An attempt has been made to determine and quantify the accumulation of creep damage in Cr-Mo-V steel. Damage is defined in terms of post exposure rupture data and hardness measurements which are shown to be interrelated. Correlation between the post exposure rupture life and the initial exposure parameters are achieved by simple parametric techniques which then form the basis for graphical representation of damage accumulation as a function of exposure time, stress, and temperature.
The way in which these curves may be used to predict failure under nonsteady conditions of stress and temperature is described. In general the linear life fraction cumulative damage rule is unrealistic for this material, although under certain restricted conditions it may give a reasonable prediction of life. Apart from the measured nonlinearity with time of damage accumulation, the present results confirm that the common assumption of a unique failure criterion independent of the test conditions cannot be justified.
The paper selects two well recognized problem areas of high temperature material behavior, namely, creep and fatigue, and considers those factors which contribute to the prediction of the useful service life of the component. The information available is obtained almost entirely in a laboratory environment, in which simplified load conditions and specimen geometries and controlled environments are employed for material evaluation. We do not consider transferring this information to actual design, other than to identify the importance of such problems as the analytical determination of stresses and strains in the structure, the effect of highly complex loading, or the behavior in a complex environment, but we will draw on several practical problems occurring in high temperature power generating machinery as examples of the topics we wish to discuss namely, creep, fatigue, and creep-fatigue interaction.
The topics of creep and fatigue are considered separately, the discussion being built around three central topics: (1) real problems in prediction, (2) complicating instabilities including the effects of environment, deformation, and microstructure; and (3) some current predictive methods. Consideration of the less well defined problem of creep-fatigue interactions is also included.
A Task Group of the Joint Committee on Effect of Temperature on the Properties of Metals (sponsored by the American Society for Testing and Materials, the American Society of Mechanical Engineers, and the Metal Properties Council) did an extensive study of methods for the extrapolation of short-time rupture data in an effort to decide if any one method might serve as a basis for a standard practice. Emphasis was on the time-temperature parameter.
The following conclusions were reached: 1. Among the computerized techniques the newly introduced minimum-commitment method (MCM) is on the average about as good as the best of the four standard time-temperature parameters studied. 2. In addition, the MCM has other potential merits which make it a candidate for the sought-after standard. Further study and investigative work are required to realize this potential. 3. Experienced analysts can get good results by manual extrapolation methods. These methods are subjective, and because of the limited availability of experienced analysts such methods probably should not be considered for standard practice.
Many recommendations are given in the report for future work toward the goal of defining a standard practice.
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