Brian T. Lester

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

Modeling of the Effective Actuation Response of SMA-MAX Phase Composites With Partially Transforming NiTi

Shape Memory Alloy (SMA) composites are being increasingly investigated to address a variety of engineering problems. An application of growing interest is an SMA-MAX phase ceramic composite for use in extreme environments. By joining these two constituents, it is intended that the martensitic transformation of the SMA phase may be used with the unique kinking behavior of the MAX phases to improve the composite response. One particular intended outcome of this utilization is the development of residual stress states in the composite. These residual stress states are generated due to the formation of irrecoverable strains resulting from the interaction of the inelastic mechanisms in the system. By tailoring this stress state, the improved mechanical response of the ceramic phase under compression may be taken advantage of. These residual stress states and their effect on the effective thermomechanical response of the composite are explored in this work. To this end, a finite element model of the composite is development. Specifically, a recent 3D phenomenological constitutive model of the SMA phase is incorporated to describe the effects of martensitic transformation and a constitutive assumption for the MAX phase response associated with kink band formation is introduced. An additional non-transforming NiTi phase is noted and the role of its constitutive response is considered. This model is used to study the micromechanics of the associated composite residual stress states. The influence of these residual stresses on the effective actuation response is then investigated and the on the associated composite behavior determined. Specifically, it is shown that the variation in inactive NiTi leads to an altered actuation response.Copyright

Modeling of Residual Stresses in Shape Memory Alloy - Ceramic Composites

Metal-ceramic composites combining Shape Memory Alloys (SMAs) with MAX phase ceramics are gaining interest due to their unique combination of inelastic constituents. Specifically, these composites, intended for extreme environments, are being explored to take advantage of the reversible martensitic transformation associated with SMAs and inelastic deformations associated with kink band formation in the MAX phases. By using the combination of these mechanisms, a superior composite response may be observed. Specifically, residual stress states may be developed in these composites to take advantage of specific material responses (i.e. improved mechanical response of ceramics under compression). In this work, a microstructurally informed finite element model of this composite is used to study the effect and development of these residual stress states. A recent thermomechanical constitutive model is used for the reversible transformation of the SMA phase while an constitutive approximation is utilized to describe the response of the MAX phases. The effect of additional irrecoverable strains from another phase on the residual stress state is studied and the interaction of the different mechanisms discussed. In this way, it is shown that such deformations increase the residual stress magnitude in the ceramic phase. Furthermore, the impact of the loading path is examined and the dominance of the cooling cycle in developing the residual stresses in the composite is shown. In this context, the micromechanics of the developed residual stress states are discussed.

Modeling of Hybrid Shape Memory Alloy Composites Incorporating MAX Phase Ceramics

A new hybrid shape memory alloy (SMA)-ceramic composite is being developed for use in extreme environments. The proposed composition is intended to address past issues with brittle failure in the ceramic phase by generating a compressive residual stress state in that phase. This biasing load takes advantage of the superior mechanical properties of ceramics when loaded in compression. Past investigations of SMA composites with an elasto-plastic second phase have shown that such a residual stress state may be developed in the second phase through the generation of irrecoverable, plastic strains. To take advantage of this characteristic, a class of ceramics known as the MAX phases are being used. These ceramics have a unique response characterized by the formation of kink bands which can allow for both recoverable and irrecoverable inelastic strains. A model of the hybrid composite incorporating this response and the effects of the microstructure is developed to explore the ability of this material system to generate such stress states. To this end, an approximation of the MAX phase response is introduced to describe the irrecoverable kink band formation. The effects of the microstructure are accounted for through the generation of a finite element mesh from microtomography results of the considered composite. Finite element simulations of the hybrid composite are performed using the assumed MAX phase response and a recent 3D phenomenological SMA constitutive model. The effective stress-temperature response of the composite is determined and the interaction of the different phases is discussed. Specifically, it is shown that composite still exhibits a hysteretic response although with a decreased hysteresis height and shifted transformation temperatures. The effect of the microstructure on the composite response is discussed. Finally, it is shown that through an actuation loading path a compressive residual stress state is developed in the ceramic phase.Copyright

Computational Micromechanical Modeling of Ceramic-SMA Composites

these two inelastic materials provides the possibility of developing residual stress fields in the composite. These stress fields could be used to apply a prestress on the ceramic phase which takes advantage of the superior mechanical response exhibited under compressive loadings. To explore the development of these residual stress fields and the e↵ective actuation response of the considered composite, a computational micromechanical model is developed. To account for the heterogeneous microstructure in such material systems, image-based techniques are used to develop finite-element meshes based on characterization of an actual composite microstructure. A recent SMA constitutive model is used to describe the response of that phase while an elastic-plastic approximation of the MAX phase is introduced. The resultant model is then used to explore the e↵ective actuation response and interaction of the inelastic phases. An interesting shift in the transformation temperatures necessary to induce transformation is observed and the e↵ect explained. It is also demonstrated that when subjected to su cient loads, irrecoverable strains are generated in the composite. The e↵ect of these strains and the residual stress fields they cause are then explored and discussed. It is demonstrated that the desired compressive residual stress may be generated through an appropriate thermomechanical processing path.

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