Yves Chemisky

Transformation characteristics of shape memory alloy composites

Shape memory alloy (SMA) composites are being used in an ever-expanding set of applications. For new applications, SMA composites are being developed incorporating a wide variety of matrices. The effect of these new compositions on the transformation behavior of the SMA inhomogeneities and on the effective composite behavior is explored here. An analytic methodology combining micromechanical methods with an SMA constitutive model is developed to determine the overall transformation properties of the composite. Specifically, the effective phase diagram, the effective transformation strains and the composite stress state before and after transformation are determined. The results obtained from the analyses of an SMA–ceramic composite show that after transformation the stress distribution between the two phases is modified such that the stress in the direction of applied loading in the SMA phase is reduced while the stress in the same direction in the ceramic phase increases. This stress redistribution decreases the local transformation strain in the direction of loading and results in an increase of the applied stress necessary to initiate and complete the forward and reverse transformation at a specific temperature. The effects of the elastic modulus of the matrix and volume fraction of the SMA inhomogeneities on the transformation behavior are explored through a parametric study to understand their influence on SMA composite design.

Virtual Processing of Hybrid Shape Memory Alloy Composites

The capability of using recoverable martensitic transformation to modify the residual stress-state of hybrid Shape Memory Alloy (SMA) composites is explored. It is shown that through careful selection of a thermomechanical loading path the composite can be “processed” such that the constituent phases have a beneficial residual stress-state. Specifically, for materials which have preferred loading conditions (i.e., compression versus tension) resulting in improved material properties, such processing places the considered phase into a preferred stress state. This processing is explored here by considering composites with an SMA phase whose constititutive behavior is described by a recent phenomenological model and an elasto-plastic second phase. To consider realistic microstructural effects, a 3D numerical representation of the composite is generated using microtomography. It is shown that through an actuation (isobaric) loading path, the martensitic transformation of the SMA phase generates irrecoverable strains in the elasto-plastic phase which, upon unloading, results in a favorable residual stress-state. To consider the applicability of this methodology for a variety of composites, the effect of thermal residual stresses due to thermal expansion mismatch is identified and matrix phases with different elastic moduli and plastic hardenings are considered. Specifically, it is shown that martensitic transformation is the driving force behind the generation of the new composite residual stress-state. Through computational simulation, it is shown that increased elastic moduli or plastic hardening coefficients of the elasto-plastic phase yield small increases in residual stresses.© 2011 ASME

Continuum Mechanics and Constitutive Laws

The study of composites requires an in-depth knowledge of the material response of each individual component that appears in the microstructure. Local conservation laws apply in each material point, and constitutive laws must be formulated in a manner that allows us to determine the local and global response of a heterogeneous medium. This chapter presents a summary of the basic continuum mechanics concepts, i.e. the kinematics, kinetics and conservation laws. Moreover, there is a detailed presentation of the second thermodynamic law and the various energy and dissipation potentials that permit us to identify proper associated constitutive laws. Several examples from classical dissipative materials, i.e. materials in which thermodynamical irreversible processes take place, are presented. Finally, a methodology for identifying material parameters through appropriate experimental protocol is illustrated at the end of the chapter.

Composites with Random Structure

Nowadays, it is well established that the overall behavior of a composite strongly depends on the properties of the material constituents and the microscopic geometry, i.e. the volume fraction, shape and orientation of constituents. Homogenization methods, as pioneered by Hill, Hill and Rice and Hashin, allow us to study the overall mechanical behavior of polycrystalline metals and composites with a microstructure that contains particles or/and fibers. Many homogenization methods rely on the famous Eshelby’s equivalent inclusion theory, which gave rise to the micromechanics of media with random microstructure. Since then, more advanced techniques, most popular among them the self-consistent and the Mori-Tanaka techniques, have been developed and extended in order to study composites with many types of particles and various particle orientations. The differences between the classical mean-field theories (Eshelby dilute approach, Mori-Tanaka and self-consistent) have been extensively discussed in several books.

Composites with Periodic Structure

Material science has grown tremendously in the recent years to meet the extensive needs of several engineering applications. The automotive and aerospace industries require novel, innovative and multifunctional composite materials that can be utilized in complicated structures with high demands in strength, durability and long lifetime during repeated loading cycles.

Concepts for Heterogeneous Media

Abstract: All real materials present an internal structure that often varies spatially and possesses significant complexity: this internal structure is generally referred as the material’s microstructure. In the microstructure, local variations of the matter appear and these are defined as the heterogeneities. The presence of heterogeneities induces highly non-homogeneous physical fields like strain, temperature, etc., in the matter.

Virtual processing of hybrid SMA composites through martensitic transformation

The capability of Shape Memory Alloys (SMAs) to modify the reference configuration of an SMA-composite through martensitic transformation is explored. It is intended that through careful selection of a thermomechanical loading path the composite can be processed such that the constituent phases are in a preferential reference configuration. Specifically, for materials which have preferred loading conditions (i.e., compression versus tension), such processing results in a residual stress state which takes advantage of the improved properties. The composite under investigation is assumed to be composed of an SMA phase and an elasto-plastic second phase. For analysis of such a composite, a Finite Element (FE) mesh based on a realistic microstructure is constructed by using the results of X-ray tomography. The resultant microstructure is analyzed using FE techniques. It is shown that through an isobaric loading path, transformation generates plastic strains in the elasto-plastic phase which modify the composite reference configuration. The effect of different applied loads is considered.

Numerical Prediction of Effective Transformation Properties of Hybrid SMA-Ceramic Composites

Metal-ceramic composites are being increasingly explored in an effort to find new materials for use in extreme environments. Via functional grading of of the volume fraction of the constituant phases and other techniques, the material can be optimized to incorporate the mechanical properties of the metal phase with the thermal properties of the ceramic phase. To get further benefit of the metal phase, a new area being investigated is the incorporation of Shape Memory Alloys (SMAs). In order to predict the phase transformation features of an SMA embedded in a stiff ceramic matrix, a micromechanical approach is developed to find the effective phase diagram of the ceramic-SMA composite. From this analysis, other composite characteristics such as stress in each phase and the evolution of tranformation strain in the SMA can be determined in order to improve the design of such new composite materials.Copyright

Modeling of Precipitation Effects on the Thermomechanical Behavior of NiTi SMA

The effect of precipitation on Ni-rich NiTi Shape Memory Alloys on the thermomechanical properties is investigated. Particulary, the relation between the chemical evolution of the alloy and phase transformation characteristics are studied in order to evaluate the impact on the global mechanical response. The evolution of chemical composition of the alloy is computed using a diffusion law, according to the precipitates population characteristics. The mechanical response is simulated using a Representative Unit Cell which takes into account the structural effect of the precipitates as well as the effect of the composition change. The results show an important effect on the global mechanical behavior even for a small volume fraction of precipitates.Copyright