<p>As a key lightweight structural component in the aerospace field, aluminum alloy plates with nonuniform thickness are prone to fracture during bending due to their complex local geometries, which severely restricts their formability and production efficiency. Therefore, accurate prediction of fracture defects during the forming process is of great importance for the structural design and optimization of aircraft components, as well as for improving forming quality and reducing material waste. In this study, three types of tensile samples with different initial stress triaxiality levels were designed, and quasi-static uniaxial tensile tests were conducted at room temperature with strain rates ranging from 0.0001 to 0.01 s⁻¹ (0.0001, 0.0005, 0.001, and 0.01 s⁻¹) to characterize the material response under different stress states. To address the limited accuracy of the classical Johnson–Cook (JC) damage model under low stress triaxiality conditions, a modified Johnson–Cook (MJC) damage model incorporating the Lode parameter associated with the maximum shear stress was proposed. Furthermore, quasi-static three-point bending experiments on nonuniform-thickness plates were carried out, and the fracture behavior was predicted using the ductile model, the classical JC model, and the proposed MJC model. Comparisons between numerical simulations and experimental results show that the MJC model provides more accurate predictions of both the load–displacement response and fracture initiation location, with the predicted fracture patterns showing good agreement with the experimental observations, particularly under low stress triaxiality conditions. In addition, the fracture mechanism of nonuniform-thickness plates under bending was further analyzed through the distributions of stress triaxiality, normal stress, and Lode parameter. The results demonstrate that the proposed MJC damage model offers an effective approach for predicting bending-induced fracture of nonuniform-thickness aluminum alloy plates.</p>