Fracture analysis of a cutting tool used in a straightening and twin bended system for coiled re-bars machine

• A study on the failure of a machine designed for the processing of prefabricated stirrup cages constructed of steel bars.
• Poor machine design is considered as the main cause of the failure.
• Some recommendations are given in order to avoid similar future failures.
• Alternatives for material selection are proposed in order to increase the machine efficiency in an economical way.

A company produces prefabricated stirrup cages for the reinforcement of concrete. The process consists of straightening, cutting and twin bending of coiled steel re-bars, in a custom produced machine. The punch of the cutting system failed repeatedly. Its cutting edges lose their functionality much earlier than expected. Two representative pieces were analyzed. The study included visual inspection, chemical analysis, hardness and micro-hardness measurements. Optical metallographic examination on representative samples was carried out. The fracture areas were analyzed using a scanning electron microscope (SEM) and the crack initiation and propagation were discussed. The punches failed by chipping wear mechanism due to low-cycle fatigue. Their repeating failures provoked by faulty machine design. Problem solving suggestions are proposed. In addition, alternatives related to material selection are provided in order to reduce crack susceptibility, in an economical way. Moreover, conclusions and recommendations can be useful, resulting in higher productivity of prefabricated stirrup cages.