Isothermal, in-phase and out-of-phase axial-torsional fatigue experiments have been conducted at 760°C on uniform gage section, thin-walled tubular specimens of a wrought cobalt-base superalloy, Haynes 188. Test control and data acquisition were accomplished with a minicomputer. Fatigue lives of the in- and out-of-phase axial-torsional fatigue tests have been estimated with four different multiaxial fatigue life prediction models, a majority of which were developed primarily for predicting axial-torsional fatigue lives at room temperature. The models investigated were: (1) the von Mises equivalent strain range, (2) the modified multiaxiality factor approach, (3) the modified Smith-Watson-Topper parameter, and (4) the critical shear plane method of Fatemi, Socie, and Kurath. In general, life predictions by the von Mises equivalent strain range model were within a factor of 2 for a majority of the tests, and the predictions by the modified multiaxiality factor approach were within a factor of 2, while predictions of the modified Smith-Watson-Topper parameter and of the critical shear plane method of Fatemi, Socie, and Kurath were unconservative and conservative, respectively, by up to factors of 4. In some of the specimens tested under combined axial-torsional loading conditions, fatigue cracks initiated near extensometer indentations. Two design modifications have been proposed to the thin-walled tubular specimen to overcome this problem.
The deformation and damage mechanisms in wrought, double-aged, Inconel718 superalloy (AMS 5663D) tested under monotonic tensile strains of 2% and lo%, fully-reversed fatigue, and tensile strain (2% or 10%) followed by fully-reversed fatigue conditions were investigated by examining the microstructures of representative specimens. All tests were conducted in air at room temperature. The specimens were sectioned and examined by transmission electron microscopy to reveal typical microstructures as well as the active deformation and damage mechanisms. Specific mechanistic features addressed include the type of slip, interaction of dislocations with y", y' and the carbides (precipitated during solidification and the subsequent heat treatment received by the superalloy), twinning, and microcracking. In all cases the microstructure of the as-received superalloy is employed as the reference to establish the nature and distribution of the secondary phases before the superalloy is subjected to different types of mechanical loading. Results of the investigation and comparisons of the mechanisms of deformation and damage observed under monotonic tensile strain, fully-reversed fatigue, and tensile strain followed by fully-reversed fatigue in Inconel 718 superalloy are reported.Superalloys 718,625,706 and Various Der~at~ves
Starting from James Rice’s classical work on cyclic plastic stresses and deformations in the plastic zone of a Mode III loaded crack, it will be shown that the crack tip opening displacement of a Mode I crack in a work hardening material can be written in analytical form. This result was then used to formulate the blunting line for J-integral testing and to estimate the fatigue crack propagation rate of a number of materials. In a similar manner—based on the strain distribution within the plastic zone of a work hardening material—the initiation of crack extension under static loading was estimated. The stress distribution ahead of a crack and the Ritchie, Knott. and Rice model were applied to the ductile-to-brittle transition of ferritic steels as well as the transition temperature shift due to neutron irradiation. Inspired by Fong Shih’s contribution to the Electric Power Research Institute Handbook, a simple but straightforward method for expressing the δ5 crack opening displacement as a crack driving force for fully plastic conditions was developed, finally ending up in a comprehensive assessment method for cracked components. The application to mismatched welded joints was demonstrated to be possible if the yield load for mismatch is available; this latter task was performed using both slip line theory and finite element (FE) analyses. Application examples of these models will be shown, and it will be seen that estimates using these models are in reasonable agreement with experimental results and FE analyses. Several elements of these models have made their way to international codes and standards.
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