In cases in which very high rates of heat transfer exist, as in nuclear reactors, severe thermal stresses can be developed in structural metals. These stresses can be relieved by plastic flow and, consequently, are not regarded as serious when steady-state conditions prevail. Under the action of thermal oscillations, however, cyclic thermal stresses are developed. Depending on the frequency of thermal cycles and severity of the thermal stress, fatigue failure can result. Because of its low thermal conductivity and high thermal expansion, 18 per cent chromium-8 per cent nickel stainless steels are particularly prone to this effect. An investigation of this problem has been under way for some time. The object has been twofold: (a) To obtain information so that a particular material may be assessed critically for its resistance to thermal stress-fatigue damage under specified conditions, and (b) to investigate the fundamental aspects of the problem in order to learn more about the fatigue phenomenon. The paper describes the test apparatus developed for this study. A companion paper gives the test results to date and their interpretation.

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