A microstructure-based plasticity-damage model is used to predict the mechanical behavior of commercially available AISI 4140 steel. Monotonic tension, compression and torsion tests were performed to obtain the set of plasticity and damage constants required for model calibration. Then, tension tests on Bridgman notched specimens were undertaken to study the damage-triaxiality dependence. Three different notch radii generated different levels of triaxiality at the notch. The modeled triaxiality-damage correlation was validated with SEM fracture surface analysis. Stress-strain correlations under different strain rate and temperature testing conditions were also studied. Little influence of the strain rate was observed. A preliminary study in high-porosity LENS materials was later performed, with satisfactory stress-strain correlation at two different temperatures on tension tests. Finally, a multistage fatigue model was used to predict life in AISI 4140 steel. The goal was to create a baseline for future application of these mathematical models into LENS manufactured materials in component design