Abstract: Power torsional springs of clutch pedal fractured at the same location after service for about 20 000 kilometers.The failure cause of the springs was analyzed by chemical composition analysis,microstructure inspection,and fracture surface analysis.In addition,mechanical properties testing and surface defects inspection were carried out with the same batch of steel wires.The results show that the fracture mode of the torsional springs is fatigue fracture caused by large unidirectional bending stress.The cracks initiated from the surface defects of the torsional springs.The chemical composition,microstructure,and surface defect status can all meet the requirements of the standard.The mechanical properties of the same batch of torsional spring steel wires can meet the requirements of the standard,too.Comprehensive analysis shows that the design insufficiency is the primary cause for the fracture,and the secondary cause is the surface defects of the torsional springs.The design of the springs has been optimized based on assembly matching and torque values,and subsequent tests have shown that the design improvement is effective.