| Fretting fatigue damage has been known to be the origin of premature failure in some of the aerospace engine components. The blade/disk assemblies, for example have been particularly susceptible to fretting induced failure. Several nondestructive evaluation techniques are being used to detect the cracks due to fretting fatigue damage. Although partial success has been achieved in detection of cracks, research is lacking in the area of detection of precursors to the development of cracks due fretting fatigue damage. The goal of the research presented in this thesis is to develop a methodology based on x-ray diffraction residual stress measurements for quantitative nondestructive characterization of accumulated fretting fatigue damage.; To achieve the goal a systematic experimental study of the characteristics of the residual stress due to surface treatments of shot peening (SP), Laser Shock Peening (LSP) and Low Plasticity Burnishing (LPB), used in the aerospace industry was conducted. The residual stress in LSP and LPB was found to be complex involving shear stress and spatial non-uniformity. On the other hand in shot peening it was found to be least complex. More over it is the most cost effective and hence often used surface treatment in the industry.; In order to gain an understanding of the effect of shot peening parameters on the fretting fatigue life, experiments were conducted on samples with four different peening intensities (0, 4, 7 and 10 A) and two surface coverage (100% and 400%). It was observed that the fretting fatigue life increases with the increasing peening intensity, and increase in surface coverage beyond 100% has virtually no effect. Scanning Electron Microscopic (SEM) observation of fractured surface was utilized to identify crack initiation.; On all of the fretting fatigued specimens relaxation of residual stress was observed and it increased with increasing number of cycles. A complete relaxation was observed before failure. To obtain an understanding of the mechanisms leading to the residual stress relaxation samples with varying number of fretting fatigue cycles were prepared and the microstructure of the damaged region was examined using SEM and White Light Intereference Microscopy (WLIM). The SEM images show formation of platelet like structures due to delamination at early stages. On further continuation of fretting the delaminated regions act as stress concentrators and generate micro-cracks. As the fretting fatigue damage is increased the micro-cracks appear to coalesce together to create a large crack to drive the sample to failure. |
