| This dissertation represents the results of an investigation of fatigue in titanium alloys and intermetallics.; The results of a combined experimental and analytical study of the probabilistic nature of fatigue crack growth in Ti-6Al-4 V are presented. A simple experimental fracture mechanics framework is presented for the determination of statistical fatigue crack growth parameters from two fatigue tests.; A probabilistic framework is introduced for the modeling of fatigue damage in three gamma-based titanium aluminides: Ti-45Al-2Mn-2Nb+0.8vol.%TiB 2, Ti-47Al-2Mn-2Nb+0.8vol.%TiB2 and Ti-48Al-2Cr-2Nb. This includes empirical stress-life and fracture mechanics approaches to the estimation of material reliability or the risk of failure.; A combined experimental and analytical study of fatigue crack growth and dwell-fatigue crack growth in forged Ti-6Al-2 Sn-4Zr-2Mo-0.2%Si (Ti-6242) is presented. Following an initial characterization of microstructures and basic mechanical properties, the micro mechanisms of long fatigue crack growth are presented for three microstructures. The fatigue crack growth rates in the near-threshold, Paris and high ΔK regimes are then characterized with empirical linearized crack growth laws. Finally, the results of dwell-fatigue crack growth experiments are presented for the three microstructures. The underlying mechanisms of dwell crack growth are compared with the mechanisms of fatigue crack growth before discussing the implications of the work for the prediction of dwell or fatigue crack growth in Ti-6242. The effects of different frequencies on dwell fatigue are also explored.; The results of an experimental study of fatigue crack nucleation and growth in a Ti-6Al-4V/TiB in-situ whisker-reinforced composite are presented. The onset of crack nucleation is shown to correspond to ∼20% of the total life at a stress range of 480MPa. The long fatigue crack growth rates in the Paris regime in the Ti-6Al-4V/TiB composite are comparable to those of Ti-6Al-4 V processed under nominally identical conditions. However, the fatigue crack growth rates in the composite are faster than those in the matrix alloy at lower ΔK values. Cyclic deformation of the composite is associated with strain softening, presumably as a result of progressive interfacial decohesion around the TiB whiskers early in the fatigue deformation process. |
