Engine components are subjected to both high temperatures and cyclic loads resulting in fatigue and creep effects. Directionally-solidified (DS) Ni-base superalloys were developed in order to produce favorable creep properties in the primary stress axis of turbine blades by casting the grains longer along this axis. Doing so causes the material to exhibit anisotropic behavior, which allows for improved fatigue and creep strength but also adds complexity to modeling the material. To predict the life of components accurately, it is necessary to use a high-fidelity constitutive model to relate the loading and the deformation of the material. The dual-phase microstructure of these DS superalloys evolves with time, rendering the yield surface of the material a challenge to track. Furthermore, components made from these materials are subjected to complex loading conditions, often seeing cycling temperature in addition to loads, known as thermomechanical fatigue (TMF), and cyclic loads with dwells, known as creep-fatigue (CF). Viscoplasticity models are able to capture the complex behaviors of these materials under complex loading conditions, including the hysteresis effects, rate-dependence, and stress relaxation, etc., making them attractive models to use with critically heated and loaded parts. These models, originally designed for equiaxed materials, have been adapted for use with anisotropic materials, such as DS superalloys. An isothermal anisotropic viscoplasticity model and parameter identification framework has been calibrated within a dedicated parameter identification framework. Principally, the constitutive model is based on the Chaboche viscoplasticity model featuring Armstrong-Frederick kinematic hardening. The performance of a preliminary model is presented for both an equiaxed (i.e., conventionally cast, CC) and DS materials within the same strength class, though more data is needed for validation. With regard to the stress relation associated with creep-fatigue, a fitting-technique for the static recovery model that has shown promise in isotropic materials is expanded to capture the behavior of the DS alloy. Previously developed methods for finding kinematic hardening constants for isotropic material based on Ramberg-Osgood constants at various orientations are expanded to an anisotropic case. These techniques allow for the capturing of more complex loading conditions with a limited number of tests, allowing for cost savings when developing the constitutive model. The model is implemented with three non-linear kinematic hardening terms with static recovery, allowing for the capture of rate and hold time effects, and non-linear isotropic hardening, allowing for the capture of cyclic hardening. The ability to capture the low cycle fatigue (LCF) behavior of both the equiaxed and DS alloys are examined through comparisons with test data.

The aerospace propulsion industry has seen strides in the use of the additive manufacturing (AM) technology in the rapid prototyping and geometric design flexibility of aerospace parts, with concurrent efforts on 3D printing turbine engine blades of Inconel 718 material [1] for use in aircraft engines. The tensile, compressive and axial fatigue response of AM Inconel 718, along with associated constitutive modeling of the material response exhibited under these mechanical test conditions have been reported. However, in addition to understanding the axial behavioral response exhibited by this material, assessing the role of cyclic shear stresses, through experimental testing and constitutive modeling can provide preliminary insight into the mechanical behavior of AM Inconel 718 under multiaxial loading conditions. This study has presented a novel approach to constitutively model the experimental cyclic shearing deformation of as-built direct metal laser sintered (DMLS) Inconel 718, manufactured along varying build orientations in the xy, yz and xz planes, compared with wrought annealed Inconel 718. Specimens were subjected to completely reversible torsional fatigue tests at room temperature, under angle of twist control. The experimental cyclic shearing response was modeled through the use of the Chaboche model, from which optimized constants are reported with build orientation; and the specimen deformation, under angle of twist control, was captured through a finite-element simulation model of the cylindrical gauge section of the specimens. Overall this study yields a comprehensive understanding of the experimental and modeled cyclic shearing response of an additively manufactured metal, which is vital to develop these components to be conducive for the multiaxial fatigue conditions to which they are subjected to in the gas turbine industry.

Engineering materials are often subjected to high temperatures and varying mechanical load, making them display both creep and fatigue. Viscoplastic models have been an attractive way to simulate the behavior of materials under these conditions due to their ability to capture the hysteresis, load rate, and stress relaxation effect exhibited by materials. Nickel-base alloys experience widespread use throughout pressure vessels and turbomachinery due to their ability to retain high strength at high temperature. In this study, the low-cycle fatigue (LCF), thermo-mechanical fatigue (TMF), and creep-fatigue (CF) of candidate material IN617 is modeled using the Chaboche viscoplasticity model. The Chaboche model is implemented using three non-linear kinematic hardening terms with static recovery included as well as nonlinear isotropic hardening. Methods for finding the kinematic and isotropic hardening from Ramberg-Osgood constants are presented to allow for model development to encompass all available data instead of fitting to specific data sets. A novel method for fitting the static recovery portion of the model from creep data is presented, allowing for the capture of complex loadings such as CF from standard creep tests. New temperature-dependent formulations of the Chaboche constants are presented for IN617. The model is compared to available test data and simulations are presented to show future experiments that could be run to further validate or improve the model.

Rapid prototyping has led to strides in improved mechanical part design flexibility and manufacturing time. Along with these advances, however, is the extremely high costs associated with additively manufacturing components that can limit a comprehensive understanding of the mechanical performance of these materials. This can be circumvented through the use of constitutive models which can both support experimental findings in addition to providing approximations of expected material behavior. The present study has demonstrated the influence of build orientation on as-built direct metal laser sintered (DMLS) stainless steel (SS) GP1/17-4PH, manufactured along varying orientations in the xy build plane, through strain-controlled tension and completely reversed low cycle fatigue experiments. Experimental findings from monotonic tension testing are used to model failure surfaces, which can be used to approximate failure regions for DMLS SS GP1 manufactured along varying build orientations within the horizontal xy build plane. Further, a Chaboche model is used to simulate the cyclic response of this material based upon experimental findings through low cycle fatigue testing. Conclusive findings from these models are used to assess the vital role that build orientation plays in affecting the mechanical performance of additively manufactured materials.