<b>DEFECT INTERACTION AND MULTI-SITE CRACK PROPAGATION IN FATIGUE FAILURE OF ADDITIVELY MANUFACTURED PARTS</b>

<p dir="ltr">Fatigue-life prediction of additively manufactured metals is challenging due to the random distribution and interaction of process-induced defects, which significantly influence crack initiation and growth. Conventional fatigue-life models based on single-crack growth often overestimate fatigue life by neglecting defect interactions and multi-site crack propagation, particularly in low cycle fatigue and mid-life regimes, where crack coalescence and stress field interactions play a crucial role. Compression pre-cracking constant-amplitude and ASTM load-reduction methods were used to generate large-crack growth rate data in the near-threshold (low-rate) regime on standard compact tension, C(T), specimens. In addition, single-edge-notch-bend, SEN(B), specimens were used to generate fatigue and small-crack-growth-rate data. Uniaxial fatigue tests were also conducted on flat, K_T_{<em> </em>}= 1, dogbone specimens. The data from these tests was integrated with defect characteristics, including size and spacing obtained from X-ray computed tomography, to estimate fatigue life of Ti-6Al-4V fabricated via laser directed energy deposition. The plasticity-induced crack closure model, FASTRAN, was used to simulate crack propagation and compare single-crack and multi-crack approaches in fatigue-life prediction. The multi-site damage approach, which explicitly considers defect spatial distribution through nearest neighbor distance analysis, significantly improves fatigue-life predictions in low cycle fatigue and mid-life regimes, aligning well with experimental data.</p>