RICERCA ITALIANA     

Characterization and modeling of mechanical behavior of polycrystal and single crystal nickel based superalloy at high temperatures for gas turbine applications

Università degli Studi di Cassino

Abstract

Mechanical components operating at high temperatures are, in most of the cases, subjected to the action mechanical and thermal stresses induced by the action of mechanical loads and temperature gradients or partially constrained thermal deformations. Since the intensity of these stresses varies with time according to the load cycle, the material undergoes to a complex deformation process, which may leads, sooner or later, the material to failure, that is commonly identified as thermo-mechanical fatigue (TMF). The present research proposal is focused in the development of a constitutive framework to predict life of structural components under TMF regime. Starting from the research units competences in the field of damage mechanics, experimental testing at high temperature and numerical simulation of deformation processes, constitutive models specific for each of the basic damage mechanisms that concur in the TFM will be developed. As far as concerns creep process, an identification procedure based on the theta projection method (TPM) will be developed for nickel based superalloy in the temperature range of interests of industrial applications (750°-1050°C). This methodology will be integrated with a damage formulation as proposed by Bonora et al. (2001), which has been successfully applied to creep failure prediction in polycrystalline metals, appropriately reformulated for single crystal. As far as concerns mechanical fatigue at elevated temperature, a Manson-Coffin based >>>

Principal Investigator

Nicola BONORA Università degli Studi di CASSINO

Research Objectives

Mechanical components operating at high temperatures are, in most of the cases, subjected to the action mechanical and thermal stresses induced by the action of mechanical loads and temperature gradients or partially constrained thermal deformations. Since the intensity of these stresses varies with time according to the load cycle, the material undergoes to a complex deformation process, which may leads, sooner or later, the material to failure, that is commonly identified as thermo-mechanical fatigue (TMF). The extension of durability of components under TMF regimes is manly controlled by three dominant damage mechanisms: mechanical fatigue at high temperature, creep and oxidation. Even though, nickel based superalloys have been specifically developed for high temperature applications, today, only the very general aspects of their behaviour under the combined action of stress and temperature are known, while reliable design tools, able to predict material performance under real work cycle conditions and to ensure transferability with respect to temperature, stress level and geometry, are still lacking. The major aim of the work is to build a constitutive framework developing specific model for each of the basic damage mechanisms in the TMF. The present research proposal is focused in the development of a constitutive framework to predict life of structural components under TMF regime. Starting from the research units competences in the field of damage mechanics >>>

Timescale

24 months

National and international background

The possibility to deal with materials able to perform at high temperatures is a fundamental and strategic need in a number of industrial fields ranging from material processing and production to transport industry, gas turbine design for both ground and aeronautics applications, (Gallardo et al., 2002). As far as concern gas turbine based power and propulsion systems a number of indicators for the performance evaluation of the system are available. The combustion efficiency is an important indicator in the power or thrust production and an increase of the combustion temperature usually correspond to a general increase in the system efficiency. Similarly, a reduction of the air volume needed, increases the system efficiency since the work lost in the compression phase for cooling the hotter components is also reduced. Consequently, increasing the operational temperature for the material of the hotter points can results in a general increase of the efficiency and performance of the entire system. Another important factor is the reliability in service, that is defined as the system service time or the flight time period between two successive maintenance operations. Increasing the material operational temperature has a direct impact in the overall operational life that will require less repair and maintenance actions with respect to material or alloy of lower performances. In the case of rotating machines, the increase of performance can be obtained increasing the specific >>>