PSI - Issue 45

Dylan Agius et al. / Procedia Structural Integrity 45 (2023) 4–11 Dylan Agius et al. / Structural Integrity Procedia 00 (2019) 000 – 000

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1. Introduction Fatigue crack nucleation sites in titanium alloys have been shown to exist along slip bands (Zhang, Zhang et al. 2018), therefore accurately predicting the accumulation of strain along these bands is vital in the step towards correctly predicting crack initiation. Digital image correlation (DIC) experiments have provided visual representation of the slip behaviour occurring across titanium alloys, and has advanced the current understanding on the influence of microstructural features on material properties. This includes observing how strain localisation is affected by features such as 2 content in bimodal Ti-6Al-4V (Lunt, Xu et al. 2018), volume fractions of primary and secondary in additively manufactured Ti-6Al-4V (Cao, Meng et al. 2022), and transformation phase size in bimodal near- titanium (Dichtl, Lunt et al. 2022). Due to the importance of accurately predicting strain localisation in order to investigate the influence of microstructural features on mechanical properties, the computational approach most utilised in this effort is crystal plasticity modelling (Zhang, Lunt et al. 2018, Ganesan, Yaghoobi et al. 2021, Isavand and Assempour 2021). A common issue with the crystal plasticity predictions is the inability to accurately produce the degree of heterogeneity apparent in the experiments, instead predicting homogeneous strain localisation. This was somewhat overcome through the efforts of Hardie, Thomas et al. (2022) who predicted discrete slip by imposing displacements measured by High Resolution-DIC at the edges of the region of interest (ROI). However, such an approach requires prior knowledge of the local displacements, which limits the models predictive capability. An additional important consideration when investigating strain localisation in + titanium alloys is the possible formation of regions of similarly orientated grains referred to as macrozones. Macrozones can develop during processing and have been shown to impact the local deformation of the material. This includes through the formation of intense transgranular strain banding (Lunt, Thomas et al. 2021) and stress concentrations at hard and soft macrozone boundaries (Xu, Joseph et al. 2020). In this study, slip band interactions in a sample of -annealed Ti-6Al-4V was investigated using DIC. This was undertaken to observe the occurrence of strain banding within and across the macrozones formed from to grain transformation during material processing. A representative nonlocal CP-FFT model is applied to explore the possibility of using computational approaches to investigate the influence of microstructural features on strain localisation. Through a qualitative and quantitative analysis of experimental and simulation results, it is found that the influence of the grain environment on strain localisation is being considered in the simulations, as well as predicting comparable levels of strain heterogeneity. Therefore, the use of a blended experimental and computational approach to rapidly assess material failures is one step closer to fruition. Nomenclature Total displacement gradient

Elastic part of total displacement gradient Plastic part of total displacement gradient Lattice strain Cauchy stress tensor Schmid tensor of slip system s ̇ Increment of shear strain on slip system s Resolved shear stress on slip system s Backstress on slip system s Critical resolved shear stress on slip system s Norton flow rule exponent Norton flow rule coefficient 0 Initial critical resolved shear stress on slip system s Maximum softening on slip system s 0 Parameter used to adjust the rate of slip system softening 0 Backstress parameter with dimensions of stress Backstress parameter with dimensions of length