Failure analysis of the damper spring in a B-segment passenger car

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. As an important machine element, the performance of springs depends on many factors, such as the spring wire material and its manufacturing history. In general, springs made of high carbon and usually alloyed steels (Cr-Si, Si-Mn or Cr-V) are used to meet the high resilience expectation. Sometimes, cold-drawn wires can also meet expectations in spring construction. The microstructure and cleanliness of steel directly affect the mechanical properties. In addition to the heat treatment history of the spring (such as tempering process temperature, decarburization effect, and grain coarsening), the surface treatments applied after manufacturing, grinding, shot blasting and coating processes (type and method of coating) also have a significant effect on the performance of the spring. The fact that spring wire is usually made of high-strength steel also means it has a high-notch sensitivity. For this reason, in some cases, manufacturing defects on the surface can accelerate the crack formation phase and worsen the part life. 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 part under investigation is a front suspension coil spring. It is made of wire with a diameter of 12 mm, the height of the spring is 350 mm, and the pitch is 75 mm. The damaged spring belongs to a B-segment vehicle with over 220,000 km. It was found that the fracture started towards the end of the first winding, in a region of about 20 degrees between the top surface of the wire and the outer surface facing the centre. The car was mainly used on asphalt roads and partially on village roads. It was understood from the personnel interviews that the vehicle was used in geography where rain and snow are seen in the winter season (spreading salt on the road in the winter snow). Spectral analysis was performed to determine the composition of the spring material. The results show that the chemical composition of fractured spring is within the specified range in the standard for AISI 9254 steel. SEM samples for microstructure were prepared by grinding, diamond paste polishing, and chemical etching with Nital 4%. It is understood that the microstructure of the fracture spring, the structure has tempered martensite. The hardness measurements near the surface and core region vary from 590-600 HV10 (average hardness is 594 HV10). In addition, the microhardness (HV0.5) measurement results from the surface to the inside did not indicate the presence of decarburization in the hardness profile. Low-magnification fractography studies were performed for macro examinations. A scanning electron microscope was used for xxiv detailed fracture surface examinations. The helical shape of the fracture surface indicates that the damage is typical fatigue fracture of high-strength materials under variable torsional stress. The presence of traces on the fracture surface and the corrosion-affected area strengthens the suspicion that the crack initiation is most likely caused by corrosion. An electron microscopy study will be useful for detailed examination in determining the origin of the crack. From these micrographs, it is clear that fatigue crack nucleation is directly related to corrosion pitting. The presence of a large number of pits on the side surface of the specimen, some of which are clustered together, is an important indication that fatigue cracking starts from corrosion pits that cause stress concentration. The fact that the spring is made of high-strength steel has possibly increased the notch sensitivity, as expected. The literature has reported that the stress concentration coefficient (SCF) of a hemispherical cavity under shear stress is directly proportional to the aspect ratio of the pit. Accordingly, it is expected that the pits at the crack origin will produce a much more severe notching effect, leading to crack formation. It is useful to investigate the effect of corrosion pitting on the stress distribution of the spring wire by finite element analysis. In the finite element analysis performed in Ansys Workbench, a linear elastic and isotropic material model was used as the material model. Poisson's ratio of 0.3 and modulus of elasticity of 200 GPa were chosen for the spring steel. The location of the cavity in the model was determined by considering the crack initiation point of the fractured spring, and a 0.5 mm diameter hemispherical cavity (0.25 mm depth) was created to represent the corrosion pit. A total of 186760 tetrahedron elements were used in the finite element model. The element density was increased to improve the mesh quality in the region where the cavity is located. Firstly, the equivalent and shear stress distribution across the spring for the 50 mm displacement model without a cavity was investigated. The stresses in the inner part of the spring are higher than in the other parts. Both stresses increase outward from the centre of the wire and also reach their maximum values on the surface of the wire facing the centre of the spring. It also confirms the results of the studies that carried out finite element-based stress analysis on helical springs. In the model with pit, the maximum value of the equivalent stress in the pit (361.3 MPa) is 1.34 times higher than the stress on the inner surface of the wire. Also the maximum stress occurs in the pit, which is 1.82 times higher than the stresses in and around it. Although the Von Mises stress distribution reveals the stress concentration effect caused by the pit, in some cases, the maximum principal tensile stress can be decisive for fatigue damage. In contrast to the distribution of the shear stress, the maximum principal stress takes the largest value (242.4 MPa) not at the bottom of the pit but at the sidewall and is greater in magnitude than the shear stress. Considering that the stress at a point close to the pit is 91.3 MPa, the stress concentration factor of the pit reaches a value of 2,65. Furthermore the maximum principal stress is the most indicative stress component and can be useful in fatigue evaluations of springs. It should be noted that it is of great benefit to consider the maximum principal stress in the evaluation of the mechanical behaviour of springs with a high spring diameter/wire diameter ratio.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 pits between the top of the spring and the outward-facing sides. 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.