Crack initiation, small crack growth, and stress intensity factor in the very high cycle fatigue (VHCF) of wire arc additive manufactured (WAAM) nickel aluminum bronze (NAB)

Wire arc additive manufacturing (WAAM) has emerged as a favorable method for industrial manufacturing due to its high productivity and cost-effectiveness in producing large metal components. This study employs a fracture mechanics approach to investigate the very high cycle fatigue (VHCF) behavior of WAAM nickel aluminum bronze (NAB) alloy, a material prized for its high strength and corrosion resistance, making it ideal for marine and naval applications. WAAM NAB specimens were fabricated using optimized process parameters, followed by annealing to minimize residual stresses and enhance mechanical properties. Ultrasonic fatigue testing (USF) at 20 kHz was utilized to evaluate fatigue life up to 109 cycles. Advanced fractography analysis enabled the quantification of small crack growth, facilitating fatigue life prediction through detailed assessments of stress intensity factors (SIF) at critical crack initiation sites. The study also investigates the influence of WAAM-induced volumetric defects on crack initiation and propagation in the VHCF regime. A comparison with conventional S-N data (limited to 5 × 106 cycles) revealed a distinct step in the S-N curve, indicating a transition to secondary fatigue strength when moving from the high cycle fatigue (HCF) to the VHCF domain. This research offers valuable insights into the long-term fatigue durability of WAAM NAB, reinforcing its potential for high-frequency cyclic load applications in marine and industrial environments.