Shape Memory Alloy Active Elements Research Articles

Development of an alternating heat engine, actuated by shape memory alloys

The paper introduces a new type of heat engine actuated by shape memory alloy (SMA) commercial wires. The originality of the design consists in using two SMA active elements that are martensitic at room temperature (RT) and compensate each other in such a way that each element becomes, on a turn, austenitic while the other is martensitic and vice-versa. The SMA wires were manufactured from Ti50Ni45Cu5 SMAs undergoing thermoelastic martensitic transformation characterized, among other things, by superior stiffness of austenitic state as compared to martensitic state. This feature has enabled thermoelastic SMAs to develop work generating shape memory effect (SME), while undergoing reverse martensitic transformation, during heating. Based on this principle, solid state heat engines have been built even since the discovery of SMAs. The paper reviews the most prominent types of SMA driven solid state engines, before introducing a new type of engine. In the present heat engine constructive solution, solar energy is used to directly heat up the active element, which has a straight austenitic “hot shape” and was bent into a curved shape at RT, while being in martensitic state. During heating, the element becomes fully austenitic, partially recovers its straight shape, by decreasing its curvature radius, and bents the bias martensitic element which is softer. The latter will be brought with its most deformed area into the focus of an optical lens, thus being instantly heated up. It develops, in its turn, work generating SME. Based on this principle, a linear-alternative heat engine has been patented. Its functioning principle was tested for different numbers of heating–cooling cycles. By means of an original testing setup, using hot air heating and sprayed water cooling, the variations of displacement with temperature were recorded and the critical temperature for then start of reverse martensitic transformation (As) was determined.

Hybrid energy harvesting based on cymbal and wagon wheel inspiration

The demand for self-sufficient electronic devices is increasing as well as the overall energy use, and such demands are pushing technology forward, especially in effective energy harvesting. A novel hybrid energy harvesting system has been proposed and analysed in this article. It has been demonstrated that the energy harvesting system is capable of converting enough energy to power a typical micro-electro-mechanical system device. This has been achieved through unification of the nine–cymbal energy harvester array, as an energy harvesting core, and shape memory alloy active elements, acting as a source of force stimulated by the environmental changes. A finite element model was developed for the cymbal energy harvester, which was verified and used for the analysis of cymbal energy harvester’s response to the change of the end-cap material. This was followed by the finite element model for the energy harvesting system used for analysis of the location of shape memory alloy wires and force generated by each wire individually and then all together. As a further optimisation of the energy harvesting system, a novel wagon wheel design was explored in terms of its energy harvesting capabilities. As expected, due to the increased displacement, an increase in the power output was achieved.

Novel High-Speed High Pressure Torsion Technology for Obtaining Fe-Mn-Si-Cr Shape Memory Alloy Active Elements

This paper introduces an adapted high-speed high pressure torsion (HS-HPT) method of severe plastic deformation applied for obtaining shape memory alloy (SMA) active elements with revolution symmetry, able to develop axial displacement/force. Billets with circular crown forms were cut from Fe-28Mn-6Si-5Cr (mass%) SMA ingots and, by means of HS-HPT technology, were directly turned into modules, with truncated cone shell configurations. This process was performed, during time intervals of seconds, under the effect of high pressure (up to 1 GPa) cumulated with high rotation speed (hundreds of rotations per minute) applied on the active surfaces of sintered-carbide anvils, specially designed for this purpose. Due to pressure and friction, generated by rotation, the entire sample volume is heated and simultaneously deformed to final shape. During the process, microstructure fragmentation occurred enabling to obtain (ultra)fine grains and nanocrystalline areas, in spite of the heat developed by friction, which was removed by conduction at the contact surface between sample and anvils, before the occurrence of any recrystallization phenomena. When compressed between flat surfaces, the truncated cone modules developed a superelastic-like response, unique among Fe-Mn-Si base SMAs and, when heated in compressed state, they were able to develop either axial strokes or recovery forces by either free or constrained recovery shape memory effect (SME), respectively. By means of optical (OM) and scanning electron microscopy (SEM) marked structural changes caused by HT-HPT were revealed, along with fine and ultrafine crystalline grains. The presence of stress-induced e-hexagonal close-packed (hcp) martensite, together with nanocrystalline areas were confirmed by x-ray diffraction.

Contactless and selective energy transfer to a bistable micro-actuator using laser heated shape memory alloy

Contactless energy transfer (CET) methods offer great flexibility in the design of complex micro-systems. This paper reports a laser based contactless and selective energy transfer method. A compliant bistable micro-actuator (curved beam of size 25 mm × 1.5 mm × 0.508 mm) is actuated between its two stable positions using the laser heated shape memory alloy (SMA) active elements (size: 3 mm × 1 mm × 0.1 mm). The switching time of the actuator turns out to be 0.5 s for d0 equal to 700 μm and a laser power of 90 mW (d0 is half of the total stroke length). The paper also demonstrates the selective energy transfer technique to the SMA active elements by depositing silver based optical filters directly onto the SMA active elements. A successful demonstration is presented for four wavelengths, 532, 660, 785, and 980 nm, using different values of d0 for the bistable micro-actuator. Finally, a long-term test is performed to highlight the thermo-mechanical effect on the selective addressing capability of the optical filters.

Design of Active Bias Sma Actuators for Morphing Wing Applications

Shape Memory Alloys (SMAs) can provide compact and effective actuation for a variety of mechanical systems. Generally speaking, SMA-driven actuator systems can be divided into three subsystems: a) SMA active element, b) the transmission and c) a bias element. In respect to the type of bias, two actuator configurations can be distinguished: passive bias actuators where the SMA active element is coupled with an elastic bias element (spring), and active bias actuators in which two SMA active elements are connected together. This work is focused on designing an SMA actuator using active bias elements for morphing wing applications. Keywords: SMA actuator, active bias, antagonist, design, morphing wing

Debonding Characterization of SMA/Polymer Morphing Structures

morphing structures are expected to play an increasingly important role in aeronautic applications, among others. Shape memory alloys (SMA) are one of the most promising candidates to date. However, work remains to be done before these structures meet the stringent requirements of their successful integration in an aeronautic context. Research has shown that SMA/Polymer interface strength can be a limiting factor in active deformable structure performance. In this study, the effect on the SMA/Polymer interface strength of various surface treatments, wire geometries and resin types are evaluated. SMA wire geometry is modified through a specific combination of cold rolling and post-deformation annealing, which is capable of maintaining SMA actuating properties while achieving required cross-section area reductions. The most promising thermomechanical processing is finally proposed but results show that further work is required before SMA active elements can safely be used in an active structure.

Experimental Bench for Shape Memory Alloys Actuators Design and Testing

Shape memory alloys (SMAs) are used as active elements in novel actuation devices. Two generic types of SMA actuators can be distinguished according to the type of bias passive-bias actuators where an elastic component serves as a bias and active-bias actuators where two SMA elements are connected together. This paper describes an experimental testing bench developed for the characterization of SMA active elements and their testing in a real actuation environment. The characterization of SMA active elements is performed under three complementary testing modes: (a) constant-stress, (b) fixed-support, and (c) elastic-bias recovery modes. Force, displacement and temperature data acquired during testing of a given SMA active element are then used to assess the mechanical work-generation potential of this active element and, ultimately, for the design of an SMA actuator containing this element. Finally, a case study is presented to illustrate the experimental design methodology and results.

Educational stand using shape memory alloys to enhance teaching of smart materials

The goal of this paper is to present a simple but effective stand used for demonstration of Shape Memory Alloys (SMA) behaviour. The hands-on device is developed to offer experiences, previously unavailable, in order to introduce SMAs to anyone, but especially to undergraduate mechanical or materials engineering students.The experimental set-up was designed in order to test SMA active elements in the Device Designing and Robotics Laboratory of Center of Advanced Research, Design & Technology - CARDT from Eftimie Murgu University Resita,. The device features an SMA wire weight-lifter and an actuated spring to demonstrate the force magnitude exerted by the active element under the action of temperature. The set-up is used as a teaching tool and as an outreach demonstration, being an effective tackle in assisting students to learn about SMAs.Many other experiments and demonstrations, such as identifying thermal transfer functions, predicting and observing the displacement and understanding the phase transition can be derived from this setup. It is found that these demonstrations help to bridge the gap between theory and intuition during a student's learning process. These demonstrations can motivate students to pursue further studies in smart materials and structures.

Design of Shape Memory Alloy Actuators for Morphing Laminar Wing With Flexible Extrados

An active structure of a morphing wing designed for subsonic cruise flight conditions is composed of three principal subsystems: (1) flexible extrados, (2) rigid intrados, and (3) an actuator group located inside the wing box. The four-ply laminated composite flexible extrados is powered by two individually controlled shape memory alloy (SMA) actuators. Fulfilling the requirements imposed by the morphing wing application to the force-displacement characteristics of the actuators, a novel design methodology to determine the geometry of the SMA active elements and their adequate assembly conditions is presented. This methodology uses the results of the constrained recovery testing of the selected SMA. Using a prototype of the morphing laminar wing powered by SMA actuators, the design approach proposed in this study is experimentally validated.

Shape memory alloy microactuators

Shape memory alloys (SMA) have been investigated as transducing materials for the development of actuators for robotic end-effectors. In this paper, basic concepts on the design and development of SMA actuating systems for robotic applications are discussed. The thermomechanical characterization of the SMA material is the basis on which the successive steps for the design of the actuating system have been developed. Isometric tests conducted on SMA wire specimens led to the definition of the limit curve concept, i.e. the locus of equilibrium points in the space of the status variables (stress, strain, temperature) which limit the SME behavior of the material. The second step in the design of SMA actuating systems involved the definition of appropriate geometries of SMA active elements in order to obtain the best performances in terms of the required forces and displacements. The definition of the whole actuating system started from the identification of the agonistic-antagonistic configuration as the most appropriate for SMA technology. As an example of the realization of practical SMA actuating systems, the performances obtained with two different SMA actuators incorporating an active element shaped by coiling thin SMA wires and ribbons, respectively, and arranged according to an agonistic-antagonistic configuration, are presented.