Failure analysis of automotive helical spring

As it is known, the suspension helical or coil spring contributes to the vehicle's driving comfort and protects the vehicle parts by softening the impacts depending on the road topography and keeping the suspension system stable without shaking. In this study, the cause of coil spring fracture in a B-segment vehicle was investigated systematically. Within the scope of the investigation, microscopy studies (optical and scanning electron), hardness, and metallographic studies were carried out, and finite element analysis was used to determine the stress distribution in the spring. The failure analysis showed that the fracture was fatigue induced, and the crack initiation was interestingly not on the inner surface of the spring where the stress was maximum but in the corrosion between the upper surface and the outer surface of the wire and in a region that makes an angle of approximately 20 degrees to the vertical. Considering the properties of the steel (AISI 9254) used in the spring, such as microstructure, hardness, and the stress values obtained from the finite element analysis (FEA), it is concluded that there was no problem in terms of design, materials, heat treatment, and processing history of the spring. However, FEA results simulating corrosion-induced pitting show that the pitting has a significant stress concentration, confirming the crack initiation assessment. In this context, it was understood that the final painting process applied to the part surface was insufficient to protect the spring against deteriorating effects such as rainwater / salty snow water from the road surface. It was concluded that improving the paint quality and/or installing a polymeric part to protect the spring against the effects of the road would be beneficial in solving the problem. Finally, in order to achieve high-performance springs, the use of steels with a combination of high strength and high fracture toughness (e.g. micro-alloyed) and/or the application of a precision shot peening process such as an ultrasonic method to the surface after heat treatment will contribute significantly to the corrosion fatigue life of the spring.