A new method is proposed for multiaxial fatigue life prediction using correlation parameters based on virtual strain energy as a measure of fatigue damage on critical planes of fracture. The virtual strain-energy parameters are physically associated with two different modes of fatigue fracture planes. The critical plane leading to Mode I fracture is driven by the principal stress and strain, and the other, leading to Mode II fracture, is driven by the maximum shear stress and strain. The mode of crack initiation and propagation depends on material, temperature, strain range, and stress and strain histories, but not on the relative magnitude of the virtual strain-energy parameters. Biaxial fatigue data obtained from the literature were analyzed for Type 304 stainless steel tested at room and elevated temperatures and for SAE 1045 steel tested at room temperature under in-phase and 90° out-of-phase loading conditions. Comparisons are made between experimental data and theoretical predictions to show the effectiveness of the proposed method.
This paper summarizes recent experimental results, obtained at Oak Ridge National Laboratory (ORNL), on creep behavior and creep rupture of a commercial grade of Si3N4 ceramic in the temperature range of 1150°C to 1300°C. A uniaxial model capable of describing the behavior under general thermomechanical loading is introduced and compared with existing models. An exploratory extension of the new model to a multiaxial form is then discussed. Issues are also discussed concerning the standardization of data analysis methodology and future research needs in the area related to development of creep database and life prediction methodology for high temperature structural ceramics.
This paper demonstrates use of subsize fatigue specimens for testing highly irradiated specimens to generate reliable information for alloy development and structural design applications. Essential specimen design features needed to accomplish irradiation and stable cyclic fatigue loading at elevated temperature and in high vacuum are discussed in detail. As examples, test results are presented for Type 316 stainless steel tested at 430 and 550°C. Comparisons are made between data generated from full-size and subsize specimens tested at 430°C with total strain ranges as high as 2% without buckling; results indicated a good reliability of the specimens and testing methods.
Cyclic lives obtained from strain-controlled fatigue tests at 593°C of specimens irradiated in the experimental breeder reactor II (EBR-II) to a fluence of 1 to 2.63 × 1026 neutrons (n)/m2 (E > 0.1 MeV) were compared with predictions based on the method of strain-range partitioning. It was demonstrated that, when appropriate tensile and creep-rupture ductilities were employed, reasonably good estimates of the influence of hold periods and irradiation damage on the fully reversed fatigue life of Type 316 stainless steel could be made. After applicability of this method was demonstrated, ductility values for 20 percent cold-worked Type 316 stainless steel specimens irradiated in a mixed-spectrum fission reactor were used to estimate fusion reactor first-wall lifetime. The ductility values used were from irradiations that simulate the environment of the first wall of a fusion reactor. Neutron wall loadings ranging from 2 to 5 MW/m2 were used. Results, although conjectural because of the many assumptions, tended to show that 20 percent cold-worked Type 316 stainless steel could be used as a first-wall material meeting a 7.5 to 8.5 MW-year/m2 lifetime goal provided the neutron wall loading does not exceed more than about 2 MW/m2. These results were obtained for an air environment, and it is expected that the actual vacuum environment will extend lifetime beyond 10 MW-year/m2.
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