The normalized coffin- manson plot in terms of a new damage function based on grain boundary cavitation under creep- fat
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I.
INTRODUCTION
relationship:
Low-cycle
fatigue (LCF) behavior of metals at high temperatures is of considerable interest in the design of many components used in power generation and aeronautics. Many components operating in power-generating plants are subjected to complex stress-loading cycles at high temperatures. In order to simulate such loading cycles under laboratory conditions, numerous tests have been carried outI1-4I in which creep and fatigue deformation are applied simultaneously. Among them, a constant strain-controlled tensile dwell test (i.e., creep-fatigue interaction condition) is frequently used to simulate the practical loading cycles. It has become apparent that the fatigue lives under creepfatigue interaction condition depend not only on testing temperature but also on wave shape with hold time. Generally, in studies on total strain-controlled creep-fatigue testing, the fatigue life is observed to be decreased with increasing test temperature and/or hold time.I4-sl It is well known that the tensile hold time may have the more pronounced effect on cyclic life in austenitic stainless steels, and the life reduction occurs as a result of grain boundary cavitation.t7,s,gI Under LCF conditions with or without hold time, the relation between the number of cycles to failure (Ns) and the plastic strain range (Aep) is known to have the following
SO0 WOO NAM, Professor, and BAIG GYU CHOI, Ph.D. Candidate, are with the Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology, Taejon 305-701, Korea. YOUNG CHEOL YOON, formerly Graduate Student, Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology, is with SamSung Motors Inc., Kyungi-do 445-970, Korea. JE MIN LEE, formerly Graduate Student, Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology, is with the Defence Quality Assurance Agency, Kyungnam 641-160, Korea. JIN WAN HONG, formerly Graduate Student, Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology, is with the Samchunly Heat Treatment Co. Ltd., Kyungi-do 425-I 10, Korea. Manuscript submitted August 29, 1994. METALLURGICAL AND MATERIALSTRANSACTIONS A
Aep. N] = const. which is known to be the Coffin-Manson plot.rio.-] In LCF tests, as the tensile hold time is increased, the fatigue life is observed to be decreased at a fixed test temperature and a given plastic strain range. And, if the test temperature is varied with a given plastic strain range or hold time, the fatigue life is also changed. Therefore, if one plots the plastic strain range with respect to the fatigue life obtained under different test temperatures and durations of hold time, one may get as many straight lines as combinations of temperature and hold time. Accordingly, using a given CoffinManson relation, one cannot predict the expected fatigue life for other conditions. The present work attempts to derive a "damage function" which can normalize all
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