Aeropropulsion components, such as disks, blades, and shafts, are commonly subjected to multiaxial stress states at elevated temperatures. Experimental results from loadings as complex as those experienced in service are needed to help guide the development of accurate viscoplastic, multiaxial deformation models that can be used to improve the design of these components. Typically, past studies investigated model materials, concentrating, for experimental simplicity, on room temperature. This study provides first-of-a-kind data by conducting axial-torsional tests at service-related temperatures on a popular aeropropulsion material, namely INCONEL 718 (IN-718, Inco Alloys International, Inc., Huntington, West Virginia). The data from this type of study can be used to develop the evolution equations necessary for this and similar engineering materials.
Yield loci were determined for aged IN-718 at 650 °C. This was done using experimental procedures developed over the past several years in a cooperative program between Penn State University and the NASA Lewis Research Center. This program represents an ongoing effort to develop test methods necessary for the validation of viscoplastic models. The von Mises ( J 2) yield criterion was found to not fit the data well because of a strength differential in tension and compression. That is, the initial yield strength in compression was greater than it was in tension. The strength differential was present for the entire range of loading (up to at least strains of ±1 percent), but it decreased as plastic flow increased. In addition, the strength differential increased as the temperature increased in the range of 20 to 650 °C.
Yield surfaces also were obtained after preloading the material to various levels. After applying an axial-shear prestrain to an equivalent inelastic strain of 500 microstrain, the yield surface translated in the direction of the prestrain (see the following figure). In addition, the yield surface elongated in the direction of prestraining and flattened on the back side. Thus, there is a component of distortional hardening that needs to be accounted for in the theoretical evolution equations.
Yield locus subsequent to an axial-shear inelastic prestrain of 500-microstrain for IN-718 aged at 650 °C. Offset for individual probes, eoff, 30 microstrain.
We are presently developing techniques to transform the yield surface data to flow surfaces, which are similar to yield surfaces except that they are rate-dependent, and thus more meaningful for viscoplastic models. Since the von Mises yield criterion was inadequate for IN-718, experiments and associated modeling work are planned to determine the dependence of initial yielding on the stress invariants ( I 1, J 2, and J 3) and to establish the importance of these invariants. In addition, we plan to examine load path dependence for the evolution of state and compare these results with predictions from viscoplastic models.
Gil, C.M.; Lissenden, C.J.; and Lerch, B.A.: Determination of Yield in Inconel 718 for Axial-Torsional Loading at Temperatures up to 649 °C. NASA TM-1998-208658, 1998.
Gil, C.M.; Lissenden, C.J.; and Lerch, B.A.: Investigation of Anomalous Behavior in Metallic-Based Materials Under Compressive Loading. NASA TM-1998-206640, 1998.
Lissenden, C.J., et al.: Robinson: Experimental Determination of Yield and Flow Surfaces Under Axial-Torsional Loading. Multiaxial Fatigue and Deformation Testing Techniques. ASTM STP-1280, S. Kalluri and P.J. Bonacuse, eds., 1997, pp. 92-112.
Lewis contact: Dr. Bradley A. Lerch, (216) 433-5522,
Bradley.A.Lerch@grc.nasa.gov
Authors: Dr. Bradley A. Lerch, Prof. Clifford J. Lissenden, and Christopher Gil
Headquarters program office: OAT
Programs/Projects: HITEMP
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