48 research outputs found
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Inverse method for parameter optimisation in superalloy tertiary creep equations
A new methodology has been devised for the optimisation of material parameters in equations that govern the tertiary creep deformation of single crystal superalloys. Such information is ordinarily extracted by conducting a series of mechanical experiments over a range of appropriate environmental conditions, e.g. at various fixed stresses and temperatures. However, the current technique allows material behaviour to be characterised from a limited number of tests of short duration performed under non-uniform stress. A strategy is presented in which the time dependent strain response under a distributed stress gradient is measured using a novel testpiece geometry incorporating a concave gauge length profile. Spatial strain distribution is determined by accurate post-deformation measurement of specimen shape. Both spatial and temporal deformation are then simulated using a well founded mechanistic damage model, and the agreement between model results and experimental data is optimised by systematic perturbation of model parameters using the Nelder-Mead direct search method, i.e. an inverse modelling approach is applied. The overall strategy has been successfully, validated for SRR99 by direct comparison with a database of more conventional tensile creep data, but it has the potential for broad application in cost effective and efficient prototyping of new materials generally
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Characterization of superalloy tertiary creep by inverse modeling
The design of new alloys for high-temperature engineering applications frequently requires the rapid characterization of material mechanical behavior, particularly creep resistance. Such information is ordinarily extracted by conducting a series of time-consuming and costly experiments under uniform load or stress. To expedite material prototyping we have developed a technique that allows estimation of the stress-dependence of material behavior from a single, short-duration, tensile test performed under non-uniform stress. The approach involves inverse modeling of experiments conducted using a novel tensile testpiece with a concave gauge-length profile. Temporal strain and post-deformation spatial strain distribution are simulated using a well-founded mechanistic creep damage model, and agreement between model results and experimental data is optimized by systematic perturbation of model parameters. This inverse strategy has been validated by examining high temperature tertiary creep in three generations of nickel superalloy single-crystal materials, but has wider application to materials characterization generally
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A state-variable model for the hot deformation of Waspaloy
A semi-empirical history-dependent constitutive model for the high temperature deformation of Waspaloy is presented. Model equations have been formulated in terms of the evolution of state variables, in particular an internal back stress, directly related to the physical processes of deformation. Below the solvus temperature of the strengthening gamma' phase, deformation is modelled in terms of the hardening and recovery associated with the formation of a dislocation network. Above the gamma' solvus, modelling of material softening incorporates the kinetics of dynamic recrystallisation. Constitutive equations have been directly validated against uniaxial compression test data collected at temperatures and strain rates relevant to disc forging
Dutch experience in the utilization of evaluation research. The procedure of reconsideration
Doing Public Good? Private actors, evaluation, and public value
This book examines the contributions of non-public organizations, such as foundations, philanthropies, charities, non-governmental organizations, private businesses, and entrepreneurs to public goods and services. Too often the impact of the contributions of such private actors are overlooked. However, they are playing an increasing role in meeting societal needs across the developing world. Doing Public Good? lays out key elements that need to be considered in evaluating the net results achieved by these private actors. It uses case studies and analysis to show how to answer such questions as: Is it working? How do they and the public know they are doing good? And how to improve? Such questions are particularly important since little is known about the net results of private avenues for delivering public value.The contributors conclude that "doing good" organizations need to be more transparent and accountable regarding their operations and achievements. The book suggests perspectives on how better monitoring and evaluation systems can improve their accountability