Publication Date
2016
Document Type
Thesis
Committee Members
Joy Gockel (Committee Member), Adam Pilchak (Committee Member), Raghavan Srinivasan (Advisor)
Degree Name
Master of Science in Mechanical Engineering (MSME)
Abstract
Ambient temperature dwell sensitivity is known to be deleterious to the fatigue response of near-alpha titanium alloys. Dwell fatigue refers to the presence of a sustained hold at peak stress as opposed to the continuous variation of normal cyclic fatigue loading. This reduction in failure life-times from dwell loading is attributed to early crack nucleation and faster crack propagation. The degradation is the result of plastic anisotropy on the microstructural scale along with tendency of titanium alloys to creep at low temperatures at stresses well below the 0.2% offset yield strength. Despite being the most widely used titanium alloy, Ti-6Al-4V has not been the subject of most dwell fatigue research. Generally, dwell sensitivity is microstructurally dependent and believed to only affect Ti-6Al-4V when severe crystallographic texture is present and under high peak stress loading. Recent studies, however, have suggested that small clusters of preferred crystal orientations, known as micro-textured regions (MTR), can have a significant effect on the dwell sensitivities in Ti-6Al-4V even without severe overall texture in the material. In this study, smooth-bar fatigue specimens were subjected to uniaxial fatigue at 20 Hz cyclic and 2-min dwell loading conditions under load-control at stresses representative of service conditions, until failure occurred. A reduction in specimen life-times by approximately a factor of three was observed under dwell conditions, which was less than for the highly susceptible near-a titanium alloys such as Ti-6Al-2Sn-4Zr-2Mo, where the dwell debit is often in excess of a factor of ten. Measurement of fatigue and dwell fatigue crack growth rates revealed a significant acceleration of the dwell crack growth rates in certain cases. Backscattered electron imaging and electron backscattered diffraction were utilized to quantify the interaction between the cracks and local microstructure. Though no correlation was found between crack growth rate and the local microstructure and crack trajectory, strong correlation was found between crack growth and the presence of grains with [0001] axes at small angles (
Page Count
138
Department or Program
Department of Mechanical and Materials Engineering
Year Degree Awarded
2016
Copyright
Copyright 2016, some rights reserved. My ETD may be copied and distributed only for non-commercial purposes and may not be modified. All use must give me credit as the original author.
Creative Commons License
This work is licensed under a Creative Commons Attribution-Noncommercial-No Derivative Works 3.0 License.
