A constrained domain-switching model for polycrystalline ferroelectric ceramics. Part I: Model formulation and application to tetragonal materials

A micromechanical model is proposed to study the constrained domain-switching process in polycrystalline ferroelectric ceramics. It is assumed that the depolarization field induced by domain switching is completely compensated by free charges, while the stress caused by non-180° switching is considered in an Eshelby inclusion manner. The model assumes that each grain contains multi-domains and the domain-switching criterion is based on potential energy density. Two switching options, which are based on Hwang et al. [Hwang SC, Lynch CS, McMeeking RM. Acta Metall Mater 1995;43:2073] and Berlincourt and Krueger [Berlincourt D, Krueger HHA. J Appl Phys 1959;30:1804], are used in the model development. Details of the switching process are analyzed for tetragonal ferroelectric/ferroelastic ceramics under electric loading or uniaxial compression (tension) by using an inverse-pole-figure method. Numerical results show that during electric poling, only a few per cent 90° switching can occur in BaTiO₃ ceramics, which agrees well with experimental observations.     

A temperature-dependent two-step domain-switching model for ferroelectric materials

Piezoelectric stack actuators are promising candidates for use in fuel injection technology. Experimental results for stack actuators have shown that a significant amount of heat is generated when they are driven under high electric-field magnitudes and/or high frequency, both of which occur in fuel injectors. They also exhibit hysteretic nonlinear behavior when driven under high electric-field magnitudes. In this paper, a new domain-switching model for PZT materials is developed. The model is based on changes in potential energy, and accounts for the temperature effect on domain switching. It also accounts for full thermo-electro-mechanical coupling. Additionally, different energy levels are assumed for different domain-switching types. It is assumed that 180° switching is a two-step process caused by two 90° switchings. A finite element implementation of a thermopiezoelectric continuum based on the proposed switching model is presented. The model shows good agreement with experimental results at different temperatures and loading conditions.     

A polycrystalline mechanical model for bulk nanocrystalline materials using the energy approach

To systematically understand the grain size, strain rate and defect development dependent mechanical behavior of bulk nanocrystalline materials, a new constitutive model is proposed to describe the deformation mechanism, microstructure evolution and mechanical response of bulk nanocrystalline materials using the energy approach. In this model, the grain interior and grain boundary were not taken as two independent phases with different volume fractions, but as an integral object sustained dislocation and accommodated grain boundary sliding mechanisms. Meanwhile, defect creation and evolution and their effects on the overall stress–strain relation as well as the failure process of bulk nanocrystalline materials were considered in the model. For experimental verification, we have prepared nanocrystalline Ni powder by the DC arc plasma evaporation method. Bulk nanocrystalline Ni samples were then made by compaction and hot sintering. Experimental measurements on the mechanical response of bulk nanocrystalline Ni were performed under different strain rates and grain sizes. Comparison between experimental data and model predictions show that the method developed appears to be capable of describing the mechanical response of bulk nanocrystalline materials. The model applications to nanocrystalline Mg and Cu have shown that it can reflect the asymmetric defect development between tension and compression under quasi-static conditions; this results in its good capacity to describe the dynamic strain rate sensitivity and strain hardening behavior over a relatively large strain range under both compression and tension conditions.     

A dynamic model of the rolling process. Part II: inhomogeneous model

Mathematical models representing the static rolling process have attracted considerable attention in the past, resulting in analytical, numerical, or graphical solutions obtained under various degrees of simplification and constraints. Most models, however, can only be used under steady conditions, and therefore are not suitable for the study of rolling chatter. An enhanced analytical process model that can handle dynamic variations exerted by roll vibrations in multiple directions is proposed in this paper, and its linearized form established for easy analysis. Finally, experiments are presented that verify the accuracy of the proposed dynamic model of the rolling process.     

多晶材料细观力学建模与模拟软件的开发与应用

Thermal Prophet-RVE是最近自主开发的一款基于实际微观组织数据的多晶材料细观力学模拟软件.本文着重对其软件架构、基本功能及技术特点进行介绍,并模拟了铁素体/马氏体双相钢生产过程马氏体相变引起的变形和双相组织在拉伸载荷作用下的细观力学行为.结果表明,基于材料实际微观组织数据构建的微结构模型能够高度还原组织...     

A sintering model for plasma-sprayed zirconia thermal barrier coatings. Part II: Coatings bonded to a rigid substrate

The sintering model described in Part I, which relates to free-standing plasma-sprayed thermal barrier coatings, is extended here to the case of a coating attached to a rigid substrate. Through-thickness shrinkage measurements have been carried out for coatings attached to zirconia substrates, and these experimental data are compared with model predictions. The model is then used to explore the influence of the substrate material (zirconia vs. a nickel superalloy), and of the in-plane coating stiffness. Both differential thermal expansion stresses and tensile stresses arising from the constraint imposed on in-plane shrinkage can be relaxed via two diffusional mechanisms: Coble creep and microcrack opening. This relaxation allows progression towards densification, although the process is somewhat inhibited, compared with the case of a free-standing coating. Comparison of the stored elastic strain energy with the critical strain energy release rate for interfacial cracking allows estimates to be made of whether debonding is energetically favoured.¹     

A new thermomechanical model of cutting applied to turning operations. Part II. Parametric study

In Part I of this work, Molinari and Moufki [Int. J. Mach. Tools Manufact., this issue], an analytical model of three-dimensional cutting is developed for turning processes. To analyse the influences of cutting edge geometry on the chip formation process, global effects such as the chip flow direction and the cutting forces, and local effects such as the temperature distribution and the surface contact at the rake face have been investigated. In order to accede to local parameters, the engaged part in cutting of the rounded nose is decomposed into a set of cutting edge elements. Thus each elementary chip, produced by a straight cutting edge element, is obtained from an oblique cutting process defined by the corresponding undeformed chip section and the local cutting angles. The present approach takes into account the fact that for each cutting edge element the local chip flow is imposed by the global chip movement. The material characteristics such as strain rate sensitivity, strain hardening and thermal softening, the thermomechanical coupling and the inertia effects are considered in the modelling. A detailed parametric study is provided in this paper in order to analyse the effects of cutting speed, depth of cut, feed, nose radius and cutting angles on cutting forces, global chip flow direction and temperature distribution at the rake face. The influence of friction at the tool–chip interface is also discussed.     

Materials for ultrasupercritical coal power plants—Turbine materials: Part II

The efficiency of conventional boiler/steam turbine fossil power plants is a strong function of the steam temperature and pressure. Research to increase both has been pursued worldwide, since the energy crisis in the 1970s. The need to reduce CO₂ emission has recently provided an additional incentive to increase efficiency. Thus, steam temperatures of the most efficient fossil power plants are now in the 600 °C (1112 °F) range, which represents an increase of about 60 °C (108 °F) in 30 years. It is expected that steam temperatures will rise another 50 to 100 °C (90 to 180 °F) in the next 30 years. The main enabling technology is the development of stronger high-temperature materials, capable of operating under high stresses at ever-increasing temperatures. Recently, the EPRI performed a state-of-the-art review of materials technology for advanced boiler/steam turbine power plants (ultrasupercritical power plants). Results of this review pertaining to boilers are reported in a companion paper in this volume. This paper describes the results relating to steam turbines.     

Small fatigue crack growth in metallic materials: A model and its application to engineering alloys

Characterization of the growth behavior of small fatigue cracks is important for materials used in structurally demanding applications such as aircraft turbine discs and some automotive engine components. Here, we present a general, dislocation-based fracture mechanics approach to predict the growth rate of small fatigue cracks in metallic materials. The applicability of the model to the small fatigue crack growth behavior of four engineering alloys was examined. Small fatigue cracks were initiated and propagated, in a controlled manner, from micronotches fabricated by femtosecond pulsed laser micromachining. The results suggest that a methodology consisting of crack-tip damage accumulation and fracture provides a common framework to estimate the fatigue crack propagation lifetime of structural materials.     

A framework for automated analysis and simulation of 3D polycrystalline microstructures. Part 2: Synthetic structure generation

This is the second of a two-part paper intended to develop a framework for collecting data, quantifying characteristics and subsequently representing microstructural information from polycrystalline materials. The framework is motivated by the need for incorporating accurate three-dimensional grain-level morphology and crystallography in computational analysis models that are currently gaining momentum. Following the quantification of microstructural features in the first part, this paper focuses on the development of models and codes for generating statistically equivalent synthetic microstructures. With input in the form of statistical characterization data obtained from serial-sectioning of the microstructures, this module is intended to provide computational modeling efforts with a microstructure representation that is statistically similar to the actual polycrystalline material.     

A sintering model for plasma-sprayed zirconia TBCs. Part I: Free-standing coatings

A sintering model is presented for prediction of changes in the microstructure and dimensions of free-standing, plasma-sprayed (PS) thermal barrier coatings (TBCs). It is based on the variational principle. It incorporates the main microstructural features of PS TBCs and simulates the effects of surface diffusion, grain boundary diffusion and grain growth. The model is validated by comparison with experimental data for shrinkage, surface area reduction and porosity reduction. Predicted microstructural changes are also used as input data for a previously developed thermal conductivity model. Good agreement is observed between prediction and measurement for all these characteristics. The model allows separation of the effects of coating microstructure and material properties, and captures the coupling between densifying and non-densifying mechanisms. A sensitivity analysis is presented, which highlights the importance of the initial pore architecture. Predictions indicate that the microstructural changes which give rise to (undesirable) increases in thermal conductivity and stiffness are very sensitive to surface diffusion.¹     

An impedance model for the estimation of water absorption in organic coatings. Part II: A complex equation of mixture

An impedance model for the estimation of water absorption in organic coatings is presented. In part I, the model was developed taking a high frequency approximation. In part II the model is generalized by removing that approximation and by treating the film as a multicomponent system. Comparison with weight variation data has shown an improvement of the estimates made with the classical model of Brasher and Kingsbury. A low frequency approximation of the model has given good results with freestanding films.     

A dynamic model of the rolling process. Part I: homogeneous model

Mathematical models representing the static rolling process have attracted considerable attention in the past, resulting in analytical, numerical, or graphical solutions obtained under various degrees of simplification and constraints. Most models, however, can only be used under steady conditions, and therefore are not suitable for the study of rolling chatter. An enhanced analytical process model that can handle dynamic variations exerted by roll vibrations in multiple directions is proposed in this paper, and its linearized form established for easy analysis. Finally, experiments are presented that verify the accuracy of the proposed dynamic model of the rolling process.     

On the structure formation in polycrystalline metals and alloys: II. Recrystallization

Effects of the difference in the thermal expansion coefficients of grains and intergranular interlayers on the process of recrystallization in metals and alloys are considered.     

Analytical models for high performance milling. Part II: Process dynamics and stability

Chatter is one of the most important limitations on the productivity of milling process. In order to avoid the poor surface quality and potential machine damage due to chatter, the material removal rate is usually reduced. The analysis and modeling of chatter is complicated due to the time varying dynamics of milling chatter which can be avoided without sacrificing the productivity by using analytical methods presented in this paper.     

A new thermomechanical model of cutting applied to turning operations. Part I. Theory

In this paper, an analytical approach is used to model the thermomechanical process of chip formation in a turning operation. In order to study the effects of the cutting edge geometry, it is important to analyse its global and local effects such as the chip flow direction, the cutting forces and the temperature distribution at the rake face. To take into account the real cutting edge geometry, the engaged part in cutting of the rounded nose is decomposed into a set of cutting edge elements. Thus each elementary chip produced by a straight cutting edge element, is obtained from an oblique cutting process. The fact that the local chip flow is imposed by the global chip movement is accounted for by considering appropriate interactions between adjacent chip elements. Consequently, a modified version of the oblique cutting model of Moufki et al. [Int. J. Mech. Sci. 42 (2000) 1205; Int. J. Mach. Tools Manufact. 44 (9) (2004) 971] is developed and applied to each cutting edge element in order to obtain the cutting forces and the temperature distributions along the rake face. The material characteristics such as strain rate sensitivity, strain hardening and thermal softening, the thermomechanical coupling and the inertia effects are taken into account in the modelling. The model can be used to predict the cutting forces, the global chip flow direction, the surface contact between chip and tool and the temperature distribution at the rake face which affects strongly the tool wear. Part II of this work consists in a parametric study where the effects of cutting conditions, cutting edge geometry, and friction at the tool–chip interface are investigated. The tendencies predicted by the model are also compared qualitatively with the experimental trends founded in the literature.