PSI - Issue 39

Keke Tang et al. / Procedia Structural Integrity 39 (2022) 387–392 Author name / Structural Integrity Procedia 00 (2021) 000–000

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1. Introduction Titanium alloy Ti-6Al-4V is widely applied in the field of aerospace engineering due to its excellent strength and fatigue resistance. Inclusions or microscopic defects existing in alloys can be approximately treated as geometrical microdefects or notches in CPFE modelling. Under the external fatigue loading, crack is prone to initiate from the neighboring area of microdefects. Further crack propagation leads to the decrease of fatigue life. It is of great significance to investigate fatigue performance of dual-phase titanium alloys within the framework of CPFE modelling. According to the composition of lamellar structure, dual-phase titanium alloys are mainly categorized as equiaxed structure, bimodal structure and full lamellar structure such as Ti-5553 alloy. It is found by Wu et al (2020), Ren et al (2021), Zeng et al (2018) that, with the increase of lamellar structure composition, fatigue resistance of dual-phase titanium alloys is appreciably improved. Micro-notch parameter is introduced by Owolabi et al. (2016) to obtain the quantitative relationship between notch size and fatigue failure. By considering the influence of geometric microstructure, numerical statistical methods are adopted by Guerchais et al. (2015; 2017) to study the evolution around notch region. Experiments and numerical simulations with different notch shapes and sizes are carried out for 316L stainless steel. It shall be pointed out that above research is mostly focused on single-phase alloy or equiaxed alloys. Work on dual-phase alloys with lamellar structure is rather surprising. It is noteworthy some work is experimentally carried out by Yuan et al. (2020), or numerical dual-phase model is established by Asim et al. (2019), with emphasis laid on crystal phase and orientation. Nevertheless, CPFEM modelling on lamellar structure effect in bimodal titanium alloys are rarely found. In this regard, the present work is focused on the effect of lamellar structure distribution around geometrical microdefects such as elliptical notches in RVE model of Ti-6Al-4V. Under external loading, strain accumulation around elliptical notch is particularly studied for fatigue resistance performance. 2. RVE model of bimodal titanium alloy In this section, a crystal plasticity RVE model is established. Notice the crystal plasticity constitutive model adopted here is formulated by Huang (1991). The constitutive model has been proved to be effective in CPFE metal alloys. The bimodal parameters are quoted from published literatures by Han (2016) and Kapoor et al. (2021). The 2D RVE model is randomly generated by Voronoi tessellation MATLAB, with a centred elliptical notch. Grain homogenization is subsequently performed. Dimension of the RVE model is 200 μm *200 μm, and it contains 225 grains and the average grain size is 15 μm after grain homogenization. For the sake of simplification, the parameters of primary α phase and secondary α are considered to be the same. Composition of primary α and ( α+β) phase respectively are 66% and 34%. T he width of β phase and secondary α phase in the lamellar structure are 0.5 μm and 2 μm. There are 24 slip systems considered in α phase, 12 possible slip systems in β phase, and the crystal plasticity parameters are tabulated in Tab.1. Uniaxial cyclic loading is applied to the top edge. Fixed boundary condition is imposed along the bottom edge, see Fig.1.

Load

h= 200μ m

Fig.1 Bimodal RVE model and boundary condition ( max F =800MPa, R=0)