PSI - Issue 2_A

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

2945

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1. Introduction

Coil springs, or helical springs, are widely used in motor vehicle industry as one of the primary elastic members of the vehicle suspension system. Used as a connection between the wheel and the body of the vehicle, they tone down the shocks that would otherwise be transmitted from uneven surface of the road to the body. Absorption and subsequent release of the external loads comes in a form of elastic energy and, due to their material and design, springs tend to return to their initial length when unloaded. Trends in the motor vehicle industry aim for continuous weight reduction along with improvement in performance and coil springs are no exception. Nowadays, coil springs are subjected to significantly larger stresses compared to ones used in previous generations of vehicles. Roughness of the material surface and inclusions are two important stress raisers in the springs. Beside the presence of stress concentration raisers, deficient microstructure is another main cause of spring failure. With increased stress levels in springs, suitable material properties and manufacturing quality become more important. To assure improvement of coil spring design and enhance their integrity, failure analysis of broken coil springs comes as an appreciable tool for manufacturers and car parts suppliers. There has been a considerable amount of research on the topic of coil springs. Paper of Prawoto et al. (2008) can serve as an introduction to automotive suspension coil springs, their fundamental stress distribution, material characteristic, manufacturing and causes of common failures. Dealing with the failure of motor vehicle coil springs, some of the recent work includes Das et al. (2007) experimentally investigating premature failure of a passenger car coil spring caused by inherent material defects coupled with deficient processing. Further, Kosec et al. (2014) discovered that failure of motor vehicle coil spring had been caused by simultaneous activity of cycling loads and corrosion attack onto the material surface. Zhu et al. (2014) analyzed reasons why a compression coil spring mounted on a heavy vehicle fractured at the transition position from the bearing coil. Fractured surface was analyzed by employing experimental methods while numerical methods served to determine contact points between the coils from which crack originated. Looking to improve performance of springs, it is important to understand the behavior of spring material. Angelova et al. (2014) gave comprehensive overview of fatigue behavior of spring steels DIN 17223C and 55Si7 followed by mathematical models of typical da/dN diagrams. Murtaza and Akid (2000) studied crack initiation and growth behavior in Si-Mn spring steel to develop empirical corrosion fatigue life prediction models. Sag resistance of Si-Cr spring steels was investigated under influence of alloying additions and tempering temperatures by Nam et al. (2000). Barani et al. (2006) tried to improve ductility of Si-Cr spring steel refining grain boundary carbides by thermomechanical treatment. Heat and surface treatments can significantly improve fatigue life of springs, a statement proved by Fragoudakis et al. (2014) on 56SiCr7 spring steel by heating, quenching and tempering. Microshot peening process can be efficiently employed to improve the fatigue life of spring steel, as confirmed by the study of Harada et al. (2014) on SUP9 spring steel. Surface defects can become the predominant origin of spring steel failures under very high cycle fatigue regime (Li et al., 2014), stressing the need for proper surface treatment of coil spring steels. Although there seems to be a substantial track of previous work on the topic of failed springs and spring material, research is still ongoing. New designs and new spring materials along with changing and unexpected service conditions give reason for further failure analysis of fractured coil springs. This paper presents a step in that direction, where a broken coil spring has been extracted from a vehicle in order to determine possible causes of failure. Local car dealership confirmed that there has been a noticeable amount of failure cases on the particular vehicle type from which the broken spring has been extracted. Results of the research presented here can be taken as a reference in further improvement of mentioned coil springs so that the failure incidents can be minimized.

2. Experimental procedures

2.1. Visual observations

A motor vehicle’s front suspension coil spring fractured after 145.000 km and 7 years of service, at the transition point from the lower bearing coil to the first upper coil. Geometry and dimensions of considered closed-end compression spring are shown in Fig. 1.