The results of a study of cyclic strain and fatigue failure arising from cyclic thermal stresses are reported. By means of a test apparatus described in a companion paper, a cyclic temperature is imposed on a thin tubular test specimen subjected to complete longitudinal constraint. Hence, the cyclic strain is the independent variable. The following studies are reported: (a) Effect of thermal-stress cycling on strain hardening and life-to-failure for a fixed mean temperature, (b) effect of degree and kind of previous cold work on strain hardening and cycles-to-failure, (c) effect of mean temperature on thermal-stress cycling, (d) effect of period of cycle on cycles-to-failure, and (e) effect of prior strain cycling on stress-strain characteristics. The significance of factors such as hysteresis, Bauschinger effect, strain hardening, strain aging, and fatigue-crack formation is discussed, and a mechanism is described to relate these factors. Evidence is presented to show that strain hardening is not an important factor in the problem. The concept of total plastic strain is discussed.

This report was prepared as the keynote address given at the 1972 Symposium on Fatigue at Elevated Temperatures at the University of Connecticut, 18–23 June, 1972. It describes the high-temperature fatigue problem as a failure process in a notch in some structure involving nucleation and early growth at the notch root, high-strain crack propagation through the plastic zone of the notch, and elastic crack growth to ultimate failure. Several of the important disciplines bearing on these three steps in the failure process are discussed. Particular attention is given to a description of the high-temperature phenomenology, distinctions between high- and low-cycle fatigue effects at high temperature, failure criteria including frequency and holdtime effects, the importance of the environment vis-a-vis creep in considering time effects on fatigue behavior, high-strain crack propagation, elastic crack growth, ratchetting effects, and methods for treating notches.

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This lecture reviews the significant developments in fatigue over the last twenty years as they pertain to the prediction of life in high-temperature service. Particular attention is given to the interpretation of fatigue test results for both low and high cycle fatigue at room and elevated temperatures. Emphasis is given to such effects as environment, frequency and strain rate, metallurgical factors, wave shape and thermal cycling, and some attempt is made to sort out their relative importance. Applicability of low cycle fatigue information to notch geometries is discussed. Lastly considered is the significance of these several factors to the current state of life prediction as well as to future directions for development of this important topic.

Experimental data from cyclic strain fatigue tests on twelve structural materials are compared on the basis of a low cycle fatigue equation as suggested by an ASME task group. This equation S=Ec2N1/2+Se where Se is the endurance limit, E the modulus of elasticity, N the cycles to failure, and c a constant derived from reported values of reduction in area, gives the stress amplitude S for comparison with elastically computed stresses. A satisfactory or good correlation with experiments for up to 105 cycles of failure is found.