Journal of Intelligent Material Systems and Structures

This article is aimed at the experimental characterization and modelling validation of shape memory alloy Negator springs. A Negator spring is a spiral spring made of strip of metal wound on the flat with an inherent curvature such that, in repose, each coil wraps tightly on its inner neighbour. The main feature of a Negator springs is the nearly constant force–displacement behaviour in the unwinding of the strip, mounted on a rotating drum. Moreover, the stroke is very long, theoretically infinite as it depends only on the length of the initial strip. A Negator spring made of shape memory alloy is built and experimentally tested to demonstrate the feasibility of this actuator. The shape memory Negator spring behaviour is predicted both with an analytical model and with a finite element software. In both cases, the material is modelled as elastic in austenitic range while an exponential continuum law is used to describe the martensitic behaviour. The experimental results confirm the applicability of this kind of geometry to the shape memory alloy actuators, and the analytical model is confirmed to be a powerful design tool to dimension and predict the spring behaviour both in martensitic and austenitic ranges, as well as the finite element model developed.

Spinal fusion surgery is performed to alleviate low back pain, and a cage implant is a spacer that sits in between two vertebrae to allow for bone growth and fusion, all while relieving compression of the spinal cord. This paper presents the design, modeling, and experimental evaluation of a minimally invasive cage which utilizes superelastic Nitinol elliptical shaped hinges. The actuation mechanism is presented and the employed additive manufacturing technique is discussed. The modeling approach is also introduced and experimental results are presented to demonstrate the performance of the model.

This article presents two innovative adaptive solutions for the ankle–foot orthosis based on mechanical and structural stiffness control of shape memory alloys. These concepts address gait abnormality in drop foot patients for various walking conditions such as different walking speeds. In the first design, a superelastic rod provides variable torsional stiffness that is adjusted by a controlled axial load. In the second design, the active length of superelastic hinge is adjusted in order to control the bending stiffness of the element. By adjusting the stiffness, variable level of compliance is achieved at the ankle. In both concepts, during powered plantarflexion in the stance phase of the gait, energy is stored in the shape memory alloy element. Release of this energy through superelasticity enables the ankle–foot orthosis to provide the desired controlled dorsiflexion motion in the sagittal plane and to raise the foot during the swing phase of the gait. The ultimate goal is to assist the patients in achieving a more natural gait and to prevent muscle atrophy. For the presented designs, numerical simulations are carried out to evaluate the stiffness properties of the active component under different gait speeds. To this end, experimental data of human gait are used to calculate the variation in ankle stiffness. The superelastic elements mimic the experimental ankle stiffness profiles.

A thermo-mechanical model to simulate the mechanical response of a shape memory alloy heat engine has been developed. In such a kind of engine, the thermally induced shape recovery capabilities of shape memory alloys are exploited to obtain mechanical work from low-grade energies, such as warm wastewater, geothermal, and solar sources. As a consequence, these engines represent simple and environmentally friendly solutions, and several architectures have been proposed in the last years. However, none of these devices has been industrialized, mainly due to the lack of robust design tools as well as to several technological and economic issues. To this aim, a simple semi-empirical numerical model has been developed to analyze the mechanical response of a shape memory alloy–based crank heat engine. In particular, the engine output characteristics (e.g. torque, specific power, efficiency) have been studied as a function of several geometrical configurations (e.g. dimensions, number of cranks) as well as of different thermo-physical properties of the heating and cooling thermal sources.

Magnetic shape memory alloys (MSMAs) are interesting materials because they exhibit large recoverable strain (up to 10%) and fast response time (higher than 1 kHz). MSMAs are composed of martensitic variants with tetragonal unit cells and a magnetization vector that is approximately aligned with the short side of the unit cell in the absence of an external applied magnetic field. These variants reorient either to align the magnetization vector with an applied magnetic field or to align the short side of the unit cell with an applied compressive stress. This reorientation leads to a mechanical strain and an overall change in the material’s magnetization, allowing MSMAs to be used as actuators, sensors, and power harvesters. This paper builds upon the work of Kiefer and Lagoudas as well as improvements proposed by LaMaster et al. to present a thermodynamic-based continuum model able to predict the response of an MSMA to any three-dimensional (3D) magneto-mechanical loading. The 3D nature of the model requires that the three variants, associated with the three axes of an MSMA single crystal, should all be allowed to evolve. In addition, this model includes evolution rules for the three magnetic domain volume fractions and the rotation of the direction of the magnetization vectors in each variant based on thermodynamic requirements.

In this paper the dependence of the torque characteristic of a magnetorheological clutch on several working parameters is analyzed by means of a feedforward neural network. The clutch was envisaged to have the possibility to disengage the vacuum pump in diesel engine vehicles, in order to increase the overall vehicle efficiency.

A large set of test was carried out following different protocols in order to obtain a detailed characterization of the clutch. Results showed that, due to the characteristics of MR fluids and to the complex mechanism of torque transmission, the torque characteristics (both the yield torque and the torque-slip behavior) are a function of the relative speed between the input and output shafts, of the energy dissipated during the clutch slip and of the rest time between consecutive slippages. The data acquired during all tests were used to train a neural network with five input elements and one output element.

The developed neural network proved to satisfactorily reproduce the actual clutch properties and could be used in the future in an engine-vehicle simulator in order to model the torque characteristic of the clutch subjected to different operating cycles.

Magnetorheological dampers have been widely studied as versatile real-time actuators for solving vibration problems in various structures and systems. However, the inherent time delay problem of magnetorheological actuators may cause performance degradation of semi-active control systems. The primary purposes of this article are to provide a comprehensive analysis on the time delay of an impact buffer system based on a magnetorheological damper and to propose compensation methods to reduce the time delay. To this end, this study evaluated the electromagnetic circuit in the magnetorheological damper and designed an advanced correcting circuit to improve the response time. The simulation results show that the proposed circuit reduces the time delay by 5 ms. Using a magnetorheological buffer system setup, its force response times were tested. The experimental results show that the electromagnetic circuit plays a significant role in the time delay and it is highly dependent on the effectiveness of the electric parts of the control hardware. Furthermore, a proportional–integral–derivative controller is able to reduce the time delay and improve the dynamic performance of the magnetorheological impact buffer system.

In this contribution, a micro-mechanically motivated, energy relaxation-based constitutive model for phase transformation, martensite reorientation and twin formation in shape memory alloys is proposed. The formulation builds on an idealized parametrization of the austenite-twinned martensite microstructure through first- and second-order laminates. To estimate the effective rank-one convex energy density of the phase mixture, the concept of laminate-based energy relaxation is applied. In this context, the evolution of the energetic and dissipative internal state variables, that describe characteristic microstructural features, is computed via constrained incremental energy minimization. This work also suggests a first step towards the continuous modeling of twin formation within the framework of energy relaxation and can be viewed as a generalization of earlier models suggested by Bartel and Hackl (2009) and Bartel et al. (2011). More specifically, in the current model the orientation of martensitic variants in space is not pre-assigned. Variants are rather left free to arrange themselves relative to the martensite-martensite interface in an energy-minimizing fashion, where, however, it is assumed that they form crystallographically-twinned pairs. The formulation also eliminates the need to introduce specific expressions for the Bain strains in each of the martensitic variants, by relating them to a master variant and utilizing the information about their absolute orientation. The predictive capabilities of the proposed modeling framework are demonstrated in several representative numerical examples. In the first part of the results section, the focus is placed on purely energetic analysis, and the particular influence of the different microstructural degrees of freedom on the relaxed energy densities and the corresponding stress-strain responses is investigated in detail. In the second part, macro-homogeneous uniaxial strain and shear loading cases are analyzed for the dissipative case. It is shown, that the proposed model, which, compared to purely phenomenological macro-scale models, has the advantage of strong micro-mechanical motivation, is capable of qualitatively predicting central features of single crystal shape memory alloy behavior, such as the phase diagram in stress-temperature space, and pseudo-elastic and pseudo-plastic responses, while simultaneously providing valuable insight into the underlying micro-scale mechanisms.

The design of shape memory alloy actuators typically compromises between force and stroke, the two properties being inversely proportional to one another for any given shape memory alloy element. This article presents a bow-like compliant actuator aimed at improving the specific performance of shape memory wires on both accounts. Conceptually, the actuator is formed by two straight elastic beams mutually hinged at the ends with a pre-stretched shape memory alloy wire in between. Heating of the alloy shortens the wire, which in turn makes the beams to buckle outwards in a symmetric double-arched configuration. The transverse displacement of the beams amplifies the contraction of the wire while producing a favourable output force. This article develops a simple, though accurate, analytical model of the actuator upon which a step-by-step design procedure is built. The theoretical findings are compared with the outcome of a finite element simulation for a case study and with the test data gathered from a physical prototype actuator.

The pointing performance of a truss structure on orbit that is used for a large space telescope is discussed. To achieve advanced science missions, large and precise support structures such as truss structures are needed. However, the preciseness of the structure might be lost due to various disturbances on orbit. Therefore, to realize ultra-large and precise support structures, active shape control of the structures is needed. To control the shape, we use artificial thermal expansion caused by heaters instead of mechanical actuators. Control systems without mechanical mechanisms have high reliability, which is very attractive for use on orbit. However, there are some constraints regarding the usage of heaters. The control input is restricted to positive inputs because heaters can give off heat but cannot dissipate heat actively, and there will be upper limits on the heat input. To improve the control performance under such constraints, we apply “model predictive control” as a feedforward control method with preview information. In this article, we mainly show the effectiveness of model predictive control compared with proportional–integral control, which is one of the typical feedback control methods. We developed a structural mathematical model and a thermal mathematical model in order to evaluate the performance of the control system. It is confirmed through numerical simulations that the total error is reduced by model predictive control compared with proportional–integral control.

Dielectric elastomers are smart materials that can be used to conceive solid-state electromechanical transducers such as actuators, sensors, and generators. Dielectric elastomer generators, in particular, are very promising for energy harvesting applications because they potentially feature large energy densities, good conversion efficiencies, good shock and corrosion resistance, and low cost. In this article, a novel concept of parallelogram-shaped dielectric elastomer generator is presented and analyzed. Parallelogram-shaped dielectric elastomer generators are rotary variable capacitance transducers, which are made by planar dielectric elastomer membranes that are covered with compliant electrodes and clamped along their perimeter to the links of a parallelogram four-bar mechanism. First, an analytical model for the electro-hyperelastic response of the parallelogram-shaped dielectric elastomer generator is described and used to assess the maximum theoretical performances of the device. Then, an experimental case study with a parallelogram-shaped dielectric elastomer generator prototype featuring a natural rubber dielectric elastomer membrane and carbon conductive grease electrodes is presented. Simulation and experimental results demonstrate the practical feasibility of the parallelogram-shaped dielectric elastomer generator concept.