A phenomenological model describing the mechanical behavior of polycrystalline solids undergoing stress-induced phase transformations is presented in the setting of three-dimensional (3-D) media. The model, which is developed within the framework of Generalized Standard Materials, has been built on a few basic assumptions, namely an admissibility condition for thermodynamic forces and a locking constraint for phase transformations. The macroscopic kinematic consequences of stress-induced phase transformations are described by a tensor named transformation strain. Numerical integrations of the resulting set of equations show that many features of shape memory alloys obtained by certain authors for CuAlZnMn alloys under non-proportional loadings can be qualitatively described by the present model.

Three-dimensional model for solids undergoing stress-induced phase transformations

A new macro-scale model of shape memory alloys is developed within the framework of generalized standard materials with internal constraints [Moumni, Z, 1995 Sur la modelisation du changement de phase a l’etat solide PhD Thesis, Ecole Nationale Superieure des Ponts et Chaussees] It is shown that the introduction of two state variables: the martensite volume fraction and the martensite orientation strain tensor, is sufficient to account for several effects exhibited by SMAs subject to thermomechanical loading, namely: self-accommodation, orientation and reorientation or martensite, as well as superelasticity and one-way shape memory These phenomena are simulated using the same set of constitutive equations and evolution laws A simple procedure for identifying the parameters of the model is described in detail and a validation against experimental data is conducted The model is then used to analyze a 3D SMA structure representing a superelastic stent Cyclic loading and other pertaining phenomena like training and two-way shape memory are considered in the second part of this paper

A three-dimensional model of the thermomechanical behavior of shape memory alloys

The remarkable properties of shape memory alloys have facilitated their applications in many areas of technology. The purpose of this paper is to present an overview of thermomechanical behavior of these alloys, discussing the main constitutive models for their mathematical description. Metallurgical features and engineering applications are addressed as an introduction. Afterwards, five phenomenological theories are presented. In general, these models capture the general thermomechanical behavior of shape memory alloys, characterized by pseudoelasticity, shape memory effect, phase transformation phenomenon due to temperature variation, and internal subloops due to incomplete phase transformations.

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An overview of constitutive models for shape memory alloys

SUMMARY During mechanical loading-unloading cycles shape-memory alloys (SMA) are able to undergo large deformations without showing residual strains (pseudoelasticity) or recovering them through ther- mal cycles (shape memory eect ). Motivated by stress-induced solid phase transformations, these unique behaviours induce the SMA exploitation in innovative and commercially valuable applications, stimulating, consequently, the interest in the development of constitutive models. Also if many models are now available in the literature, eective three-dimensional proposals are still few and limited in several aspects. In this paper, a three-dimensional thermomechanical model recently proposed by Souza et al. (European Journal of Mechanics-A/Solids, 1998; 17:789-806.) is taken into consideration; such a model is of particular interest for its eectiveness andexibility, but it also shows some limitations and missing links in the algorithmical counterparts. This work discusses some improvements to the original model as well as the development and the implementation of a robust integration algorithm to be adopted in a numerical scheme, such as a �nite-element framework. Copyright ? 2002 John Wiley & Sons, Ltd.

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Improvements and algorithmical considerations on a recent three-dimensional model describing stress-induced solid phase transformations

The application of the eXtended finite element method (X-FEM) to thermal problems with moving heat sources and phase boundaries is presented. Of particular interest is the ability of the method to capture the highly localized, transient solution in the vicinity of a heat source or material interface. This is effected through the use of a time-dependent basis formed from the union of traditional shape functions with a set of evolving enrichment functions. The enrichment is constructed through the partition of unity framework, so that the system of equations remains sparse and the resulting approximation is conforming. In this manner, local solutions and arbitrary discontinuities that cannot be represented by the standard shape functions are captured with the enrichment functions. A standard time-projection algorithm is employed to account for the time-dependence of the enrichment, and an iterative strategy is adopted to satisfy local interface conditions. The separation of the approximation into classical shape functions that remain fixed in time and the evolving enrichment leads to a very efficient solution strategy. The robustness and utility of the method is demonstrated with several benchmark problems involving moving heat sources and phase transformations.

Solving thermal and phase change problems with the eXtended finite element method

A One-Dimensional Continuum Model for Shape-Memory Alloys

Practical sensing applications such as real-time safety alerts and clinical diagnoses require sensor devices to differentiate between various target molecules with high sensitivity and selectivity, yet conventional devices such as oxide-based chemo-resistive sensors and metal-based surface-enhanced Raman spectroscopy (SERS) sensors usually do not satisfy such requirements. Here, a label-free, chemo-resistive/SERS multimodal sensor based on a systematically assembled 3D cross-point multifunctional nanoarchitecture (3D-CMA), which has unusually strong enhancements in both "chemo-resistive" and "SERS" sensing characteristics is introduced. 3D-CMA combines several sensing mechanisms and sensing elements via 3D integration of semiconducting SnO2 nanowire frameworks and dual-functioning Au metallic nanoparticles. It is shown that the multimodal sensor can successfully estimate mixed-gas compositions selectively and quantitatively at the sub-100 ppm level, even for mixtures of gaseous aromatic compounds (nitrobenzene and toluene) with very similar molecular structures. This is enabled by combined chemo-resistive and SERS multimodal sensing providing complementary information.

Synergistic Integration of Chemo-Resistive and SERS Sensing for Label-Free Multiplex Gas Detection.

The present invention relates to a flexible transparent protective film including a colorless transparent polyimide film layer in which magnesium oxide nanoparticles are uniformly embedded. Transparent polyimide has excellent durability and can protect a flexible material from external environments so as to be very properly used in providing a transparent flexible substrate. In particular, oxide nanoparticles which can effectively block moisture by interactions with moisture in polyimide are embedded so that excellent protective film properties can be exhibited. An inorganic film can be further stacked on the top layer of a polyimide composite film in which oxide nanoparticles are installed, and a multi-layer structures on which magnesium oxide-polyimide composite films/inorganic films/magnesium oxide-polyimide composite films are continuously coated so that the same can be used as an excellent protective film material.

Sang Joon Kim

Papers

A One-Dimensional Continuum Model for Shape-Memory Alloys

Synergistic Integration of Chemo-Resistive and SERS Sensing for Label-Free Multiplex Gas Detection.

Fabrication of mgo nanoparticles embedded colorless polyimide film as encapsulation and it's multi-stacking passivation film