Experimental study of initial damage to steel bars due to bending process and the resulting performance deterioration

Brittle fracture of stirrups, due to effects such as diseased concrete expansion, is frequently reported and poses a potential threat to the load-bearing performance of structures. The brittle fracture was speculated to be directly related to the initial damage formed to bent steel bar during the bending process. However, few researches have been carried out on the formation of initial damage and its effect on mechanical performance. In this study, bending processes were conducted based on commonly used crescent-shaped steel bars; the effects of bending parameters and rib morphologies on the degree of initial damage were investigated in combination with data on bamboo-shaped steel bars, which often undergo brittle fracture. At the same time, tensile tests were carried out using V-shaped bent steel bars. The results show that as the bar diameter increases and the bending diameter decreases, the degree of initial damage in the bent steel bar increases. For example, for a crescent-shaped steel bar with a diameter phi of 16 mm, the initial damage when bending with a diameter 2.0 phi was 430 mu m, which exceeded the extent of the bamboo-shaped steel bar. At the same time, the ultimate tensile strength of bent steel bars was reduced by 20% compared to straight steel bars, and can be reduced by approximately 90% in severe cases; the deformation capacity of bent steel bars after yielding was also reduced by a maximum 90%. SEM analyses showed that the fracture surfaces of bent steel bars exhibit typical brittle fracture morphologies with the cleavage and quasi-cleavage fracture, indicating influences of initial damage and its propagation. Finally, based on the study results, some recommendations are made regarding the improvement of the transverse rib morphology, construction measures and the design of mechanical performance for bent steel bars, to prevent, for example, the premature failure of stirrups and the concrete components in which they are located.