Raasheduddin Ahmed

Constitutive Model Development for Thermo-Mechanical Fatigue Response Simulation of Haynes 230

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.Copyright

Unified Constitutive Modeling towards Enhanced Structural Integrity

Design and analysis of critical components in energy (nuclear, solar and fossil power), aerospace, automobile and chemical industries based on detailed inelastic analysis can enhance structural integrity and thereby economy. Especially for the components exposed to very high temperature thermomechanical fatigue loading, unified inelastic analysis based life prediction may enhance accuracy. A unified constitutive model (UCM) with features of strain rate-dependence, static recovery, mean-stress evolution, strain range-dependence, and finally creep damage is developed. The modified UCM is validated against simulating a broad set of strain-controlled isothermal and anisothermal fatigue and fatigue-creep responses, and stress-controlled creep responses of Haynes 230. Some of these results are presented to demonstrate improved simulations by the modified UCM. Importance of damage parameters in improving simulations in the tertiary creep regime is observed.

Haynes 230 High Temperature Thermo-Mechanical Fatigue Constitutive Model Development

Service temperatures of propulsion turbine engine combustor components can be as high as 1,800 °F. This induces a thermo-mechanical fatigue (TMF) loading which, as a result of dwell periods and cyclic loadings, eventually leads to failure of the components via creep-fatigue processes. A large set of isothermal and anisothermal experiments have been carried out on Haynes 230, in an effort to understand its high temperature fatigue constitutive response. Isothermal experiments at different loading strain rates show that the material can be considered to be rate-independent below and at 1,400 °F. However, isothermal strain hold experiments show stress relaxations below and at 1,400 °F. The out-of-phase strain-controlled TMF experiments show a mean stress response. A Chaboche based viscoplastic constitutive model with various features is under development with the final objective of predicting the strains in an actual combustor liner in service through finite element simulation for fatigue lifing. Temperature rate terms have been found to improve hysteresis loop shape simulations and static recovery terms are essential in modeling stress relaxation at temperatures where the behavior is overall rate-independent. It is anticipated that the new modeling feature of mean stress evolution will model the experimentally observed thermo-mechanical mean stress evolution.

Constitutive Modeling of Haynes 230 for High Temperature Fatigue-Creep Interactions

Non-linear stress analysis for high temperature cyclic viscoplasticity is increasingly becoming an important modeling framework for many industries. Simplified analyses are found to be insufficient in accurately predicting the life of components; such as a gas turbine engine of an airplane or the intermediate-heat exchanger of a nuclear power plant. As a result, advanced material models for simulating nonlinear responses at room to high temperature are developed and experimentally validated against a broad set of low-cycle fatigue responses; such as creep, fatigue, and their interactions under uniaxial stress states. . This study will evaluate a unified viscoplastic model based on nonlinear kinematic hardening (Chaboche type) with several added features of strain-rangedependence, rate-dependence, temperature-dependence, static recovery, and mean-stress-evolution for Haynes 230database. Simulation-based model development for isothermal creepfatigue responses are all critically evaluated for the developed model. The robustness of the constitutive model is demonstrated and weaknesses of the model to accurately predict low-cycle fatigue responses are identified.