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The design of complex parts subjected to creep conditions is generally-based on the results of relatively simple laboratory tests in which both the load and temperature are held constant. In actual service such steady conditions seldom are encountered. The load and temperature usually vary in some complex manner depending on the operating cycle. In recent years there has been an increasing effort to evaluate the influence of nonsteady load and temperature conditions on the creep characteristics of a number of alloys. The object of such tests may be considered two-fold: first to provide a fundamental understanding of the influence of nonsteady conditions on the flow and fracture characteristics, and secondly to arrive at more rational methods of design which will reduce the safety factors now employed. It is not to be expected that the results of nonsteady creep tests can be related in any simple manner to those of steady tests. If either the load or temperature change in some manner, various recovery effects and/or transient effects may influence creep flow and fracture. In addition, many engineering alloys are subject to structural changes during creep and these may be altered by the nonsteady conditions. These changes include: (a) recrystallization, (b) precipitation or changes in the form or arrangement of precipitates present in the heat treated structure, and (c) oxidation or corrosion. Structural effects such as these can influence the mechanical properties directly through their influence on the strength or indirectly through influence on ductility. Further, it is well known (1) (2) that cyclic heating alone can produce grain deformation without overall dimensional change in certain noncubic metals and that thermal cycling of uranium results in gross elongation (3). Another effect that must be considered is that of thermally induced stress due to temperature gradients throughout the body caused by alternate heating and cooling. The magnitude of these stresses and their effects will of course depend both on the geometry of the part and the temperature gradients. These various mechanisms are not mutually exclusive and in a given case they may act in combination. The observed effects of nonsteady creep conditions can seldom be clearly related to one or more mechanisms. This is particularly true in the investigations of engineering alloys where the tests were not specifically designed to reveal fundamental information of this nature.
Manson, S. S.
Chief, Lewis Research Center, National Aeronautics and Space Administration, Cleveland, Ohio
Brown, W. F.
Chief, NASA, Lewis Research Center, Cleveland, Ohio