PSI - Issue 2_A

Goran Vukelic et al. / Procedia Structural Integrity 2 (2016) 2944–2950 Author name / Structural Integrity Procedia 00 (2016) 000 – 000

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Fig. 4. SEM image of fracture surface around crack initiation point.

SEM analysis was difficult to perform because the fracture surface was damaged by corrosion. Examination of the fracture surface at 80x magnification revealed a heavily oxidized and corroded surface which obscured fracture surface details. Although an attempt was made to remove the corrosion layer using more aggressive cleaning agent, corrosion products could still be seen on SEM images.

3. Discussion

Visual examination of fractured coil spring revealed that the protective paint layer around the fracture was mechanically damaged. This damage was probably caused by the continuous contact between the lower bearing coil and first adjacent active coil. Thus, surface of spring wire was exposed to corrosive environment making way to formation of corrosion pits, Fig 2 (b). Three such pits served as crack initiation points under cyclic loading from the vehicle. Fig. 2 (c) clearly shows primary fracture zone colored in darker tone where the crack propagation was slower and opened surface was exposed to corrosion for a longer time. Crack propagated towards opposite edge of the spring wire following paths of radiating ridges. Finally, near the opposite edge of the spring wire fast fracture zone can be observed. As the analysis showed, spring was made of spring steel 61SiCr7. It is a steel with somewhat elevated content of silicone and chromium. Chromium at steels tends to increase tensile strength, hardness, toughness, resistance to wear and corrosion, while silicon is used as a deoxidizer in the manufacture of steel. It slightly increases tensile strength and can help in increasing the toughness and hardness levels. Nevertheless, undamaged paint layer comes as a must in protecting springs against long term exposure to corrosive environment. Optical and scanning electron microscopy images revealed damage to the wire surface caused by corrosion pits. This is the point form which fracture emanated. Heavily oxidized and corroded fracture surface limited SEM examination so deeper insight into the fracture surface could not be obtained. Research presented in this paper was concerned with possible causes of motor vehicle coil spring failure. Fractured spring was examined using experimental methods; namely, visual observation of fractured surface, determination of chemical composition using glow discharge spectrometer, hardness testing, optical and scanning electron microscope analysis of fractured surface. Findings suggest that this is an example of corrosion fatigue failure. Protective paint layer of the wire was damaged what opened the possibility of corrosion pits appearance. This pits served as crack initiation points from which fracture 4. Conclusion