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One of the analytical tools most used by elevated-temperature designers is the log stress-log minimum creep-rate curve. The curve is normally constructed using at least three and preferably more data points from individual tests. In this paper the possibility of constructing the stress-minimum creep-rate curve using a single specimen programmed for variable step-up and step-down loading is investigated. Both high and low-ductility Cr-Mo- V steels are studied. The results are treated both graphically and analytically to predict rupture under variable loading. Interesting metallographic features of rupture are shown and the results treated using phenomenological theory of anelasticity. It is concluded that a reasonable estimate of the stress-minimum creep-rale curve can be obtained during step-up loading of a single specimen of ductile steel. Among the various techniques for predicting rupture life under variable loading, a strain-fraction method gives good results.

In this paper a method is presented for correlating the creep and rupture strengths of a wide variety of commercial alloys. The ultimate aim of this correlation is to predict design creep properties from rupture data alone. This is of considerable interest because rupture parameter or isothermal rupture curves are frequently the only data available since relatively little creep data is taken today. It is demonstrated in this work that reasonable predictions, useful in design, can be made. The alloys studied range from aluminum base through low alloy and stainless steels and include iron-nickel, nickel, and cobalt-base superalloys. Very long time data for single heats of each of the alloy types has been taken from either the literature or sources willing to make such data available. The construction is simple, and common techniques for determining scatter in the correlation are developed. The predictions include scatter bands of strain-time data developed from the 15 data sets encompassing all the alloys. It is suggested that some refinement might be gained by studying numerous heats of a single specification material where such data is available. A complicating problem of structural instability arises and is discussed in the paper.

The use of the controversial and contradictory parameter techniques in correlation of high-temperature stress-rupture data is discussed. They are treated with a view toward their application in extrapolating short-time data to predict working stresses in serviceable heat-resistant alloys. Three parameter methods are compared on a statistical basis for their ability to reproduce the isothermal data on which they are based and in particular the longest time data currently available and usable for this purpose. Comparisons are further made with long-time extrapolations based on direct plotting of test data. The materials chosen for evaluation are alloys S-590, A-286, Nimonic 80A, and 1Cr-1Mo-1/4V steel. It is shown that prediction of long-time working stresses using parameter techniques will generally give better results than can be obtained from long extrapolations on double logarithmic plots. Among the parameters themselves, the Manson-Haferd linear parameter method gave the most reliable extrapolations. However, the results obtained using the Larson-Miller method may be improved considerably if the proper constant is selected for each set of data rather than using C = 20 for all data. For reliable extrapolations all methods require data from tests up to 1000 hr and covering adequate ranges of stress and temperature.

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