Stress relaxation of an AISI 1080 steel
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INTRODUCTION
THE focal point in the study of the mechanical prope ~ e s of materials is the understanding of the stress-strain relation, t~-4~ In recent years, the stress relaxation technique has contributed to this understanding. 15'6'71 The principal advantage of a stress relaxation test is that it scans a broad range of strain rates while straining the specimen by only a small amount. Therefore, it is possible to think that the substructure of the material remains ,approximately constant during the test and to obtain the stress as a function of the strain rate at constant structure. I5'8'9j T w o o f the most common stress relaxation methods used are the continuous relaxation of Gupta and Li t~~ and the incremental unloading technique of Gibbs. lltl The stress relaxation technique would be very useful, but as indicated by Guiu l~2J and Reed-Hill and Donoso, 171 the relaxation inherent in a tensile testing machine can be large enough to influence the tests. Most of the stress relaxation tests that have been reported in the scientific literature were conducted using screw-driven testing machines. ~5-~6~ These machines deform significantly under load, leading to difficulties concerning the effects of the machine on the test results. T w o main problems arise. (1) As shown in a previous paper, 16J the elastic behavior (machine stiffness) seems to vary from test to test, although it apparently remains almost constant during each individual relaxation. (2) The machine deformation has an anelastic component that has not been quantitatively related to any test variables. 17,~5I By anelastic, we mean deformation that depends not only on load but also on time, which is usually called the "relaxation" of the machine. One explanation for this effect was offered by Hart and Solomon, l~TI who discussed the role of lubricating grease in the lead screws of screw-driven machines. Rohde and Nordstrom 1~6] (R & N) performed stress relaxation experiments using both screw-driven Instron and servohydraulic MTS testing machines. In the MTS machine, the stress relaxation is obtained under condiRICARDO E. MEDRANO, Professor, is with the Instituto de Fisica, University of Campinas, Campinas, Sho Paulo, Brazil. PETER P. GILLIS, Professor, is with the Department of Materials Science and Engineering, College of Engineering, University of Kentucky, Lexington, KY 40506. Manuscript submitted September 28, 1988. METALLURGICAL TRANSACTIONS A
tions of constant specimen strain. While in the screwdriven Instron machine, the total of specimen strain plus machine strain remains constant. They found large differences in the parameter/3 ( = 0 In ~/Otr), which should be independent of the behavior of the machine, and attributed these differences to larger plastic deformation for the same stress change, which occurs in the screwdriven machine as compared to the servohydraulic one. Their conclusions are in opposition to several authors, [17"18A91 who claim that substructural changes are small. Rohde and Nordstrom's suggestions are difficult to under
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