Thermomechanical fatigue, oxidation, and Creep: Part II. Life prediction
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I.
INTRODUCTION
A. Background L I F E prediction models are needed to assist with the design and evaluation of components undergoing thermomechanical fatigue (TMF). The models currently available are severely limited in their generality. Often the application of the current models outside the range of experiments used to derive the model is not recommended. Since the mechanisms producing damage may differ from one set of testing conditions to another, advancement in the area of life prediction needs to be in the identification of these different damage mechanisms and the conditions which activate them. Microstructure and bulk parameters which quantify these mechanisms should be established. The model derived in this report attempts to take advantage of physical damage measurements and identifies these with a measure of lifetime. There have been data reported for isothermal fatigue tests; however, many critical components undergo a complicated temperature-strain history which can be more closely approximated by a TMF test in the laboratory, tq Therefore, it is important that the new generation of life prediction models is able to handle these strain-temperature histories and yet remain general enough so that the prediction model is not restricted to only a few temperature, strain rate, or strain range levels. When elevated temperature experiments are conducted, environmental and creep effects on the life can become significant, depending on the material and testing parameters. Both environmental effects and creep are activated simultaneously with fatigue and lower the life. These damage mechanisms change with strain, temperature, phasing, and strain rate in a complicated way, as indicated in Part 1. In most cases, the strain-temperature variation with time and the material condition will dictate which mechanism is the most damaging.
R.W. NEU, Research Assistant, and H U S E Y I N SEHITOGLU, Associate Professor, are with the Department of Mechanical and Industrial Engineering, University of Illinois, Urbana, IL 61801. Manuscript submitted May 16, 1988. METALLURGICAL TRANSACTIONS A
B. Current Life Prediction Models for High-Temperature Fatigue 1. Oxidation-fatigue models The earliest life prediction model on environmental effects is the frequency-modified strain-life equation I21which included, in addition to the plastic strain range, a frequency factor which accounted for the effect of oxidation. The early work on oxidation-fatigue interaction considered the effect of hold times on lifetime under isothermal fatigue conditions. These life prediction models are configured to handle either "crack initiation or crack growth to a certain size" with modified low cycle fatigue equations or "crack growth" with modified fracture mechanics parameters. The structures of these models are discussed here. Challenger et al.[31 developed a model which predicts the cycles to initiate a surface oxide crack. No creep effects for the 2.25 Cr-lMo alloy steel at 593 ~ were included. In the absence of hold times, the oxidation effect c
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