Failure analysis of a motor vehicle suspension helical spring

Abstract

The purpose of this work is to reveal the cause of the failure of the motor vehicle rear suspension barrel-shaped spring with the progressive elasticity characteristic and predict measures to increase the lifetime of springs of this type. The fracture of the spring occurred on the middle coil, which operates under conditions of more severe stress in comparison with other coils. The chemical composition of the spring material, determined by X-ray fluorescence spectral and microstructural analyzes, corresponded to chromium-silicon steel 54SiCr6. In terms of structure and mechanical properties, the spring material met the standards. No traces of decarburization were detected, and no crack initiation, caused by non-metallic inclusions, was found in the material of the fractured spring. Macroscopic examination of the spring surface did not reveal any cracks, scratches, dents, traces of blows with stones and marks of spring coiling tool. Instead, extensive areas of exfoliation of the protective coating were found. The metallographic analysis revealed selective corrosion in the form of pitting damage in places of exfoliation of the protective coating. The fatigue crack propagates from the certain deep pit with the reorientation of the crack plane along the spiral surface to the central axis of the coil wire. After depletion of the safety margin, the spring broke down quickly. The fast fracture zone contains steps of the river pattern formed due to the spiral reorientation of the fracture surface. The research can be used to understand the importance of adhesive strength and wear resistance of protective coatings on the spring surface. Their local exfoliation causes subsequent corrosion damage to the spring, which stimulates its fatigue fracture.