Due to frequent failures of coil springs on a specific type of motor vehicle, analysis of possible causes of failures was performed. Analysis was done on a single coil spring removed from a vehicle after failing in service. Besides visual examination that revealed fracture to happen on a first bottom coil, several other experimental techniques were used in the failure analysis. Using optical microscopy evaluation of the basic microstructure of the fractured surface was performed and possible inclusions distinguished. Detailed scanning electron microscopy (SEM) examination at suitable magnifications was employed to characterize the fine microstructure of the fractured surface and reveal flaws that served as crack initiation points. Optical emission spectrometer with glow discharge source (GDS) sample stimulation was used to determine chemical composition of material used for spring fabrication. Additionally, hardness test was performed. Using results of the performed experimental analysis, possible causes of failure were recognized. Several factors, among them inherent material defect combined with material fatigue and helped by insufficient corrosion protection, caused failure of coil spring. Obtained results are valuable in predicting behaviour of coil springs mounted in other vehicles of the same type and can be taken as a reference in improving future design. Further analysis would include employing finite element method to determine stress levels in undamaged and damaged coil spring along with numerical estimation of fatigue life.
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