Engineering stress vs true stress error
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Confidence intervals are estimated, and a comparison with the ASME Boiler and Pressure Vessel Code Case 1592 creep-fatigue design curve is made more » for a particular set of conditions. Goodness of fit with respect to these variables as well as the appropriateness of the transformations employed are discussed. The first-order variables, which include strain range, strain rate, temperature, and tensile hold time, were used in a multivariable regression analysis to describe the observed variation in fatigue life. The available elevated-temperature, strain-controlled, uniaxial fatigue data on Type 304 stainless steel (435 data points) are summarized, and variables that influence cyclic life are divided into first- and second-order categories. The irregular material behavior at 650/degree/C is attributed to dynamic strain aging as indicated by serrated stress-strain curves (the Portevin-LeChatelier effect). Prior aging had no significant influence on the rate-dependent material response. Both at room temperature and at 650/degree/C the relaxation behavior was independent of the stress and/or strain level at the beginning of the relaxation, but depended nonlinearly on the strain rate preceding the relaxation test. Test results indicate that at 650/degree/C the more » material may exhibit a strain rate history effect.
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At 650/degree/C the pattern of strain rate sensitivity was not set as soon as the plastic flow was fully established, but continued to evolve with the further straining in the plastic range. The high-temperature experimental results revealed a complex and dramatically different material behavior. At room temperature material exhibited conventional strain rate sensitivity and no strain rate history effect. The strain rate sensitivity and short term relaxation behavior of Type 304 Stainless Steel were investigated in the uniaxial strain rate jump tests with intermittent periods of relaxation at room temperature and at 650/degree/C. Carburization of the specimens appears to inhibit the intergranular failure. Metallographic examination of the fracture surfaces shows a transition from a complete ductile fracture to a partial intergranular failure as the strain rate decreases. The values of the saturation more » stress for uniform elongation predicted from the Voce model are higher for the sodium-exposed specimens than for those in the solution-annealed condition at strain rates or approximately 5 x 10/sup -5/ s/sup -1/. The results show that, when compared with solution-annealed specimens, the tensile flow behavior of the sodium-exposed specimens is characterized by a higher strain-hardening rate, which decreases rapidly with an increase in flow stress. The Voce equation was used to describe tensile flow curves for plastic strains above 0.005 at temperatures between 550 and 700/sup 0/C and strain rates of 3.81 x 10/sup -6/ to 1.90 x 10/sup -3/ s/sup -1/. An analysis of tensile behavior of Type 304 stainless steel was conducted for specimens in the solution-annealed condition and after exposure to a sodium environment.