Dwell fatigue fracture in Ti microstructures through crystal plasticity and phase field fracture frameworks O. Bulut∗, C. Erdogan, E. Günay, T. Yalçinkaya Department of Aerospace Engineering, Middle East Technical University, Ankara 06800, Türkiye ∗ orhun.bulut@metu.edu.tr Keywords: Dwell fatigue, Crystal Plasticity, Phase field fracture Titanium alloys, with their superior strength-to-weight ratios, corrosion resistance, and fatigue performance, are integral to aerospace applications, notably in aero-engines. However, their vulnerability to dwell fatigue significantly reduces their operational life which requires an in-depth analysis of micro-mechanisms [1,2]. These alloys largely consist of alpha and beta phases, with the latter becoming dominant at high temperatures. The pronounced plastic anisotropy of the HCP-structured alpha phase underscores the importance of studying cold dwell loadings [3]. This research delves into the "rogue" grain combination concept, a potential major factor behind the early failure of Ti alloys. This unique combination comprises a c-axis oriented hard grain juxtaposed with soft grains leading to evolution of local high stresses which can be analyzed through an anisotropic plasticity model at the grain scale. For this purpose, a rate-dependent crystal plasticity model is employed and coupled with a phase field fracture framework for the prediction of crack nucleation and subsequent propagation. Three-dimensional polycrystalline representative volume elements are generated and loaded under both quasi-static and dwell fatigue conditions. Additionally, a lower order strain gradient crystal plasticity model is employed enabling modeling of size effects. With this model, a failure criterion based on GND densities and stresses is introduced through the phase field. References [1] Zhang, Z., Cuddihy, M. A., & Dunne, F. P. E. (2015). On rate-dependent polycrystal deformation: the temperature sensitivity of cold dwell fatigue. Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences, 471(2181), 20150214. [2] Ozturk, D., Kotha, S., Pilchak, A. L., & Ghosh, S. (2019). Two-way multi-scaling for predicting fatigue crack nucleation in titanium alloys using parametrically homogenized constitutive models. Journal of the Mechanics and Physics of Solids, 128, 181-207. [3] Dunne, F. P. E., D. Rugg, and A. Walker. "Lengthscale-dependent, elastically anisotropic, physically-based hcp crystal plasticity: Application to cold-dwell fatigue in Ti alloys." International Journal of Plasticity 23.6 (2007): 1061-1083. 108
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