Thermal-imaging technologies for detecting damage during high-cycle fatigue
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INTRODUCTION
FATIGUE of materials has been studied extensively due to its importance in engineering applications. With the ability to in-situ monitor the fatigue processes, nondestructive evaluation (NDE) methods are extremely useful to the fatiguedamage analysis, especially for studying the structural integrity of engineering components in service. Current NDE technologies include acoustic emission, eddy current, neutrons, ultrasonics, and X-ray.[1–12] In the recent two decades, with the breakthrough in the infrared (IR) technology, thermography has been developed as a new NDE method, which will be explored in this article. While most other NDE technologies rely on special sample preparations or sensors attached to the targets, thermography does not have these limitations. In this article, an IR-imaging system that can obtain detailed two-dimensional (2-D) temperature maps of the target has been applied to “watch” the in-situ temperature variations of different materials during fatigue testing. At the same time that the self-generated heat history is providing information for bioprocesses and chemical reactions,[13–18] it is also a fingerprint of the fatigue-damage processes. In 1934, Taylor and Quinney[19] demonstrated that the major work of plastic deformation in metals was transferred into heat. In 1969, Rice and Levy[20] estimated the temperature rise at a crack tip and its potential to change crack-propagation behaviors. Recently, with the 2-D IR technology, more research has shown the potential of thermography in monitoring mechanical and B. YANG, G. WANG, and W.H. PETER, Graduate Students, P.K. LIAW, Professor and Ivan Racheff Chair of Excellence, R.A. BUCHANAN, Professor and Department Head, and D.E. FIELDEN, Staff Member, are with the Materials Science and Engineering Department, The University of Tennessee, Knoxville, TN 37996–2200. Contact e-mail: [email protected] Y. YOKOYAMA, Professor, is with the Materials Science and Engineering Department, Himeji Institute of Technology, Shosha 2167, Himeji City, Japan. J.Y. HUANG and R.C. KUO, Researchers, are with the Institute of Nuclear Energy Research (INER), Lungtan, Taiwan 325, Republic of China. J.G. HUANG formerly with the Taiwan Power Company, Taipei, Taiwan, R.O.C. is now Professor, Department of Electrical Engineering, Chien-Kuo Institute of Technology, Changhua City, 500, Taiwan. D.L. KLARSTROM, Director, is with Haynes International, Inc., Kokomo, IN 46904. Manuscript submitted June 30, 2003. METALLURGICAL AND MATERIALS TRANSACTIONS A
fatigue damages.[21–24] Models for the short-time measurement of fatigue limits by observing the steady-state temperature during fatigue are also suggested.[25] However, detailed investigations and comprehensive analyses are needed to develop a more practical thermography technique in characterizing the fatigue behavior. In the present article, observations of plastic deformations, such as Lüders bands and plastic zones, made using thermography, are discussed. Theoretical models, including thermoelastic, inelastic, and
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