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The use of alloys under conditions where creep occurs results in continuous accumulation of “damage” to the mechanical properties. Design must therefore consider what amount of creep can be tolerated during the required service life. In some cases the allowable creep is considerably less than one percent plastic strain. In other applications the governing criterion is failure by rupture, which may or may not be accompanied by relatively large amounts of creep strain. Because creep is a continuing process leading to failure, it can be considered to introduce increasing amounts of “damage” with increasing time in service. The damage accompanying creep may result from: (1) the plastic flow of creep, (2) the structural changes, which are thermally or strain induced, (3) the loss of elements (for example, decarburization), or the introduction by reaction and diffusion of contaminants from the environment, or surface damage arising from corrosion. With extensive use of alloys under creep conditions, and especially with more and more applications accumulating prolonged service times approaching design life, the following questions are raised with increasing frequency and urgency: (1) What is the remaining creep-service life of the material? (2) What damage to other mechanical properties arises from creep service? (3) Can the service life be extended by re-heat treatment during creep service? This paper presents a survey of the literature on creep damage. It was designed to produce answers to these three questions and to cover other pertinent aspects of the problem. The effects of corrosion and other surface reactions were omitted in so far as possible because they are highly dependent on the specific conditions of service or testing. Also no attempt was made to analyze the data in the light of the extensive literature on the fundamental processes of creep in metals and alloys.
Freeman, J. W.
Voorhees, H. R.