Abstract: To obtain a constitutive and failure description of D406A steel under quasi-static loading, SHPB dynamic loading, and elevated-temperature conditions, quasi-static tensile tests at room and high temperatures as well as split Hopkinson pressure bar (SHPB) dynamic compression and dynamic tensile tests were conducted. In addition, tensile specimens with notch radii of 3, 6, and 9 mm were designed to obtain fracture data over different stress triaxialities. Based on a stepwise decoupled calibration strategy, a Johnson–Cook (J–C) strength–damage model for D406A steel was established and calibrated, yielding the strength parameters A = 1.344 GPa, B = 1.833 GPa, n = 0.392, C = 0.013, and m = 1.350, and the damage parameters D₁ = 0.072, D₂ = 12.161, D₃ = −8.422, D₄ = −0.022, and D₅ = 1.555. Based on the calibrated parameters, the calculated plastic-stage response shows good agreement with the corresponding experimental results, validating the applicability and stability of the identified parameters within the loading conditions considered in this study. The fracture characterization results indicate that quasi-static tension is dominated by ductile cup-and-cone fracture, with the fracture surface comprising a central fibrous region and an outer shear lip, and dimpled morphology prevailing microscopically. With increasing temperature, the fibrous region area decreases from 4.52 mm² to 1.81 mm², and the dimples evolve from coarse and sparse to fine and uniform, suggesting that elevated temperature promotes plastic flow and alters the fracture morphology. A smaller notch radius enhances constraint and stress concentration, leading to coarser and more scattered dimples and locally mixed fracture features. The calibrated strength and damage parameters can improve the predictive capability of failure simulations for D406A steel under impact and thermo-mechanical coupling conditions, providing material-parameter support for predicting engine-casing failure and fragmentation and for safety assessment under thermal-stimulus scenarios.