Abstract: In order to clarify the mechanism for the brittle fracture of bent steel bars under the action of concrete expansion, starting from the presumed origin of brittle fracture - the initial damage of bent steel bars, bending processing tests and numerical simulations were carried out for crescent-shaped steel bars under bending parameters and gradient increasing bending angles. The following topics were explored, including the influence of bar diameter and bending core diameter on the formation of initial damage, the deformation behavior of the transverse rib in the bent steel bar at each subdivision stage, the evolution of the initial crack scale, and the residual stress response inside the bending arc, in order to analyze the law and mechanism of initial damage formation of ribbed steel bars due to bending processing, and further to obtain the better transverse rib morphology and processing parameters to inhibit initial damage. The results show that: a larger bar diameter and bending angle, a smaller bending core diameter will significantly increase the degree of initial damage; the initial crack of crescent-shaped rebar under severe conditions (the diameter of the steel bar and the bending core is 16 mm and 32 mm respectively, and 90° bending) can reach 400 μm (2.5% of the bar diameter), which exceeds the level of the frequently fractured bamboo-shaped steel bars in Japan, and this increases the possibility of propagation for initial damage of the bent steel bar under the action of stress; with increasing bending angle, the degree of extrusion deformation and root stress concentration of the transverse rib in the steel bar becomes more significant, and after unloading at the 90° bending, the internal bending arc part changes from compressive to tensile stress state, which aggravates the formation of initial damage and the susceptibility of bent steel bars such as stirrup to brittle fracture; and the initial damage can be substantially mitigated by setting an appropriate transition circle between the crescent transverse ribs and the base circle.