Shape-Adaptive Robotics: Programmable Morphing through SMA and SMP Integration.

Though the conventional pyrotechnic fastener mechanisms are widely used in spacecraft for their reliable releasable-fastening function, they still have several unavoidable problems: physical shock, high maintenance cost, to name a few. This paper introduces a new type of smart releasable mechanism based on a Shape Memory Alloy (SMA) spring and its corresponding heating apparatus. To obtain the high heating efficiency and increase the response speed, the SMA spring is transitionally fit with the heating apparatus shell instead of directly heating by electric current. As soon as the heating apparatus begins to work, the SMA spring will provide an un-locking force to release the fastening device within the standard time, which also realizes the similar releasable-fastening function comparing with the conventional pyrotechnic fastener mechanisms. In order to ensure the reliability of space products, the heating apparatus is composed of two identical ceramic heating elements which can be controlled independently or synchronously. Finally, the experimental results clearly show that, under the satellite power supply at the constant value of 28 V, the SMA spring can reach the desired 30 N unlocking force within 93 s and 51 s by single or dual heating elements, respectively. The maximal output force can even be increased as large as 40 N under the limited volume of the releasable mechanism.

In this study, we developed a variable-stiffness and deformable link using shape-memory material and the jamming transition phenomenon. Above its glass transition temperature (Tg), a shape-memory polymer (SMP) can be deformed by applying a small load. SMPs maintain the deformed shape after they have been cooled below Tg, and they return to their original shape when heated above Tg. The reversible change in the elastic modulus between the glassy and rubbery states of SMPs can be on the order of 100–1000 times. We exploited the characteristics of SMPs to develop robot components with variable stiffness and sensitivity. The jamming transition phenomenon for granular material has been widely used as a method to change the stiffness of robots. This phenomenon is the change from fluid-like to solid-like conditions by removing air from a space containing particles. In this study, we developed a variable-stiffness link by combining the SMP and the jamming transition phenomenon. Moreover, by replacing the SMP with shape-memory alloys (SMAs), whose recovery force and elastic modulus are larger than those of SMPs, we prepared a second prototype with variable stiffness. We evaluated the performance of both prototypes, using the SMP or the SMA, with experiments and confirmed the motion principle of the proposed link (e.g., shape recovery and shape fixity). Moreover, it was confirmed that the stiffness of these links can be changed among four states.

This study details the design and construction of an Analog Water Heater Chamber (AWHC) specifically tailored for tensile testing of Shape Memory Alloy (SMA) rods and springs. The AWHC is designed to simulate various temperature conditions, enabling precise control over the thermo-mechanical behavior of SMA specimens. Energy conversion from the electrical to heat form is achieved using an electrical heating element of 1000 watts. This heat is transferred to the SMA spring/wire using convection using a water medium, and phase transformation occurs. The chamber's unique design allows for real-time monitoring of deformation and recovery processes under controlled temperature and loading conditions. Different test results for both the SMA rod and SMA spring at various loading rates and constant temperature, and continuous loading rate and different temperatures showed a good thermo-mechanical coupling result The AWHC's performance was validated through tensile testing of SMA rods and springs, demonstrating its efficacy in evaluating the mechanical properties and phase transformation temperatures of these materials. The results show excellent correlation with theoretical predictions, confirming the AWHC's suitability for characterizing SMA behavior under diverse thermal and mechanical conditions. This research contributes to the development of efficient testing protocols for SMA components, paving the way for their enhanced integration into various engineering applications, thus the aim of which AWHC is designed for is achieved.

A transmission method of magnetorheological(MR) fluid in disc type driven by shape memory alloy (SMA) spring with the thermal effect was proposed. The torque changed at difference temperatures. Based on the thermal effect of SMA, the expression for displacement of SMA spring was established and the rheological properties of MR fluids under the magnetic fields was described by Herschel-Bulkley model. The shear flow of MR fluid between two discs was analyzed by the momentum equation and the expressions of flow velocity and torque were obtained. The experimental results show the displacement of SMA spring and the transmit torque of MR Fluid are directly proportional to the temperature and the applied magnetic field, respectively. The transmit torque of MR Fluid can be continuously controlled by the SMA spring in difference temperatures. Keywords-thermal affect; SMA spring; MR fluid; transmission property

Smart material based variable internal connectivity has the potential to overcome the inherent conflicting requirements of lightweight shape adaptable structures by reducing the actuation force required for shape adaptation, and, consequently, volume and mass of the actuation system, by increasing the compliance of the structure temporarily. In this paper, we consider shape adaptable sandwich panels with a truss core and implement smart material based variable connections, called mechanical switches, in the trusses of the core. We present three concepts of mechanical switches based on electro-bonded laminates, dielectric elastomer actuators and shape memory alloys, and evaluate their performance in terms of variation in stiffness and in maximum transferable force. Based on these results, we show how the implementation of variable connections in the core trusses can be used to reduce the actuation force by different orders of magnitude for three simple types of deformation of the panel: change in thickness, in transverse shear angle and bending. Further, considering the influence of the mechanical switches on the out-of-plane stiffness of the core, we demonstrate that this approach can be used to obtain changes in thickness of the panel, a kind of deformation that is not possible to obtain with other approaches found in literature. Moreover, we show that mechanical switches can be used to selectively vary the shear stiffness of the panel in the desired direction. Finally, we analyze the limitations of this technology in terms of mechanical properties in order to identify possible applications.

This paper presents a gripper actuated by a pair of differential shape memory alloy (SMA) springs with the tactile feedback controller of the polyvinylidene fluoride (PVDF) films. The SMA actuator is heated by the electric current and cooled by the fans, and a proportional-integral (PI) controller is proposed to control the output position of the gripper. In order to handle soft, deformable or slippery objects very well, the PVDF sensor is added to the finger of the gripper to perform the tactile feedback when it grasps the objects. The hardware system is established including data acquisition and signal processing parts, the control system is programmed. Experiments of grasping a soft object are carried out, and the gentle grasp ability of the gripper is also tested. Experiments results show that the SMA gripper with the PVDF tactile sensor could grasp a soft object through controlling tactile forces.

This paper designs a new composite transmission device for improving the transmission torque by squeezing magnetorheological fluid (MRF) with an electrothermal shape memory alloy (SMA) spring. Based on the finite element method, a numerical analysis of the magnetic circuit and magnetic field distribution of the magnetorheological (MR) transmission is presented, as well as a theoretical derivation and calculation of the squeezing force output by the electrothermal SMA spring and the transfer torque of the variable thickness MR transmission. In addition, the output characteristics of the electrothermal SMA spring at different temperatures are analyzed, as are the torque characteristics of the variable thickness MR transmission. The research shows that the electrothermal SMA springs exhibit a highly non-linear squeezing force output during the temperature rise, and the increase in current affects the martensite phase transition quality, and thus the phase transition temperature. The squeezing force generated by the springs increases significantly when the temperature is within the martensitic phase transformation interval, with a maximum squeezing force of 318.43 N when the SMA temperature reaches 100 °C. The proposed variable thickness MR transmission can increase the maximum torque by 4.88 times under SMA spring squeezing force, and its maximum transmitted torque is increased from 15.08 to 73.56 N·m. By squeezing the MRF with an electrothermal SMA spring, the torque of the variable thickness MR transmission can be increased quickly and effectively.

Multifunctional shape memory composites composed of shape memory polymers and shape memory alloys exhibit superior shape memory properties; therefore, it is of great interest to researchers to model their thermo-mechanical behavior numerically. Although a number of constitutive models of shape memory alloys and shape memory polymers have been developed, very few models have been developed for shape memory composites and validated with experimental data. In this study, we first review separately constitutive models of shape memory alloys and shape memory polymers developed in previous studies. Both models were validated with thermo-mechanical tests conducted on a shape memory alloy fiber and shape memory polymer, respectively. A constitutive model for the shape memory composite was then developed utilizing the homogenization scheme. Shape memory composites containing 0.5% or 1% of the shape memory alloy fiber volume content embedded in the shape memory polymer matrix were fabricated. Thermo-mechanical tests were carried out on these shape memory composites to validate the proposed constitutive model. The experimental results showed that the proposed shape memory composite model was able to predict the general trend of the thermo-mechanical behavior of the shape memory composites.

4D printed and multi-stimulus responsive shape memory polymer nanocomposites developed on hydrogen bonding–metal-phenolic sacrificial network: Application for hazardous chemical operations soft robots

The use of Shape Memory Alloy (SMA) materials for actuation represents a technological opportunity for the development of innovative applications. The issue of using the SMA actuators is an enticing solution that has attracted the interest of specialists in recent years, due to their numerous advantages (i.e. reliability, compactness and flexibility, reduced cost, linear or angular movement etc.). In addition, their unmatched energy density, easy integration into various structures and the simplicity in terms of design, have all motivated the authors to develop a barrier structure model, actuated by three SMA helical springs. The actuator used in this structure works as a linear actuator, contracting with great strength and speed, thus exerting the necessary force to lift the barrier arm in the SMA springs heat-activated, austenitic state. The first part of this paper provides a description of the accomplished model and of its functioning principle. The second part of the study presented in this paper focuses on the authors' investigation on how the performances of the actuator inserted in the barrier structure—namely the actuator's heating time, cooling time, actuation time and stroke—are affected by the SMA spring activating direct current values. The tests were conducted while keeping the geometrical parameters of SMA springs and the weight of the barrier arm constant.

Shape memory alloys (SMAs) have several unique characteristics, including their Young's modulus-temperature relations, shape memory effects, and damping characteristics. The Young's modulus of the high-temperature austenite of SMAs is about three to four times as large as that of low-temperature martensite. Therefore, a spring made of shape memory alloy can change its spring constant by a factor of three to four. Since a shape memory alloy spring can vary its spring constant, provide recovery stress (shape memory effect), or be designed with a high damping capacity, it may be useful in adaptive vibration control. Some vibration control concepts utilizing the unique characteristics of SMAs will be presented in this paper. Shape memory alloy springs have been used as actuators in many applications although their use in the vibration control area is very recent. Since shape memory alloys differ from conventional alloy materials in many ways, the traditional design approach for springs is not completely suitable for designing SMA springs. Some design approaches based upon linear theory have been proposed for shape memory alloy springs. A more accurate design method for SMA springs based on a new nonlinear thermomechanical constitutive relation of SMA is also presented in this paper.

Materials design involving the thermomechanical constitutive modeling of shape memoryalloy (SMA) and shape memory polymer (SMP) composites is a key topic in thedevelopment of smart adaptive shape memory composites (SASMC). In this work, aconstitutive model for SASMC is developed. First, a one-dimensional SMA model, whichcan simulate the pseudoelastic (PE) and shape memory effects (SME) is presented.Subsequently, a one-dimensional SMP model able to reproduce the SME is addressed. BothSMA and SMP models are based on a single internal state variable, namely themartensite fraction and the frozen fraction, which can be expressed as a function oftemperature. A consistent form of the analytical solution for the SMP model is obtainedusing the fourth-order Runge–Kutta method. Finally, the SASMC constitutivemodel is proposed, following two analytical homogenization approaches. Oneapproach is based on an equivalent inclusion method and the other approachis the rule of mixtures. The SMA and SMP constitutive models are validatedindependently with experimental results. However, the validation of the composite modelis performed using the two homogenization approaches and a close agreementin results is observed. Results regarding the isothermal and thermomechanicalstress–strain responses are analyzed as a function of SMA volume fraction. Further, it isconcluded that the proposed composite model is able to reproduce consistentlythe overall composite response by taking into consideration not only the phasetransformations, variable modulus and transformation stresses in SMA but alsothe variable modulus, the evolution of stored strain and thermal strain in theSMP.

Morphing structures have the potential to significantly improve vehicle performance over existing fixed component designs. In this paper, we examine new composite material design approaches to provide combined high stiffness and large reversible deformation. These composites employ shape memory polymers (SMP) matrices combined with segmented metallic reinforcement to create materials with variable stiffness properties and reversible accommodation of relatively large strains. By adjusting the temperature of the sample, the storage modulus can be varied up to 200x. We demonstrate the segmented composite concept in prototype materials made using thermoplastic polyurethane SMP reinforced with interlocking segmented steel platelets. Measured storage moduli varied from 5-12 GPa, below SMP T_g, and 0.1-0.5 GPa above SMP T_g. The samples demonstrated more than 95% recovery from induced axial strains of 5% at 80°C. Viscoelastic effects are dominant in this regime and we investigate the rate dependence of strain recovery.

Recent advances in shape memory alloys (SMAs) and shape memory polymers (SMPs) have inspired researchers to create intelligent textiles as self-regulating shape-changing structures responding to environmental variation, contributing to a new field in the scientific frontier of smart materials. This paper shows how appropriate shape memory training can engineer fabric designs to enhance their performance and their aesthetic appeal. Main technologies include the training procedure of shape memory effect (SME) in SMAs and SMPs, the regulation of phase transformation temperatures in SMAs, the manufacture of SMP yarns and SMP coating on fabrics for specific requirements of fabric design. Further attention is also paid to the combination of SMPs or SMAs with textile design of different fabric yarn for promoting specific dynamic aesthetic effects.

Neurogenic bladder, a bladder dysfunction resulting from injury or disease affecting the central or peripheral nervous systems, significantly compromises patient quality of life and poses potentially life-threatening risks. Currently, no reliably effective clinical treatments are available. In this context, medicine-engineering integrated approaches aimed at developing artificial detrusor systems to facilitate voluntary voiding demonstrate unique value and advantages. However, existing artificial detrusor system designs face significant challenges, including structural complexity and suboptimal biomechanical compatibility. This study proposes a novel artificial detrusor system, inspired by the physiological mechanisms of human urine storage and voiding, driven by shape memory alloy (SMA) springs and powered via transcutaneous energy transfer (TET). The system employs the shape memory effect of SMA springs to mimic detrusor contraction and utilizes TET for non-contact energy transmission. The driving performance and feasibility of the system were evaluated using simulation experiments and animal studies. Results indicate that excitation voltage, SMA spring specifications, and energization duration significantly influence the voiding rate and temperature increment of the artificial detrusor. The animal-tested prototype effectively induced bladder voiding. This study presents a promising artificial detrusor system that combines SMA-driven actuation with TET to address neurogenic bladder dysfunction. This system features a simple structure and a feasible working principle, providing a basis for subsequent optimization of artificial detrusor designs and offering a new technical pathway for assistive solutions addressing neurogenic bladder dysfunction.