@article{ahmed_hassan_2017, title={Constitutive modeling for thermo-mechanical low-cycle fatigue-creep stress-strain responses of Haynes 230}, volume={126}, ISSN={["1879-2146"]}, DOI={10.1016/j.ijsolstr.2017.07.031}, abstractNote={• Unified constitutive modeling for thermomechanical fatigue-creep (TMFC) responses. • Illustration of the need of advancing constitutive modeling for TMFC responses. • Experimental validation of the modified model against a broad set of TMFC responses. • Discussion and performance illustration of the modified modeling features. • Demonstration of significant progress in unified constitutive modeling. Haynes 230 (HA 230), a Nickel-based superalloy, is the primary material of combustor liners in airplane gas turbine engines. This component operates in the temperature range between ambient to as high as 1000 °C. Such thermal cycles together with the resulting strain cycles in a combustor liner may induce thermo-mechanical fatigue-creep (TMFC) damage and initiate cracks earlier than the estimated life. Use of a robust unified constitutive model (UCM) for nonlinear analysis based design may improve fatigue life estimation of high temperature components. Available UCMs in the literature or commercial software packages are unable to simulate the TMFC responses reasonably. Hence, this study developed a UCM incorporating the modeling features of rate and temperature dependence, static-recovery, various kinematic hardening evolutions, and strain-range dependence, and validated the model against a broad set of TMFC experimental responses of HA 230. This modified UCM is capable of simulating the mean-stress evolution under both the out-of-phase and in-phase TMFC loading cycles. The modified UCM can adequately simulate most of the characteristic cyclic phenomena of HA 230 including the influence of maximum temperature on the out-of-phase and in-phase TMFC hysteretic responses, stress amplitude, and stress relaxation during strain-dwell. The time-derivative of the elastic modulus is an essential modeling feature for accurately simulating the inelastic strains under TMFC loading. These simulations demonstrate the progresses made in UCM.}, journal={INTERNATIONAL JOURNAL OF SOLIDS AND STRUCTURES}, author={Ahmed, Raasheduddin and Hassan, Tasnim}, year={2017}, month={Nov}, pages={122–139} }
 @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} }
 @inproceedings{morrison_ahmed_hassan_2017, title={Thermomechanical fatigue response and constitutive modeling for Haynes 230}, DOI={10.1115/pvp2016-63283}, abstractNote={Design by analysis is usually performed by commercially available finite element analysis (FEA) software. Constitutive models available in the FEA software are developed and validated using limited experimental data. Hence, a broad set of thermomechanical fatigue experiments with strain dwell at compressive peaks are performed to understand local fatigue failure responses of high temperature components. This study developed a unified viscoplastic model based on nonlinear kinematic hardening of Chaboche type with added features of strain range dependence, rate dependence, temperature dependence, static recovery, and mean stress evolution. The robustness of the constitutive model is demonstrated by comparing its simulations against the experimental responses.}, booktitle={Proceedings of the ASME Pressure Vessels and piping conference, 2016, vol 5}, author={Morrison, M. and Ahmed, R. and Hassan, T.}, year={2017} }
 @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{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{ahmed_menon_hassan_2012, title={Constitutive model development for thermo-mechanical fatigue response simulation of Haynes 230}, DOI={10.1115/pvp2012-78221}, abstractNote={Turbine engine combustor components are subject to thermo-mechanical fatigue (TMF) during service. The combustor liner temperatures can sometimes reach as high as 1800°F. An accurate estimate of the strains at critical locations in the combustor liner is required for reliable lifing predictions. This demands the need for a detailed analysis of the TMF responses and a robust constitutive model capable of predicting the same. A large set of experiments have been carried out on the liner material, a nickel based alloy, HA 230, in an effort to understand its thermo-mechanical fatigue constitutive response. The out-of-phase strain-controlled TMF experiments with a negative mean strain show a positive mean stress response, while the in-phase TMF experiments with a positive mean strain show a negative mean stress response. A Chaboche based viscoplastic constitutive model is under development. It will have several essential features such as nonlinear kinematic hardening, isotropic hardening, strain range dependence, rate dependence, temperature dependence and static recovery. The constitutive model being developed for accurately calculating the stress-strain response is being carried out with the final objective of predicting the strains in an actual combustor liner in service through finite element simulation for fatigue lifing.}, booktitle={Proceedings of the ASME Pressure Vessels and Piping Conference 2012, PVP 2012, vol 9}, author={Ahmed, R. and Menon, M. and Hassan, T.}, year={2012}, pages={171–179} }