Hastelloy X and stainless steel 304 are alloys widely used in industrial gas turbines components, petrochemical industry and energy generation applications; In the Pressure Vessel and Piping (PVP) industries they are used in nuclear and chemical reactors, pipes and valves applications. Hastelloy X and stainless steel 304 are favored for these types of applications where elevated temperatures are preferred for better systems’ efficiencies; they are favored due to its high strength and corrosion resistance at high temperature levels. A common characteristic of these alloys, is its rate-dependent mechanical behavior which difficult the prediction of the material response for design and simulation purposes. Therefore, a precise unified viscoplastic model capable to describe Hastelloy X and stainless steel 304 behaviors under a variety of loading conditions at high temperatures is needed to allow a better and less conservative design of components. Numerous classical unified viscoplastic models have been proposed in literature, to predict the inelastic behavior of metals under extreme environments. Based on Miller and Walker classical unified constitutive models a novel hybrid unified viscoplastic constitutive model is introduced in the present work, to describe the inelastic behavior caused by creep and fatigue effects at high temperature. The presented hybrid model consists of the combination of the best aspects of Miller and Walker model constitutive equations, with the addition of a damage rate equation which provides a description of the damage evolution and rupture prediction capabilities 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. Material constants were calculated using the recently developed Material Constant Heuristic Optimizer (MACHO) software, to ensure the use of the optimal material constants values. This software uses the simulated annealing algorithm to determine the optimal material constants in a global surface, by comparing numerical simulations to an extensive database of experimental data. 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 amplitudes; besides the consideration of two alloys in the present work, would demonstrate the model’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.

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