Investigation of service failures in automobile shock absorbers

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Investigation of service failures in automobile shock absorbers

Gallardo, J.M. / Soria, L. / Herrera, E.J.

Several service failures had occurred at the head of shock absorbers manufactured by the same company. Three specific cases were known of. In one case, the service time is unknown; in the other two cases, the failures occurred after 1400 and 12,000 km of use, respectively. Fracture had occurred at the head of the damping device through a welded zone at the piston-rod/ring junction. Failure analysis (mechanical tests, metallographic and fractographic examinations) has been carried out. It has been found that undesirable porosity, cracks, inclusions and low-strength microstructures developed during welding. Failure, nevertheless, is mainly attributed to a poor alignment of the junction parts during the welding operation. This means that the weld nugget was not positioned in the centre, but rather to one side. Less heat was applied, since the electrical resistance of the contact area was lower, and the heat distribution was different, affecting the heating and cooling sequence of the system. The result was a weak mechanical joint with large hollow spaces or cavities. In addition, pores, cracks and inclusions were formed, due mainly to prior contamination of the surfaces to be welded, mainly with sulphur compounds. These imperfections in turn facilitated the failure of the shock absorbers, albeit only in a secondary role. To avoid these welding defects, it is recommended that the surfaces of the parts to be joined be cleaned beforehand, and that the stroke of the piston rod should be extended in order to increase the forge pressure. In this way, lateral extrusion would force a substantial part of the contaminated material sideways out of the joint.