A strain energy-based approach to the low-cycle fatigue damage mechanism in a high-strength spring steel
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INTRODUCTION
LOW-CYCLE fatigue (LCF) evaluation has received increasing attention recently, not only due to the ease of estimating the LCF life exclusively by monotonic tensile properties,[1] but also because of a close link between the fundamental LCF properties and the fatigue crack growth (FCG) behavior.[2,3] Analysis of the LCF behavior is conventionally based on strain parameters, since LCF tests have been mostly strain-controlled and the well-known Coffin–Manson[4,5] strain-life relationship has been conveniently adopted for data evaluation. However, due to the different transient behavior of different materials, it is not always possible to locate the stable (saturated) state of the cyclic deformation based on the variation in the strain or stress parameters. As in the case of fracture mechanics, a strain energy–based criterion may serve as one of reasonable alternatives to stress- or strain-based criteria. The energy-based approaches to fatigue virtually date back to early in this century, when the role of the hysteresis energy in the fatigue process was qualitatively described.[6] Then, in the middle of the century, came some hypotheses for cumulative damage in fatigue based on hysteresis energy[7,8] and on empirical approaches relating the total plastic strain energy required for fatigue failure to fatigue life.[9,10] The analytical work of Morrow[11] and Halford[12] based on cyclic hysteresis energy received fairly general recognition regarding its quantitative evaluation of both the average strain energy dissipated per cycle and the total dissipated strain energy. In recent decades, there has been a tendency toward increased application of the plastic strain energy– based criterion in LCF data evaluation.[13–18] The theoretical D.M. LI, formerly Senior Research Associate with the School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, People’s Republic of China, is Senior Research Associate with the Center for Advanced Aerospace Materials, Pohang University of Science and Technology, Pohang 790-784, Korea. W.J. NAM, Research Group Director, is with the Technical Research Laboratory, Pohang Iron and Steel Co. Ltd., Pohang 790-785, Korea. C.S. LEE, Professor, is with the Center for Advanced Aerospace Materials, Pohang University of Science and Technology, Pohang 790-784, Korea. Manuscript submitted April 15, 1997. METALLURGICAL AND MATERIALS TRANSACTIONS A
frame for these energy-based investigations has been basically retained as the one proposed previously,[11,12] while modifications[13] have been made to calculate the hysteresis energy for materials showing an asymmetrical tensile-compressive hysteresis loop configuration. To better understand the underlying damage mechanism of fatigue, however, a unified physical interpretation of data obtained from the energetic approaches is still required. For the microscopic aspects of fatigue damage, the microstructural evolution, especially the dislocation substructural variation as a result of cyclic straining, is found to be
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