@article{barrett_hassan_2020, title={A unified constitutive model in simulating creep strains in addition to fatigue responses of Haynes 230}, volume={185}, ISSN={["1879-2146"]}, DOI={10.1016/j.ijsolstr.2019.09.001}, abstractNote={• Evaluation of modeling features in simulating creep and fatigue responses. • Need of coupling continuum damage model to unified constitutive model (UCM). • Issues in simulating creep strains at low temperatures by the modified UCM. • Issues in simulating fatigue responses at high temperatures by the modified UCM. • Challenges of UCM in simulating elevated temperature creep and fatigue responses. A unified constitutive model (UCM) specifies that its flow rule for inelasticity computes both the plastic and creep strains as a single state variable. A Chaboche framework based UCM with the modeling features of strain range-dependence, strain rate-dependence, static recovery and mean stress evolution was developed and experimentally validated against a broad set of fatigue and fatigue-creep responses of Haynes 230 (HA 230) under isothermal and anisothermal temperature conditions. This article demonstrates that this advanced Chaboche-based UCM can simulate the secondary minimum creep strain rates reasonably, but is unable to predict the tertiary creep strain responses. To simulate the tertiary creep strain responses a continuum damage model is needed to be coupled to the UCM. This study also evaluated three different unified flow rules, Norton's power law, exponential Norton and sine-hyperbolic Norton for calculating the inelastic strain rates. It is found that the choice of flow rule is important in simulating the stress amplitude saturation rate of fatigue responses, but has minimal effect in simulating the tertiary creep strains. However, the damage coupled UCM independent to the unified flow rules listed above can adequately simulate fatigue, fatigue-creep including the stress relaxation during strain dwell, and creep strain up to the tertiary range for HA 230. The drawbacks of the damage coupled UCM are the hysteresis loop softening at very high temperatures and asymptotic simulation at low creep temperatures, which are identified as challenges to be overcome towards developing a universal UCM for robust design and analysis of high temperature components.}, journal={INTERNATIONAL JOURNAL OF SOLIDS AND STRUCTURES}, author={Barrett, Paul R. and Hassan, Tasnim}, year={2020}, month={Mar}, pages={394–409} }
 @article{ahmed_barrett_menon_hassan_2017, title={Thermo-mechanical low-cycle fatigue-creep of Haynes 230}, volume={126}, ISSN={["1879-2146"]}, DOI={10.1016/j.ijsolstr.2017.07.033}, abstractNote={• Developed a broad set of thermo-mechanical fatigue-creep (TMFC) responses of a superalloy. • Demonstration of the influence of TMFC loading on mean stress evolution. • Demonstration of the influence of TMFC loading on the elastic modulus rate change. • Discussion on the challenges in constitutive model development for TMFC responses. • Comparison of isothermal and thermo-mechanical fatigue-creep lives. Combustor liners of airplane gas turbine engines experience premature thermo-mechanical fatigue-creep (TMFC) failure under operational loading conditions. The loading history of combustor liners encompass temperature fluctuation between ambient to as high as 1000 °C, and concurrent strain or stress fluctuation with load peak dwell-periods of 30 min to as long as 16 h of flying time. Repetition of such an anisothermal loading history leads to crack initiation in components via TMFC damage accumulation processes. In an effort to investigate such TMFC failures, a set of anisothermal experiments, both in-phase and out-of-phase, with peak dwell periods, were carried out for Haynes 230, a nickel-based superalloy used in constructing combustor liners. Analysis of the responses from out-of-phase experiments with compression dwells show mean-stress evolution in the tensile direction, while that from in-phase experiments with tensile dwells show mean-stress evolution in the compression direction. The total stress relaxation during peak strain dwell in the TMFC loading in general decreases with cycle, whereas that in the isothermal low-cycle fatigue-creep (LCFC) increases with cycle. The cyclic hardening-softening response is found to depend on the maximum temperature in the TMFC loading cycle. While calculating the inelastic strain in the experiments, it was found that the time derivative of the elastic modulus needed to be considered to prevent anomalous shifting of the hysteresis loops with cycles. The fatigue lives in the TMFC experiments are adversely affected by higher maximum temperatures and longer dwell-periods. These experimental responses are presented and analyzed, and challenges in developing a unified constitutive model for simulation of these responses are identified.}, journal={INTERNATIONAL JOURNAL OF SOLIDS AND STRUCTURES}, author={Ahmed, Raasheduddin and Barrett, Paul Ryan and Menon, Mamballykalathil and Hassan, Tasnim}, year={2017}, month={Nov}, pages={90–104} }
 @article{barrett_ahmed_menon_hassan_2016, title={Isothermal low-cycle fatigue and fatigue-creep of Haynes 230}, volume={88-89}, ISSN={["1879-2146"]}, DOI={10.1016/j.ijsolstr.2016.03.011}, abstractNote={Service temperature of airplane gas turbine engine combustors fluctuates between ambient to as high as 982 °C, during which structural constraints induce cyclic stresses and strains resulting in thermo-mechanical fatigue damage accumulation in the combustor liner. In order to substantially improve the current design methodologies or low-cycle fatigue (LCF) life predictions of such high-temperature components, it is essential to develop an experimentally validated advanced constitutive model. This requires a broad set of fatigue data of the combustor liner material, Haynes 230 (HA 230) – a nickel-based superalloy, to characterize its fatigue failure responses. Hence, a systematic set of isothermal experiments are conducted prescribing uniaxial strain-controlled loading cycles, with and without a compression peak strain-dwell, with and without a mean strain, at seven different temperatures in the range of 24–982 °C and at three strain rates. The experimental responses are critically examined to explore various fatigue failure responses of HA230, which is a complex material showing unique fatigue-creep, strain rate sensitivity, strain range dependence, temperature dependence and dynamic strain aging (DSA) properties. DSA is found to occur in the temperature domain 427–760 °C. Isothermal experimental responses at different strain rates show that HA 230 can be considered rate-independent at and below 760 °C. However, stress relaxation is observed at lower temperatures up to 649 °C during the peak strain-dwell period. Finally, fatigue lives of HA 230 from the isothermal experiments are found to decrease with increase in temperature. These experimental responses are presented and challenges in constitutive model development are discussed.}, journal={INTERNATIONAL JOURNAL OF SOLIDS AND STRUCTURES}, author={Barrett, Paul R. and Ahmed, Raasheduddin and Menon, Mamballykalathil and Hassan, Tasnim}, year={2016}, month={Jun}, pages={146–164} }
 @article{ahmed_barrett_hassan_2016, title={Unified viscoplasticity modeling for isothermal low-cycle fatigue and fatigue-creep stress-strain responses of Haynes 230}, volume={88-89}, ISSN={["1879-2146"]}, DOI={10.1016/j.ijsolstr.2016.03.012}, abstractNote={A robust cyclic viscoplasticity model is developed for simulating a broad set of isothermal, low-cycle fatigue and fatigue-creep responses of Haynes 230 (HA 230) under uniaxial loading. High temperature components experiencing thermo-mechanical fatigue failures can be designed considering their failure responses such that their fatigue life is predictable. Hence, design of high temperature components in aerospace, automobile, nuclear power, and chemical industries should be based on viscoplastic nonlinear analysis using a robust constitutive model. A unified viscoplasticity model based on the nonlinear kinematic hardening rule of Chaboche with several added features for strain-range dependence, rate-dependence, static recovery, and mean stress evolution is developed and evaluated against a broad set of HA 230 responses. Robustness of the constitutive model is demonstrated against predicting fatigue and dwell period stress relaxation responses under uniaxial strain-controlled loading for a broad temperature range of 25–982 °C and strain rate range of 1.1×10−2 to 2.6×10−5/s. Parameter determination of such an advanced model is discussed showing the importance of a well thought out experimental database and thereby providing physical meaning to model parameters.}, journal={INTERNATIONAL JOURNAL OF SOLIDS AND STRUCTURES}, author={Ahmed, Raasheduddin and Barrett, Paul R. and Hassan, Tasnim}, year={2016}, month={Jun}, pages={131–145} }
 @inproceedings{barrett_hassan_2015, title={A unified viscoplastic model for creep and fatigue-creep response simulation of Haynes 230}, DOI={10.1115/pvp2015-45671}, abstractNote={A Chaboche-based unified viscoplastic constitutive model, including features of strain range dependence, strain rate-dependence, static recovery, and mean stress evolution is developed and evaluated for simulating fatigue-creep and creep responses of Haynes 230. In other words, this constitutive model attempt to simulate not only strain-controlled fatigue and fatigue-creep responses of Haynes 230, but also stress-controlled creep responses. After investigating various flow rules and kinematic hardening rules, a unified viscoplastic constitutive model is developed for simulating both the fatigue-creep and creep responses. The parameter determination for this constitutive model, however, requires a robust optimization algorithm. The proposed unified constitutive model can adequately simulate fatigue-creep responses, and creep responses up to the secondary creep regimes. However, with the introduction of damage modeling features the constitutive model can simulate the tertiary creep regime responses, but with some limitations in simulating fatigue-creep responses. Nonetheless, the unified viscoplastic constitutive model with or without damage modeling features has shown to be able to capture the stress-controlled creep responses while still maintaining high fidelity in capturing the strain-controlled fatigue and fatigue-creep responses.}, booktitle={ASME Pressure Vessels and Piping Conference - 2015, vol 3}, author={Barrett, P. R. and Hassan, T.}, year={2015} }
 @inproceedings{ahmed_barrett_hassan_2014, title={Constitutive modeling of Haynes 230 for anisothermal thermo-mechanical fatigue and multiaxial creep-ratcheting responses}, DOI={10.1115/pvp2013-97248}, abstractNote={Service life analysis and design of high temperature components, such as turbine engines, needs accurate estimation of stresses and strains at failure locations. The structural integrity under these high temperature environments can be evaluated through finite element structural analysis. This requires a robust constitutive model to predict local stresses and strains. A unified viscoplastic constitutive model based on the Chaboche type nonlinear kinematic hardening rule was developed including the added features of strain range dependence, rate dependence, temperature dependence, static recovery, and a mean stress evolution. The new constitutive model was validated through critical evaluation of the simulation of a broad set of stress and strain responses of a nickel-base superalloy Haynes 230. The experimental database encompasses uniaxial strain-controlled loading histories which include isothermal low cycle creep-fatigue and anisothermal thermo-mechanical fatigue experiments at temperatures ranging from 75°F to 1800°F. Simulations from the modified model are presented to demonstrate its strengths and weaknesses, and future work is needed for developing a robust constitutive model.}, booktitle={Proceedings of the ASME Pressure Vessels and Piping Conference - 2013, vol 6B: Materials and Fabrication}, author={Ahmed, R. and Barrett, P. R. and Hassan, T.}, year={2014} }
 @inproceedings{barrett_menon_hassan_2012, title={Isothermal fatigue responses and constitutive modeling of Haynes 230}, DOI={10.1115/pvp2012-78342}, abstractNote={Constitutive models are an integral part of a lifing system because it allows for accurate estimation of stresses and strains at failure locations of interest. Constitutive models can be properly defined in a material subroutine of a finite element code. The computational capabilities of today are far higher, allowing for more comprehensive models that can provide more accurate results. Macroscopic models that are physically based, phenomenological models characterize the material behavior on a larger scale that provides invaluable insights even at such length scales which are compatible for industrial application. A unified viscoplastic model based on nonlinear kinematic hardening (Chaboche type) with several added features such as nonproportionality, multiaxiality, strain range dependence, and thermal recovery is being implemented in ANSYS through the User Programmable Features. The simulation capability of the model will be experimentally validated on a nickel based superalloy, HA230. The experimental database encompasses a broad set of low cycle fatigue, symmetric, uniaxial strain-controlled loading histories which include isothermal with and without hold times, with and without a mean strain, at temperatures ranging from 75°F to 1800°F. Simulations from the modified model compared to the experimental responses will be presented to demonstrate the strengths and weaknesses.}, booktitle={Proceedings of the ASME Pressure Vessels and Piping Conference 2012, PVP 2012, vol 9}, author={Barrett, P. R. and Menon, M. and Hassan, T.}, year={2012}, pages={71–79} }