Ziolkowski Pseudoelasticity of Shape Memory Alloys

Theory and Experimental Studies
1. Auflage 2015
ISBN: 978-0-12-801801-9
Verlag: Elsevier Science & Techn.
Format: EPUB
Kopierschutz: 6 - ePub Watermark

E-Book, Englisch, 270 Seiten

Pseudoelasticity of Shape Memory Alloys: Theory and Experimental Studies is devoted to the phenomenon of pseudoelasticity (superelasticity) exhibited by shape memory alloy materials. It provides extensive introductory content on the state-of-the-art in the field, including SMA materials development, definition of shape memory effects, and discussions on where shape memory behavior is found in various engineering application areas. The book features a survey of modeling approaches targeted at reliable prediction of SMA materials' behavior on different scales of observation, including atomistic, microscopic, mezoscopic, and macroscopic. Researchers and graduate students will find detailed information on the modern methodologies used in the process of building constitutive models of advanced materials exhibiting complex behavior. - Introduces the phenomenon of pseudoelasticity exhibited by shape memory alloy materials - Features a survey of modeling approaches targeted at reliable prediction of SMN materials' behavior on different scales of observation - Provides extensive coverage of the state-of-the-art in the field - Ideal reference for researchers and graduate students interested in the modern methodologies used in the process of building constitutive models of advanced materials

Andrzej Ziólkowski is an associate professor at the Institute of Fundamental Technological Research, Polish Academy of Sciences, Warsaw, Poland. He was awarded his PhD degree by the Scientific Council of the Institute of Fundamental Technological Research, Polish Academy of Sciences - IPPT PAN, for work devoted to 'Problems in pseudoelasticity of shape memory alloys” and was awarded a postdoctoral degree of qualification for associate professor for his dissertation 'Pseudoelasticity of shape memory alloys, experimental studies and theoretical description” (2007). His research interests are in continuum mechanics and thermodynamics, solid-solid phase transitions, special materials constitutive modeling, smart materials, unconventional energy sources. Dr. Ziólkowski's scientific publications are chiefly devoted to different aspects of shape memory alloys behavior. He was co-organizer of a Shape Memory Alloys Workshop and has been a member of the scientific committee of Symposiums, Shape Memory Materials for Smart Systems at the E-MRS Fall Meeting Conferences in Warsaw in 2005 and 2007. He is author and co-author of about 20 original scientific papers devoted to shape memory alloys.

Autoren/Hrsg.

Ziolkowski, Andrzej

Leseproben

Preface

The quest for new materials with properties superior to those already attainable, more useful for a specific purpose, or exhibiting a combination of required qualities is a continuous subject of interest to researchers and engineers all over the world. A very interesting group of metallic materials make up so-called shape memory alloys (SMA), revealing a number of extremely useful practical applications, combinations of functional properties, and utility features not observed together in other metallic materials. For that reason, these materials frequently earn such attributes as smart, intelligent, or they are sometimes called living composites. Intelligence and smartness are inherent attributes of living creatures, and they can be inherited by SMA materials probably to the extent that actual users of them exhibit these virtues, but nobody should object when SMA materials are called adaptive materials. The nice, from an engineering point of view, features of SMA materials do not come at any cost. Their behavior is very complex, and their efficient use requires comprehensive experimental knowledge about their performance when submitted to various thermomechanical loadings, as well as possessing a credible theoretical model that enables SMA materials to have consistent characterization and the quantitative prediction of appearing stresses, deformations, and/or thermal effects.

The aim of this book is to provide experimental evidence of SMA materials' pseudoelastic behavior and a theoretical framework for the efficient and accurate constitutive modeling of SMA materials' pseudoelasticity. The term pseudoelasticity is understood here as the loading-unloading cycle involving hysteresis, with the underlying assumption that the state of the material remains the same/does not change after termination of such a loading cycle (no change of material properties occurs). In the materials science literature, this range of SMA materials' behavior is frequently called superelasticity, the term stemming from the SMA property that elastic strains involved are close to two orders of magnitude larger than in the case of classical engineering materials (e.g., structural steel). The terms superelasticity and pseudoelasticity, in the sense delineated above, can be treated as equivalent.

The above definition of SMA pseudoelasticity presumes no change of properties of the material due to the loading-unloading cycle; thus, the considerations present in this book do not embrace in a direct way the Mullins effect, which can be treated as a manifestation of pseudoelastic behavior of rubber-like materials and that is caused by progressive damage to the material due to the sequence of loadings. The general philosophy adopted in this book is that we already have an SMA material with stable properties, by whatever means it is achieved. By stable we mean stable in time, in the intended operational regime, which in one case will mean stable for one cycle of work and in another stable during 100-1000 cycles or even 1 million cycles of operation. Whatever the values of modeling parameters and functions characterizing SMA material, they are assumed to be invariable. Thus, the book does not deliberately discuss strategies on how to obtain SMA material with specific features, which the author believes is the primary task of materials science specialists. The procedures of thermomechanical treatments, training procedures of SMA, and evolution of SMA materials properties are not discussed, nor are material functional or utility properties degradation due to cyclic loadings.

The book focusses on delivering experimental evidence about the essential features of macroscopic, thermomechanical behavior of SMA materials. Upon careful analyses and discussion of the experimental results presented, the concepts and ideas are introduced useful in description of macroscopic, pseudoelastic behavior of SMA materials, within the context of continuum mechanics and nonequilibrium thermodynamics. The worked-out idealizations and simplifications serve as an introduction to and development of a family of so-called RL models of pseudoelasticity, the first member of which was originally elaborated as a result of Polish-French-Japanese collaboration by Raniecki, Lexcellent, and Tanaka at the beginning of 1990s.

Shape memory effects (SME) of metallic materials, pseudoelasticity among them, originate on physical grounds from thermoelastic, martensitic phase transformation (p.t.). The latent heat of this phase transition is quite considerable in the case of some SMA materials, which results in strong coupling of two physical fields: mechanical and thermal. For example, in the case of the current commercially dominant SMA material (i.e., NiTi alloy), the thermomechanical coupling originating from latent heat of martensitic p.t. may lead, in adiabatic conditions, to an increase of temperature on the order of 50 °C; an effect that can hardly be disregarded.

The formalism of continuum, macroscopic, nonequilibrium thermodynamics with internal state parameters is employed in this book for phenomenological modeling of SMA materials behavior and characterization. Nevertheless, information from all levels of micro-, meso-, and macroscopic observation has been used in building an effective macroscopic theory of pseudoelasticity. Crystallographic and/or compound martensitic variants, objects that can be distinguished at a microscopic scale of observation, are grouped together in mesoscale to distinguish larger generic objects: self-accommodating martensite and oriented martensite. Information on specific properties and behavior of microlevel and mesolevel objects in their respective scale of observation enters the macroscopic, constitutive model only in indirect, tacit way.

For example, mesomechanical studies are executed to obtain a physical interpretation of terms present in a proposed heuristically form of SMA materials macroscopic Gibbs free energy function. The so-called representative volume element (RVE) of the SMA material, a material point with homogeneous properties in a macroscopic model, is treated as a two-phase (multiphase) elastic medium with eigenstrains, its microstructure evolving due to applied thermomechanical loadings. The assumption on continuity of the displacement field in the RVE done in these studies originates from microscopic observations of two-phase austenitic, martensitic microstructures. The incompatibility of phase eigenstrains and constraint of continuity of the displacement field required on interphase boundaries are the key sources for coherency energy stored in SMA material RVE. In order to determine the RVE macroscopic effective properties, it is actually treated as a composite with frozen—at some arbitrary stage of loading—and very complex mesostructure to which homogenization methodology is employed.

An effective description of any physical phenomenon requires reaching an amicable compromise between model intricacies and the accuracy required from it for specific purposes. It seems that such an amicable compromise has been reached in the family of RL models of pseudoelasticity thanks to the introduction of only two internal state parameters. The scalar parameter is the volume fraction of martensite (oriented one), and the tensorial parameter is the so-called ultimate phase eigenstrain. The product of volume fraction of martensite and ultimate phase eigenstrain gives macroscopic phase strain. Evolution of these two parameters with external loading of macroscopic stress and temperature allows predicting the value of macroscopic phase strain describing deformation effects of SMA materials connected with martensitic phase formation, its presence and its mesostructural evolution—reorientation of martensitic phase.

Extensive study and analysis of experimental data on the macroscopic behavior of SMA materials in the pseudoelastic range of their behavior, and the belief that matter organization is subject to some higher ordering principles, led to the formulation of rules governing the evolution of internal state parameters.

The so-called rule of optimum mesostructure rearrangement of SMA material RVE has been noticed and worked out. It is presumed valid in this entire book. Exploitation of this law leads to the property that ultimate phase eigenstrain tensor always follows/has the direction of macroscopic stress tensor. As a consequence, RL model theory predicts that macroscopic phase strain tensor always has the direction of macroscopic stress tensor in the pseudoelastic range of SMA materials behavior.

A primary question arises regarding what is actually the key reason for the appearance of pseudoelastic behavior—hysteresis loop in SMA materials behavior, taking into account the axiom posed earlier that upon the loading-unloading cycle the properties and the state of SMA material remains unchanged. In that respect, the fundamental conjecture is adopted that it is phase instability of RVE's mesostructural arrangement that causes the appearance of pseudoelastic behavior. Unstable phase equilibrium states lead to the abrupt initiation of qualitative reorganization of SMA material internal mesostructure. The conjecture is used to formulate the criteria of initiation of forward and reverse phase transitions directly, consistently, from the adopted form of Gibbs potential. No ad hoc threshold values are proposed for phase...