Date of Award
Fall 2025
Project Type
Dissertation
Program or Major
Mechanical Engineering
Degree Name
Doctor of Philosophy
First Advisor
Marko Knezevic
Second Advisor
Mark Lyon
Third Advisor
Jinjin Ha
Abstract
This dissertation presents a comprehensive experimental investigation of the mechanical behavior of an high-strength low-alloy (HSLA) steel fabricated via laser powder bed fusion (LPBF). The work focuses on fatigue strength, laser shock peening (LSP) effects, creep behavior, fatigue crack growth and fracture toughness. Fatigue strength was assessed using rotating bending tests, showing superior performance of LPBF specimens compared to as-cast materials due to the dual-variant martensitic microstructures. Laser shock peening enhanced fatigue life by inducing compressive residual stresses, delaying crack initiation and slowing crack propagation. Creep behavior was evaluated for different layer thicknesses, with thinner layers improving secondary creep resistance while thicker layers promoted better ductility in tertiary creep. Fatigue crack growth tests revealed different crack paths influenced by the dual martensitic microstructure, with tempered martensite regions aiding crack deflection and retardation. Fracture toughness and near-threshold fatigue crack growth tests showed that crack closure effects diminish at higher stress ratios, particularly in untempered condition. This dissertation advances the understanding of dual-variant martensitic microstructures in LPBF UHSLA steels, providing critical insights into their damage tolerance and durability for potential structural applications.
Recommended Citation
Kljestan, Nemanja, "Multi-level characterization and testing of fatigue, fracture and creep of the ultra-high strength low-alloy steel fabricated via laser powder bed fusion" (2025). Doctoral Dissertations. 2923.
https://scholars.unh.edu/dissertation/2923