative analysis and comparison of crack initiation and propagation in ‘as manufactured’ and shot peened material. Through this experimentation and analysis, this thesis endeavors to answer the question of what is the mechanism for shot peening enhancement for fatigue performance. An objective of this work is to understand how a cracked particle starts to incubate the short crack into the matrix within a residual stress field
