Development of a Novel Hybrid Unified Viscoplastic Constitutive Model

Gas turbines are now days used in power plants for power generation and for propulsion in the aerospace industry. In these applications gas turbines are exposed to severe temperature and pressure variations during operating cycles. These severe operating conditions exposed the turbineâ??s components to multiple deformation mechanisms which degrade the material and eventually lead to failure of the components. Nickel based and austenitic super alloys are candidate material used for these applications due to its high strength and corrosion resistance at elevated temperatures. At such temperature levels, candidate materials exhibit a rate-dependent or viscoplastic behavior which difficult the prediction or description of the material response due to deformation mechanisms. Unified viscoplastic constitutive models are used to describe this viscoplastic behavior of materials. In the present work Miller and Walker unified viscoplastic models are presented, described and exercised to model the creep of Hastelloy X and the low cycle fatigue behavior of stainless steel 304. The numerical simulation results are compared to an extensive database of experimental data to fully validate the capabilities and limitations of the considered models. Material constant heuristic optimizer (MACHO) software is explained and used to determine both models material constants and ensure a systematic calculation of them. This software uses the simulated annealing algorithm to determine the optimal material constants values in a global surface, by comparing numerical simulations to an extensive database of experimental data. A quantitative analysis on the performance of both models is conducted to determine the most suitable model to predict materialâ??s behavior. Based on the two exercised classical viscoplastic models, a novel hybrid unified model is introduced, to accurately describe the inelastic behavior caused by creep and fatigue effects at high temperature. The presented hybrid model consists on the combination of the best aspects of Miller and Walker model constitutive equations, with the addition of a damage rate equation which provides capabilities to describe damage evolution and life prediction for Hastelloy X and stainless steel 304. A detailed explanation on the meaning of each material constant is provided, along with its impact on the hybrid model behavior. To validate the capabilities of the proposed hybrid model, numerical simulation results are compared to a broad range of experimental data at different stress levels and strain rates; besides the consideration of two alloys in the present work, would demonstrate the modelâ??s capabilities and flexibility to model multiple alloys behavior. Finally a quantitative analysis is provided to determine the percentage error and coefficient of determination between the experimental data and numerical simulation results to estimate the efficiency of the proposed hybrid model