Effects of Mission Overloads on Fatigue Crack Growth in Ti-6Al-2Sn-4Zr-2Mo

Date of Award

2018

Degree Name

M.S. in Materials Engineering

Department

Department of Chemical, Materials and Bioengineering

Abstract

Aircraft turbine engines, especially military engines, experience variable amplitude loading (mission loading) during operation. Predicting the impact of overloads in turbine engines is key in interpreting fatigue damage and assessing the reliable lifetime of components. The objective of this study was to understand the effects of single and repeated overloads during fatigue crack growth in Ti-6Al-2Sn-4Zr-2Mo used in aircraft turbine engine rotor components.Experiments were conducted using compact tension specimens in a servo-hydraulic testing machine to measure the fatigue crack growth rates during the application of single overloads under stress intensity factor and load control. Additional experiments were conducted having, variable amplitude loading consisting of a controlled number of constant amplitude baseline cycles between periodic overloads.Single overload experiments revealed crack growth acceleration and not the classic retardation typically expected. Repeated overloads experiments demonstrated that Miner's rule accurately predicted realistic overload behavior. Overall, the crack growth rates during single overload or repeated overloads resulted in consistent behavior, and significant crack growth retardation was not observed throughout testing in this material. In addition, crack growth rates were similar for overload and underload block fatigue conditions. The understanding of this behavior and the impact on aircraft turbine engine life tracking using Total Accumulated Cycles (TACs) was discussed. It appeared that minor cycles were generally more damaging than currently accounted for in military turbine engine life tracking.

Keywords

Materials Science, Aerospace Materials, Engineering, Fatigue Crack Growth, Variable Amplitude Loading, Mission Loading, Ti-6242, Compact Tension

Rights Statement

Copyright © 2018, author

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