Failure analysis and finite element assessment of a torsion spring

The torsion spring is an important component for vehicle steering system. The torsion spring in this study was manufactured using high quality carbon structural steel. In order to study the failure mechanism of the spring, a series of experiments were carried out. All the collected evidences indicated that the spring fracture was due to fatigue crack propagation. At the same time, an obvious machining defect was observed on the inner surface of the ring. There are two crack initiation regions which are located at the areas near and opposite side of the machining defect. Under the cyclic loading induced by the repeated turning, the crack propagated and fractured finally. In order to analyze the effect of the machining defect on the fatigue failure, a computed tomography experiment was conducted. The precise characters of the defect were measured by computed tomography experiments. Then a finite element model was established according to the actual defect size and the stress distribution of the torsion spring was analyzed. The results showed that the local stress of the spring and the area of stress concentration are closely related to the machining defect and the gap between the spring and the two cylindrical objects. Under the actual condition of the torsion spring in service, the severe stress was located on the fracture location, which indicating that the machining defect is the efficient cause of spring failure.