Titanium alloys are often used in aerospace applications in which they are subjected to cyclic loading. It has been found that the addition of a hold at peak load greatly decreases the fatigue life of these alloys. This is referred to as "cold-dwell fatigue". In order to improve component life prediction, it is desirable to understand the mechanisms leading to initiation of fatigue cracks. Due to the high anisotropy of a-titanium, it is expected that deformation on a microstructural scale will he significantly heterogeneous. To understand the process of crack nucleation, the mecha- nisms of this deformation must first he understood. This thesis will present a series of experiments which will attempt to give some insight into this process. Two main lines of inquiry have heen undertaken. The first method uses electron hackscat- tered diffraction (EBSD) to obtain the crystal orientations within a selected region. Optical mi- croscopy and digital image correlation methods were then used to track the accumulation of strain in the region during deformation. Results were obtained for monotonic tension, cyclic fatigue, and cold-dwell fatigue. Behaviour in local strain peaks differed hetween loading modes, hut strain concentrations in the range of 2-4 times mean strain were typical. The second method uses the cross-correlation based EBSD technique developed by Wilkinson et al [1] to derive the dislocation densities present in a crystal. By tracking the changes in dislocation density after different deformation regimes, some insight can be gained into the local deformation processes that produced the dislocation densities. Strong links were found between the local crystallography and inhomogeneities in strain and dislocation density accumulation. In particular, interactions between grains of differing orientations was found to have a strong impact on deformation behaviour. Additionally, a link was found between high strain accumulation and crack initiation in both fatigue and dwell fatigue.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:556214 |
| Date | January 2011 |
| Creators | Littlewood, Philip Dilan |
| Publisher | University of Oxford |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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