Controlled shape-morphing metallic components for deployable structures

Nanoscale precipitates have been proved to affect the transformation behavior of shape memory alloys (SMA) significantly. In this work, the effects of Heusler precipitates (Ni 2 TiAl) on the Ni 50 Ti 24.5 Hf 22.5 Al 3 SMA were systematically investigated. Density functional theory (DFT) calculations showed that coherent Heusler precipitates can mechanically stabilize the austenite phase, indicating the decrease of transformation temperatures. This trend was further confirmed experimentally. The depression of transformation temperatures with the increase of heat treatment time was observed as nanoscale coherent precipitates nucleated and grew. This was followed by an abrupt increase in transformation temperatures when aging time exceeded 200 hours at 700 °C. We attribute this increase to the loss of coherency between the Heusler precipitates and matrix which weakened their mechanical coupling. Detailed microstructure characterization by high-resolution transmission electron microscopy (TEM) and energy-dispersive X-ray spectroscopy (EDS) were provided to support the finding.

This paper describes the development of fuzzy systems for modeling the hysteresis behavior of shape memory alloy (SMA) actuators. Due to their simplicity and ease of actuation, SMA actuators are very attractive for applications such as miniature robots for micro manufacturing. However, SMAs have not been widely used for motion control applications due to their nonlinear behavior and control difficulties. One approach to design a position controller for SMA systems is to employ an inverse-model of the system in the control loop to compensate the hysteresis properties of the material. Fuzzy systems, due to their nonlinear learning and adaptation abilities, are good candidates for obtaining inverse-models. In this paper two fuzzy modeling approaches are employed and compared to develop a model for a SMA wire actuator. A set of experiments are conducted to generate the training data. The test stand includes a Nickel-Titanium (TiNi) SMA wire, a position sensor, a bias spring and a current amplifier. By comparing the performance of the two employed fuzzy modeling techniques, it is revealed that the approach based on fuzzy Gustafson-Kessel (GK) clustering shows a better performance in the modeling of the hysteresis in the SMA wire. Thus, GK clustering algorithm is employed to develop the inverse-model for the SMA. The reported results demonstrate the ability of the employed fuzzy algorithm for modeling the hysteresis in the system, and the merits of the introduced inverse-model in the control of the position of the SMA.

The objective of this study is to evaluate the effect that mechanical training has on the properties of NiTi based shape memory alloys. The unique mechanical behavior of shape memory alloys, which allows the material to undergo large deformations while returning to their original undeformed shape through either the shape memory effect or superelastic effect, has shown potential for use in seismic design and retrofit applications for civil engineering structures. However, cyclic loading has been shown to degrade the energy dissipation capacity and decrease the recentering capability of the material due to fatigue effects. It has been recommended that mechanical training of superelastic shape memory alloys prior to use in applications can limit these fatigue effects. A factorial experimental design is employed to explore the optimal number of mechanical training cycles, strain level of training, and the effect of the loading rate after training in order to minimize the degradation in the loading plateau stress, residual strain, and equivalent viscous damping properties. The results presented can serve as a guide to optimizing the properties of NiTi shape memory alloys for seismic applications. The ability to obtain stable properties of shape memory alloys under a specified training schedule further supports the eventual implementation of the material into actual building and bridge systems as seismic design and retrofit devices.

Micro-mechanical behaviors of SMA composite materials under hygro-thermo-elastic strain fields

The temperature memory effect of shape memory alloy (SMA) induced by an incomplete martensitic inverse phase transformation has the potential values in various engineering fields. It is of theoretical and practical interests to establish a phase transformation model which overcomes the limitation that previous phase transformation models of SMA fail to take into account the temperature memory effect. In this paper, the curves of heat flow versus temperature of SMA during the martensitic phase transformation and martensitic inverse phase transformation are described as the quadratic functions of temperature. A new phase transformation model of SMA is developed based on the differential relationship between martensitic volume fraction and phase transformation free energy. Numerical results show that the new model well predicts the phase transformation behaviors of SMA associated with the temperature memory effect.

The stress-strain relationship and phase transformation behavior of shape memory alloy (SMA) in pure shear state are investigated by using Zhou's SMA micromechanical constitutive equation of 3-dimension and Zhou's SMA phase transformation equation of 1-diemsion respectively. Then thermo-mechanical model describing thermo-mechanical behaviors of SMA torque thin-walled tube is established based on the stress-strain relation and phase transformation model of SMA in pure shear state. The relations of torque vs. rotation, recovery rotation vs. temperature and recovery torque vs. temperature of the torque tube are numerically simulated by using the thermo-mechanical model.

Micromechanical analysis of precipitate effects on shape memory alloys behaviour

Shape memory alloys (SMA) can be used to create actuators for use in mechanical systems that carry pronounced benefits with their low weight, high strength, and low cost when coupled with advancements such as robust self-sensing. However, there exist drawbacks in the form of slow system response and complex material behavior. The design and implementation of controllers that drive SMA actuators successfully can pose a challenge, and accurate modelling of the material in software can help to optimize the system response time and power requirements. We have created a variety of tools to help implement these actuators into simulations that capture accurate thermal and mechanical responses of various SMA systems under a variety of control laws. In particular, thermo-electro-mechanical models of SMA behavior are implemented into software representations of mechanical systems such that the entire temperature, stress, and phase profiles of an SMA actuator can be accurately determined via simulation. Because analytical equations for modelling SMA behavior quickly become so complex to be untenable, we use stable numerical schemes to solve for these profiles. In particular, a finite difference scheme allows for spatial and temporally discretized temperature profiles along a shape memory actuator which can be solved for using empirically derived expressions for the heat transfer coefficient that dictated convective heat transfer. These types of models allow for a variety of boundary conditions to capture a number of SMA geometries, orientations, and applications. This paper presents the results of the numerical schemes for thermal cycling and a sliding mode controller used to drive a simple SMA actuator with varying boundary conditions and compares them to experimental results.

The three-dimensional incremental finite element formulation for the multiaxial behavior of shape memory alloy devices is proposed in the present study by considering the coupling effect of the axial and torsional behaviors of shape memory alloys. The previously proposed one-dimensional constitutive model for shape memory alloy devices is extended to take account of the multiaxial stress state introducing some new material constants. The calculated results are compared with the uniaxial, purely torsional and multiaxial test results for NiTi tubes to illustrate the validity of the proposed computational modeling.

: This project is inserted within the scope of the technology of morphing aircrafts. Specifically, the research team investigated, in a multidisciplinary framework, the design methodology of variable camber morphing wings based on the use of active materials, namely piezoelectric materials and shape memory alloys. The research work has been carried out in partnership with Prof. Daniel J. Inman, from the Department of Aerospace Engineering of the University of Michigan. After preliminary assessment of various innovative candidate morphing strategies, the research evolved to the concept of active meta-structures based on shape memory alloys and permanent magnets, which is currently in development in the context of a Ph.D. thesis jointly advised by Profs. Domingos A. Rade and Daniel J. Inman. The main accomplishments made are the following:1.development of a methodology for the multidisciplinary design of variable camber morphing wings based on finite element simulations, encompassing the thermo-mechanical behavior of shape memory alloys and aerodynamic transient analysis. This methodology enables to evaluate the evolution of the aerodynamic forces and moment during the morphing transition.2. conceptual design and tests of variable camber aerodynamic profiles combining shape memory alloy wires and permanent magnets to achieve multi-stable configuration for morphing purposes

Harvested vibration energy is typically considered as an alternative power source for sensors' networks for health and usage monitoring in civil and mechanical structures. The longevity, and hence the efficacy, of these sensors is severely limited by the levels of generated power. Piezoelectric vibration harvesters have been widely used given their energy conversion ability and relatively high mechanical to electrical coupling properties. Several techniques can be applied to improve these properties and to cancel external environmental effects such as temperature variations. In this paper, the temperature compensation of the response characteristics of a bimorph cantilever Lead zirconate titanate (PZT) piezoelectric beam, through a combination with shape memory alloys, is studied. A mathematical model, based on onedimensional linear piezoelectricity equations and one dimensional constitutive behavior of shape memory alloys, is derived. The model describes the effect of temperature deviations on the theoretical harvestable energy levels as well as the compensation methodology. Proof of concept experimental results are also presented. The voltage response transfer functions are measured at different temperatures to show the induced effect by shape memory alloys.

The utilization of novel engineering materials like shape memory alloys has significantly broadened their application scope within civil engineering. Consequently, this study examines the mechanical performance of a grooved metallic-yielding damper fabricated from shape memory alloys. The seismic performance of the new system has been studied using the nonlinear static analysis method in ABAQUS software. Also, after a parametric investigation, the applicability of this kind of damper in a steel frame has been evaluated using the nonlinear dynamic analysis and considering the accelerograms of near-fault and far-fault earthquakes. The superelastic behavior of shape memory alloys has been applied to ABAQUS software by using the Brinson model and considering phase transformations. Lastly, the effect of various geometric parameters on the steel frame’s seismic performance, including stiffness, ductility, and energy absorption capability, has been studied. The results indicate that the ductility of the presented new damper is 56 and is greater than ductility of many conventional metal damper. Also, studied damper has great energy absorption and can easily replace other metallic-yielding dampers including ADAS and TADAS. Moreover, it is evident that the proposed system exhibits substantially enhanced energy absorption and ductility equated to the braced frame equipped with the equivalent steel damper, thus rendering it suitable for engineering applications.

In copper-based shape memory alloys (SMAs), some exceptional phenomena, such as the shape memory effect (SME) or superelasticity (SE), are observable. However, commercial aluminum bronzes, Cu3Al-based alloys, do not present these functional properties (SME and/or SE) in their original state. Thus, since one of the main copper-based SMA systems is the Cu-Al-Ni alloy, this paper aims to analyze the modification of these commercial aluminum bronzes to SMA by the addition small amounts of Cu, Al and/or Ni. These modified bronzes were reprocessed by induction melting and injected by centrifugation into a ceramic coating mold. The modifications were made to determine the nominal composition for a Cu-13,0Al-4,0Ni (%wt) SMA. The effectiveness of the modifications was verified by differential scanning calorimetry (DSC) thermal analysis. All modified Cu-Al-Ni bronzes presented DSC peaks of the thermoelastic martensitic phase transformation, showing that SMA behavior was achieved, while the non-modified bronzes revealed no transformation. These results were supported by Vickers hardness (HV), X-ray diffraction (XRD), semi quantitative composition by EDS analysis and optical microscopy.

The deformation behavior of porous shape memory alloy (SMA) is determined both by the features of their inhomogeneous structure and the martensitic transformation. The present work shows the results of simulation aimed at clarifying the influence of these factors on the behavior of porous SMA. The structures of porous samples with different orientations of the pore channels are approximated by sets of beams of various configurations, and the constitutive relations of the microstructural model are used for calculations of the strain associated with martensitic transformations. The calculations are performed for a dense SMA sample, porous SMA samples with longitudinal and transverse orientation of the pore channels and similar porous samples that do not undergo martensitic transitions. Simulations of isothermal compression at different temperatures and strain variations at cooling and heating under a constant stress are carried out. It is shown that the reorientation of martensite in a dense SMA specimen begins at a higher stress than in porous specimens. Moreover, a dense sample has a higher yield stress in both austenitic and martensitic states. At the same stress level, a porous SMA sample with transverse pore channels accumulates a larger strain than a sample with longitudinal channels. The same relation is observed for similar samples of an alloy without transformations. Porous SMA accumulates greater strain in comparison with common porous metal due to the reorientation of martensite and pseudoelasticity. The largest strain accumulated on cooling under a constant stress among the samples under consideration occurs in a porous sample with transverse pore channels.

This paper focuses on the thermo-mechanical behaviors of shape memory alloy (SMA) board with a crack subjected to bending load. The stress field near the crack tip of SMA board is described according to the solution of linear elastic mechanics. The phase transformation behaviors of SMA board are formulated based on Zhou’s one-dimensional phase transformation model. The phase transformation zone equation is derived to describe the phase transformation zone near the crack tip.