Additive manufacturing (AM) is intended to be extensively employed in the aerospace and biomedical sectors. However, the fatigue performance of materials such as Wire Arc Additive Manufactured (WAAM) Ti-6Al-4V continues to present challenges due to microstructural variability. Hence, we propose a computational framework that integrates Crystal Plasticity Finite Element (CP-FE) [1] modelling with phase field methods [2] to simulate the initiation and propagation of fatigue cracks in WAAM Ti-6Al-4V, with the microstructure serving as the primary input. The proposed CP-FE model effectively captures the influence of microstructural heterogeneities, while the phase field model allows for explicit representation of crack evolution under cyclic loading. WAAM Ti-6Al-4V specimens were characterised using Electron Backscatter Diffraction (EBSD) to construct Representative Volume Elements (RVEs), which were based on statistical distributions of microstructural features. This study presents a systematic approach for integrating CP-FE and phase field modelling, contributing to the reliability assessment and certification of AM components. The results from the micro-mechanical simulations offer valuable insights into the impact of microstructure on the fatigue behaviour and damage mechanisms of WAAM Ti-6Al-4V. REFERENCES [1] F. Azhari, C. Wallbrink, Z. Sterjovski, B.R. Crawford, A. Menzel, D. Agius, C.H. Wang, G. Schaffer, Predicting the complete tensile properties of additively manufactured Ti-6Al-4V by integrating three-dimensional microstructure statistics with a crystal plasticity model, International Journal of Plasticity 148 (2022) 103127. [2] E.G. Kakouris, S.P. Triantafyllou, Phase-field material point method for brittle fracture, International Journal for Numerical Methods in Engineering 112(12) (2017) 1750-1776.