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Ductile fracture and extremely low cycle fatigue (ELCF) [1] are two common failure modes in aircraft engines and turbomachinery designs [2]; however, the linkage between these two failure modes under multi-axial loading conditions has never been systematically studied. Inconel 718 (IN718) is one type of high temperature alloys widely used in turbomachines. Specially designed specimens and tests were used to achieve desired multi-axial loading conditions. Two groups of tests were conducted: (a) round bar specimens with different notches; (b) plane strain specimens. Similar types of tests were conducted for IN718 under both types of failure modes (ductile fracture and ELCF). It is found that the ductile fracture of IN718 under multi-axial loading conditions is strongly dependent on stress triaxiality, but weakly dependent on the Lode angle parameter [3], A 3D fracture locus was calibrated using modified Mohr-Coulomb (MMC) criterion proposed by Bai and Wierzbicki [4], It is found that the same phenomenon of stress state dependency exists in the ELCF, which need to be addressed. The mechanism linkage between these two failure modes was explored.

Lightweight, nano-structured aluminum metal-matrix composites (MMCs) have been identified as a next-generation armoring material due to their low density and high strength. The properties of A359-SiCp-30% are investigated here, with the aim of reducing material loss to edge cracking during the hot rolling process through characterization of the deformation and rupture behavior at high temperatures and moderate strain rates. Multiaxial isotropic constitutive equations designed for modeling the thermomechanical processing response of a lightweight MMC are developed. The model incorporates both strain rate and temperature dependence of the inelastic response. Tensile tests were performed on A359-SiCp-30% samples to validate the model. By means of a finite element analysis, the constitutive model was applied to simulate the tensile response, and a strong correlation with the experimental data was achieved.Metallurgical analyses were carried out on tensile-tested samples to determine the microstructural mechanisms leading to tensile rupture.

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Contemporary computing packages handle a wide variety of stress analysis types, but are yet to provide an optimal way to handle certain load cases and geometries. Blades in gas turbine systems, for instance, undergo repetitive thermal and mechanical load cycles of varied shape and phasing. Complexly-shaped airfoils create non-uniform stress paths that exacerbate the problem of FEA software attempting to determine the correct states of stress and strain at any point during the load history. This research chronicles the update and integration of Miller’s original viscoplasticity model with ANSYS finite element analysis software. Elevated temperature strain-controlled LCF and strain-controlled TMF loadings were applied to single-element, uniaxial simulation runs and the results were then compared to data from duplicate experimental testing. Initial findings indicate that the model maintains significant accuracy through several cycles, but longer tests produce varying error in hysteretic response. A review of the modernized implementation of Miller’s viscoplasticity model is presented with a focus on modifications that may be used to improve future results.