Assessment of Fatigue Life of 3D-Printed Metal Components Under Cyclic Loading

Thesis Abstract

The rapid advancement of additive manufacturing technology has revolutionized the production of complex metallic components, offering unparalleled benefits such as design flexibility, weight reduction, and rapid prototyping. However, the inherent microstructural heterogeneity and the unique layer-by-layer fabrication process of 3D-printed metals introduce challenges concerning their mechanical reliability, particularly under cyclic loading conditions that simulate operational service environments. Despite extensive research into static mechanical properties, there remains a significant knowledge gap regarding the fatigue behavior and life prediction of 3D-printed metallic components, which limits their widespread industrial adoption in safety-critical applications. The primary aim of this study is to comprehensively assess the fatigue life of 3D-printed metal components, focusing specifically on samples produced via selective laser melting (SLM) of stainless steel 316L. The specific objectives include (1) characterizing the microstructural features and porosity levels of the printed samples, (2) experimentally determining the fatigue life under various cyclic stress amplitudes, (3) investigating the influence of process parameters such as laser power and scan speed on fatigue performance, and (4) developing predictive fatigue life models grounded in fracture mechanics and material behavior theories. The study also aims to compare the fatigue responses of additively manufactured samples with traditionally manufactured counterparts to evaluate the effects of layer fusion and residual stresses. A mixed-method research design is employed, combining quantitative experimental testing with qualitative microstructural analysis. A total of 120 specimens are fabricated using a standardized SLM process, with process parameters systematically varied to investigate their effects. The sample size allows for adequate statistical power to detect significant differences in fatigue life across treatment groups. Fatigue testing is conducted under controlled laboratory conditions utilizing a servo-hydraulic testing machine, adhering to ASTM E466 standards, at stress amplitudes ranging from 50 MPa to 200 MPa, with a fatigue life measurement up to 10^7 cycles. Microstructural characterization employs optical microscopy, scanning electron microscopy (SEM), and X-ray computed tomography (XCT) to assess porosity and layer interface integrity. Data analysis incorporates analysis of variance (ANOVA) to evaluate the significance of process parameters on fatigue life, complemented by regression modeling to develop fatigue life prediction equations. Fracture surfaces and crack initiation sites are examined through SEM to elucidate failure mechanisms, with results contextualized within the frameworks of Griffith fracture theory and the Coffin-Manson fatigue model. The study also applies the Miner's rule for cumulative damage assessment under complex loading cycles. Expected findings include a quantification of the relationship between process parameter variations, microstructural defects, and fatigue life, revealing that higher porosity levels significantly reduce fatigue durability. The research anticipates establishing a predictive fatigue life model that incorporates porosity fraction, residual stress levels, and surface roughness, offering a practical tool for quality assurance and engineering design. Additionally, the comparison with traditional manufacturing methods is expected to confirm the superior or comparable fatigue performance of optimized 3D-printed components when processed under controlled parameters. This research makes a substantial contribution to the existing body of knowledge by bridging the gap between additive manufacturing process optimization and mechanical performance in cyclic loading scenarios. It provides evidence-based insights to inform engineering standards and best practices for the safe deployment of 3D-printed metals in structural and functional applications. The study concludes with recommendations emphasizing process qualification, nondestructive evaluation techniques, and post-processing methods such as heat treatment and surface finishing to enhance fatigue life. Future research directions include long-term service testing and the development of nondestructive monitoring techniques for real-time fatigue assessment of additively manufactured components.