Dwell fatigue life assessment of dual-phase titanium alloy using polycrystalline plasticity

Date

2025-8-20

Author

Açar, Sadık Sefa

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In this thesis, the mechanical response of dual-phase Ti-6Al-4V microstructures is examined under various loading conditions, employing a rate-dependent local crystal plasticity model. In cold sections of gas turbine engines, titanium alloys are subjected to cyclic mechanical loading. These materials usually spend most of their service life under dwell-hold condition, which may lead to significantly reduced fatigue life. Termed dwell sensitivity, this fatigue life reduction is mainly controlled by microstructural features, including phase volume fractions, morphology, and colony structure. To capture dwell effects, representative volume elements (RVEs) with systematically varied β phase volume fractions (VFs) and lamella thickness ratios (LTRs) are generated. Simulations are carried out with a phenomenological power-law constitutive model and a spectral solver in the DAMASK framework. Monotonic strain-controlled simulations are conducted, and effects of β phase VF and colony morphology on the macroscopic response and local stress concentrations are discussed. Stress-controlled loading simulations are carried out under conventional cyclic and dwell cyclic loading conditions. Macroscopic strain accumulation, evolutions of hysteresis loops, slip system activity, and time-dependent and spatial variation of field variables are critically evaluated. A two-step life estimation method involving the Basquin-Coffin-Manson formula and the Palmgren-Miner damage rule is used for conventional and dwell fatigue life calculations, exhibiting strong dwell sensitivity at high loading amplitude for microstructures having low β phase content. This study presents a comprehensive framework to predict the dwell fatigue life of Ti-6Al-4V microstructures, thereby contributing to the development of microstructure-based structural assessment methods for gas turbine engine components.

Subject Keywords

Dwell fatigue, Crystal plasticity, Microstructure, Dwell sensitivity, Fatigue life estimation

URI

https://hdl.handle.net/11511/115568

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Graduate School of Natural and Applied Sciences, Thesis