A generalized threshold function for viscoplastic materials, which can also serve as a yield function in rate-independent plasticity, is suggested for materials that exhibit a strength differential and/or a permanent volume change. The motivation for this type of a threshold function is that experiments, at both 25 and 650°C, on the nickel-based alloy Inconel 718 indicate that higher stresses occur in compression than in tension. Similar results have been obtained for martensitic steels and other metallic alloys at 25°C. A general approach for determining the inelastic flow dependence on each of the three stress invariants (I1,J2, and J3) is to follow stress paths where only one invariant is changing. Two classical experiments that do this are hydrostatic pressure and pure torsion, however many others are possible. Unfortunately, these stress paths generally require three-dimensional stress states, which are difficult to obtain in the laboratory. Thus, for experimental expediency, tests involving axial-torsional loading of thin-walled tubes can be used to determine the significance of the first and third stress invariants, I1 and J3, respectively. [S0094-4289(00)01303-7]
Large gas turbine nickel based superalloys blades are typically manufactured using an investment casting process. In spite of superior processing and quality control, large turbine blades may contain manufacturing induced discontinuities such as porosities, segregation, chemical inhomogeneities, particles, etc in few of the manufactured parts. These manufacturing discontinuities can significantly influence the reliable component life if they are under a load condition where they will form a crack. A typical engineering approach is to treat these discontinuities as a crack from the beginning of the part life. This leads to a conservative life prediction, as crack nucleation and influencing geometrical details of the discontinuities are neglected. This paper presents a framework and path forward for a comprehensive life assessment. It includes computer tomography (CT) analysis, metallographic analysis, local stress analysis, crack formation, as well as state of the art fracture mechanics analysis. For instance, CT and destructive tests reveal details of the geometry of a porous region and thereby enabling the calculation of crack formation life. A subsequent fracture mechanics analysis by engineering tool, FRANC3D can then yield a comprehensive life assessment for comparisons to experimental findings and fleet experience. This approach enables Siemens to diligently ensure that comprehensive life predication assessment has been performed for the components for robust and reliable operation.
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