Cyclic creep and cyclic deformation of high-strength spring steels and the evaluation of the sag effect: Part I. Cyclic
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INTRODUCTION
SAG and stress relaxation are two common failure forms of mechanical springs, especially of automotive suspension springs. (Hereafter, only the term “sag” will be used to represent both phenomena in this report). The behavior of sagging usually portrays the gradual decrease in height or length of a spring within a period of time, during which the maximum service load, generally a fatigue load, has been clearly lower than the yield strength of the spring material. This type of deformation can become a major concern, especially when (1) the service fatigue load is relatively high and asymmetrical and (2) the size and, hence, weight of the springs are major factors in design and service. With the current trend of improving fuel efficiency in the automotive manufacturing industry, suspension springs have become a typical component among those vehicles that need weight reduction most urgently, but whose springs cannot be easily made from other materials. One of the major problems encountered in reducing the weight of suspension springs is how to suppress sagging behavior in service, so as to meet the bumper-height standard set by the industry. Since the reduction in spring size and weight would result in higher working stress levels, the steels suitable for suspension springs must have an excellent sag resistance while having a high hardness or strength. ZHI’AN YANG, Postdoctoral Fellow, and ZHIRUI WANG, Professor, are with the Department of Metallurgy and Materials Science, University of Toronto, Toronto, ON, Canada M5S 3E4. Manuscript submitted March 28, 2000.
METALLURGICAL AND MATERIALS TRANSACTIONS A
It must be pointed out that sagging of any actual spring is a property of the component, i.e., it depends not only on the property of the spring material, but also on the design/ configuration of the spring. Up to now, a Bauschinger test was the only method used to characterize the sag behavior of spring steels, and, in fact, more frequently in industry, sag and stress-relaxation behavior have been tested directly with springs after manufacturing.[1,2,3] Although the deformation micromechanism, in general, should be the same in coil and leaf spring materials, torsion tests are used more often in industry to test Bauschinger effects for coil spring materials, because coil springs are generally considered to fail under shear stresses in service. This is done so as not to produce a testpiece-dependent stress-strain curve.[3] On the other hand, testing of leaf spring materials specifically for sag behavior is rarely discussed in the literature. It must also be pointed out that results from Bauschinger tests can reflect the behavior of the materials under an instantaneous or static loading condition, but not under dynamic, cyclic loading conditions. In other words, from the viewpoint of deformation mechanisms, sag behavior and its fundamental mechanism(s) are far from being understood. In some of our recent related studies on the room-temperature fatigue behavior of the Al (6061) alloy[4] and some steels,[5,6,
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