Shape Memory Alloy Resistance Research Articles

Modeling and Analysis of Resistance-Sensing Characteristics for Two-Way Shape Memory Alloy-Based Deep-Sea Actuators

Deep-sea actuators based on shape memory alloys (SMAs) are an emerging frontier field of multidisciplinary crossover, and the resistive sensing characteristics are the basis for the drive control of SMA deep-sea actuators. The resistance and resistivity of SMAs are complex and highly dependent on temperature and stress, and there is no complete description of SMAs for extreme environments of high pressure, low temperature, and high salinity in the deep sea. In this study, the logistic function is introduced to improve the kinetic equation of phase transition, and the macromechanical model, the law of resistance, and the resistivity mixing rule are integrated to model and analyze the resistive self-awareness characteristics of two-way shape memory alloy deep-sea actuators. The complex coupling relationships among resistance, strain, stress, resistivity, and temperature under constant load conditions are investigated, and the validity of the resistance-sensing model is verified by the water bath cycling test. The results show that the predicted values of the model agree well with the measured values. The self-perceived relationship between the resistance and deformation of the two-way shape memory alloy can be effectively expressed, which provides theoretical model support for the design of memory alloy deep sea actuators and sensorless drive control.

Crack monitoring and repair model of SMA composite material based on strain transfer

In the process of using composite materials, crack damage is unavoidable. These cracks may cause a decrease in the structural performance of the material and may even pose a threat to the safety of the entire structure. In this paper, the shape memory alloy (SMA) is embedded into the composite material, and the strain transfer effect of the interface layer is considered. The crack monitoring and repair model of SMA composite material based on strain transfer are established using SMA resistance-sensing characteristics and SMA constitutive model. Based on this monitoring and repair model, the crack monitoring behavior of SMA under different initial states and temperature conditions is discussed, and the crack repair behavior of SMA under specific initial states and temperature conditions is also discussed. The research results show that the change in SMA resistance and the strain of the composite material crack are linearly segmented, and as the temperature increases, the composite material crack is gradually repaired. This study can provide theoretical guidance for the further engineering application of SMA composite material crack monitoring and repair theory.

Environment-Induced Degradation of Shape Memory Alloys: Role of Alloying and Nature of Environment.

Shape memory effects coupled with superelasticity are the distinctive characteristics of shape memory alloys (SMAs), a type of metal. When these alloys are subject to thermomechanical processing, they have the inherent ability to react to stimuli, such as heat. As a result, these alloys have established their usefulness in a variety of fields and have in recent years been chosen for use in stents, sensors, actuators, and several other forms of life-saving medical equipment. When it comes to the shape memory materials, nickel-titanium (Ni-Ti) alloys are in the forefront and have been chosen for use in a spectrum of demanding applications. As shape memory alloys (SMAs) are chosen for use in critical environments, such as blood streams (arteries and veins), orthodontic applications, orthopedic implants, and high temperature surroundings, such as actuators in aircraft engines, the phenomenon of environment-induced degradation is of both interest and concern. Hence, the environment-induced degradation behavior of the shape memory alloys (SMAs) needs to be studied to find viable ways to improve their resistance to an aggressive environment. The degradation that occurs upon exposure to an aggressive environment is often referred to as corrosion. Environment-induced degradation, or corrosion, being an unavoidable factor, certain techniques can be used for the purpose of enhancing the degradation resistance of shape memory alloys (SMAs). In this paper, we present and discuss the specific role of microstructure and contribution of environment to the degradation behavior of shape memory alloys (SMAs) while concurrently providing methods to resist both the development and growth of the degradation caused by the environment.

Fracture resistance of shape memory alloys under thermomechanical loading

Experimental measurements of the fracture resistance of Shape Memory Alloys (SMAs) under thermomechanical loading conditions are reported. NiTi compact tension specimens are subjected to either isothermal mechanical or isobaric thermal loading; the latter loading path is an idealization of typical loading paths that utilize these alloys as actuators. A single-parameter description of the experimental data is employed on the basis of a path-independent contour integral, which is approximated by the load–load-line displacement curves recorded from the experiments. The obtained results represent the first experimental measurement of the fracture toughness of SMAs under coupled thermo-mechanical loading, and indicate that the fracture toughness enhancement associated with crack advance under isobaric thermal loading is less pronounced than the corresponding one under isothermal mechanical loading.

Nanocomposite NiTi shape memory alloy with high strength and fatigue resistance.

Many established, but also potential future applications of NiTi-based shape memory alloys (SMA) in biomedical devices and solid-state refrigeration require long fatigue life with 107-109 duty cycles1,2. However, improving the fatigue resistance of NiTi often compromises other mechanical and functional properties3,4. Existing efforts to improve the fatigue resistance of SMA include composition control for coherent phase boundaries5-7 and microstructure control such as precipitation8,9 and grain-size reduction3,4. Here, we extend the strategy to the nanoscale and improve fatigue resistance of NiTi via a hybrid heterogenous nanostructure. We produced a superelastic NiTi nanocomposite with crystalline and amorphous phases via severe plastic deformation and low-temperature annealing. The as-produced nanocomposite possesses a recoverable strain of 4.3% and a yield strength of 2.3 GPa. In cyclic compression experiments, the nanostructured NiTi micropillars endure over 108 reversible-phase-transition cycles under a stress of 1.8 GPa. We attribute the enhanced properties to the mutual strengthening of nanosized amorphous and crystalline phases where the amorphous phase suppresses dislocation slip in the crystalline phase while the crystalline phase hinders shear band propagation in the amorphous phase. The synergy of the properties of crystalline and amorphous phases at the nanoscale could be an effective method to improve fatigue resistance and strength of SMA.

Seismic Resistant Bridge Columns with NiTi Shape Memory Alloy and Ultra-High-Performance Concrete

Reinforced concrete bridge columns often endure significant damages during earthquakes due to the inherent deficiencies of conventional materials. Superior properties of the new materials such as shape memory alloy (SMA) and ultra-high-performance concrete (UHPC), compared to the reinforcing steel and the normal concrete, respectively, are needed to build a new generation of seismic resistant columns. Application of SMA or UHPC in columns has been separately studied, but this paper aims to combine the superelastic behavior of NiTi SMA and the high strength of UHPC, in order to produce a column design with minimum permanent deformation and high load tolerance subjected to strong ground motions. Additionally, the excellent corrosion resistance of NiTi SMA and the dense and impermeable microstructure of UHPC ensure the long-term durability of the proposed earthquake resistant column design. The seismic performance of four columns, defined as steel reinforced concrete (S-C), SMA reinforced concrete (SMA-C), SMA reinforced UHPC (SMA-UHPC), and reduced SMA reinforced UHPC (R-SMA-UHPC) is analyzed through a loading protocol with up to 4% drift cycles. The use of NiTi SMA bars for the SMA reinforced columns is limited to the plastic hinge region where permanent deformations happen. All the columns have 2.0% reinforcement ratio, except the R-SMA-UHPC column that has a 1.33% reinforcement ratio to optimize the use of SMA bars. Unlike the S-C column that showed up to 68% residual deformation compared to peak displacement during the last loading cycle the SMA reinforced columns did not experience permanent deformation. The SMA-C and R-SMA-UHPC columns showed similar strengths to the S-C column, but with about 5.0- and 6.5-times larger ductility, respectively. The SMA-UHPC column showed 30% higher strength and 7.5 times larger ductility compared to the S-C column.

Atomic Layer Deposited Al2O3 Films on NiTi Shape Memory Alloys for Biomedical Applications

NiTi shape memory alloys (SMAs) have been widely applied in biomedical fields owing to their excellent shape memory effect, super-elasticity, biomedical properties and fatigue performance. These applications include the cardiovascular stent, guidewire, stone extractors used in the cystic duct, artificial heart valve and orthodontic wire, etc. However, NiTi alloys might release out the Ni atoms in the physiological environment, which will induce the serious problems of allergic reaction, poisoning and causing cancer. This study employs the atomic layer deposition (ALD) technique to deposit Al2O3 films on the surfaces of NiTi SMAs. Experimental results show that the chemical compositions of ALD-Al2O3 films are well homogeneous. The ALD-Al2O3 films exhibit a fair adhesion with the NiTi substrates and sustain a suitable deformation strain. The ALD-Al2O3 films can also effectively inhibit the release out of Ni atoms from the NiTi SMAs. Meanwhile, the corrosion resistance of NiTi SMAs can be improved by the ALD-Al2O3 films.

A Three-Phase Model Characterizing the Low-Velocity Impact Response of SMA-Reinforced Composites under a Vibrating Boundary Condition.

Structural vibration induced by dynamic load or natural vibration is a non-negligible factor in failure analysis. Based on a vibrating boundary condition, the impact resistance of shape memory alloy (SMA)-reinforced composites was investigated. In this investigation, a modified Hashin’s failure criterion, Brinson’s model, and a visco-hyperelastic model were implemented into a numerical model to characterize the mechanical behavior of glass fiber/epoxy resin laminates, SMAs, and interphase, respectively. First, a fixed boundary condition was maintained in the simulation to verify the accuracy of the material parameters and procedures by a comparison with experimental data. Then, a series of vibrating boundaries with different frequencies and amplitudes was applied during the simulation process to reveal the effect on impact resistances. The results indicate that the impact resistance of the composite under a higher frequency or a larger amplitude is lower than that under a lower frequency or a smaller amplitude.

Evaluation of Shape Memory Effect and Wear Characteristics of Cu-Al-Be-Mn Quaternary Shape Memory Alloys

Copper based Shape memory alloys (SMA’s) are extensively investigated because of their good shape memory properties and of low cost. This study has been made on Shape memory effect and the wear rate of Cu-Al-Be-Mn quaternary shape memory alloy containing 10-14.5 wt.% of Aluminum, 0.4-0.5 wt.% of Beryllium 0.3-0.4 wt.% of Manganese and remaining Copper. This Cu -Al- Be- Mn quaternary SMA’s are prepared by ingot metallurgy using an induction furnace. Shape memory effect is determined by doing the bend test, whereas Wear has been determined by using a Pin on disc tribometer at room temperature and dry sliding conditions. The experiments has been performed on cylindrical specimens having the diameter 8 to 10 mm and length 25 to 30 mm under three different parameters i.e. Applied Load, Sliding speed and Sliding distance. After this a characterization of the wear surfaces by means of Optical microscope, has also been carried out. Because of one of the main property of shape memory alloy called pseudo-elasticity, it is found that the wear resistance of shape memory alloys is mainly due to the recovery of the super-elastic deformation.

A Comparative Study on the Corrosion Performance of Ni47Ti49Co4 and Ni51Ti49 Shape Memory Alloys in Simulated Saliva Solution for Dental Applications

In this study, the corrosion behavior of Ni47Ti49Co4 shape memory alloy (SMA) was investigated in simulated saliva solution with the binary alloy Ni51Ti49 as a reference. The surface morphology and the chemical composition of the oxide films were characterized by scanning electron microscopy and X-ray photoelectron spectroscopy before and after immersion in the test solutions, respectively. The results showed that the ternary alloy was less affected by the test solution, owning to the formation of passive layer composed mainly of the oxides of titanium and cobalt in several oxidation states. Cyclic voltammetry, electrochemical impedance spectroscopy and potentiodynamic polarization measurements affirmed that the passive oxide film significantly improved the corrosion resistance of Ni47Ti49Co4 SMAs as demonstrated by the smaller corrosion current density, larger resistance and smaller capacitance. Consequently, alloying with cobalt, which has paramount importance in enhancing the passive layer, expands the use of Ni47Ti49Co4 SMAs in dental work as new nitinol alloys with high corrosion resistances.

Shape memory alloy with bi-functionality in the master system to control a slave

Application of a shape memory alloy (SMA) with bi-functionality, acting as a force sensor and as an actuator (sensor+haptics=‘sensaptics’) is proposed for the master side of a master – slave system. A 1-DOF master is instrumented with SMA wires to servo a 2-DOF slave. Antagonistic shape memory alloy wires employed in master system performs shared sensing and actuation; the operator's command to control the slave system is sensed by the variation in the differential electrical resistance of SMA and it functions as a haptic device to display the interactive force fed back from the slave to the master. From the haptic information the operator can make decisions and take appropriate action/correction in co-ordination with the requirement of the slave environment. The laboratory model of SMA based master – slave system is designed, developed and evaluated for its performance. The validity of the results proves the concept of shared bi-functionality.

Fabrication and characterization of a dual-joint smart inhaler nozzle actuated by embedded SMA wires

Shape memory alloy (SMA) wires offer a novel solution for many embedded actuator and sensor applications. Small SMA wires in particular can be heated with a relatively low electric current, cool rapidly, and serve as a sensor thanks to a measurable resistance change. However, the challenges of fabrication with SMA actuator wires as well as their hysteretic nature have prevented them from finding mainstream application. This work focuses on the process used to control the fabrication of an SMA-actuated adaptive nozzle for the previously presented Smart Inhaler application. The elements of nozzle design that facilitate fabrication are summarized and an assembly setup and procedure is presented for controlling the stress and strain in the SMA wires while they are attached to the nozzle structure via temperature-resistant adhesives. Finally, the performance of the nozzle is characterized by measuring the changes in nozzle deflection and SMA wire strain and resistance in response to a controlled Joule heating power input. Results show controlling pre-stress in the wires during assembly can lead to reproducible behavior, an input heating power serves to control nozzle deflection, and a measured resistance can provide a useful sensor of SMA wire strain and nozzle joint deflection.

Wear mechanism and tribological characteristics of porous NiTi shape memory alloy for bone scaffold

The abraded debris might cause osteocytic osteolysis on the interface between implants and bone tissues, thus inducing the subsequent mobilization of implants gradually and finally resulting in the failure of bone implants, which imposes restrictions on the applications of porous NiTi shape memory alloys (SMAs) scaffolds for bone tissue engineering. In this work, the effects of the annealing temperature, applied load, and porosity on the tribological behavior and wear resistance of three-dimensional porous NiTi SMA are investigated systematically. The porous structure and phase transformation during the exothermic process affect the tribological properties and wear mechanism significantly. In general, a larger porosity leads to better tribological resistance but sometimes, SMAs with small porosity possess better wear resistance than ones with higher porosity during the initial sliding stage. It can be ascribed to the better superelasticity of the former at the test temperature. The porous NiTi phase during the exothermic reaction also plays an important role in the wear resistance. Generally, porous NiTi has smaller friction coefficients under high loads due to stress-induced superelasticity. The wear mechanism is discussed based on plastic deformation and microcrack propagation.

경량 의수용 SMA 구동식 생체모방 손가락 모듈

This paper proposes a biomimetic finger module to be used in a lightweight hand prosthesis. The finger module consists of finger skeleton and an actuator module driven by SMA (Shape Memory Alloy). The prototype finger module can perform flexion and extension motions; finger flexion is driven by a contraction force of SMA, but it is extended by an elastic force of an extension spring inserted into the finger skeleton. The finger motions are controlled by feedback of electric resistance of SMA because the finger module has no sensors to measure length and angle. Total weight of a prototype finger module is 30g. In experiments the finger motions and finger grip force are tested and compared with simulation results when a constant contraction force of SMA is given. The experimental results show that the proposed SMA-driven finger module is feasible to the lightweight hand prosthesis.

An accurate self-sensing method for the control of shape memory alloy actuated flexures

This paper proposes an accurate self-sensing method for the control of shape memory alloy (SMA) actuated flexures. SMA actuators exhibit large strain, high energy density, and can be successfully employed in flexures for miniature automation applications. A promising approach to obtain strain feedback for motion control is through an accurate self-sensing. The presented technique builds a self-sensing model based on the SMA strain to resistance curves. To overcome the inaccuracies resulting from hysteresis, the resistance curves can be influenced by sufficient pretension force to exhibit very small hysteresis gaps. The curve shapes are shown to be robust against external stiffness and temperature variations. The curves are then modeled by fitted polynomials so that strain values are directly obtained from measured SMA resistance. Accurate self-sensing control is demonstrated through step response and sinusoidal tracking experiments. Two flexural mechanisms are illustrated to show how the technique can be successfully implemented to various contexts.

Surface treatment of NiTi shape memory alloy by modified advanced oxidation process

A modified advanced oxidation process(AOP) utilizing a UV/electrochemically-generated peroxide system was used to fabricate titania films on chemically polished NiTi shape memory alloy(SMA). The microstructure and biomedical properties of the film were characterized by scanning electron microscopy(SEM), X-ray photoelectron spectroscopy(XPS), inductively-coupled plasma mass spectrometry(ICPMS), hemolysis analysis, and blood platelet adhesion test. It is found that the modified AOP has a high processing effectiveness and can result in the formation of a dense titania film with a Ni-free zone near its top surface. In comparison, Ni can still be detected on the outer NiTi surface by the conventional AOP using the UV/H 2O 2 system. The depth profiles of O, Ni, Ti show that the film possesses a smooth graded interface structure next to the NiTi substrate and this structure enhances the mechanical stability of titania film. The titania film can dramatically reduce toxic Ni ion release and also improve the hemolysis resistance and thromboresistance of biomedical NiTi SMA.

Fabrication of (Ti-O-N-Si)/Ti Composite Coating on NiTi Shape Memory Alloy Using PIIID and Coating Evaluation

AbstractA compact and uniform (Ti, Si, O, N)/Ti composite coating was fabricated on the surface of a NiTi shape memory alloy (SMA) (containing 50.8 at.% Ni) using plasma immersion ion implantation and deposition (PIIID) with radio-frequency (RF) magnetron sputtering. The coating and coated NiTi SMA were studied using various techniques. Analysis showed that the Ni content was drastically reduced on the surface of coated samples due to coating formation. This could greatly improve the biocompatibility of NiTi SMA. There was no TiO2 or TiN formation in the coating. The shape memory ability of NiTi SMA samples was no deteriorated by the coating process. The coating significantly improved the corrosion resistance and wear resistance of NiTi SMA and also rendered the material bioactive.

Effects of Electro-Fenton Process on Blood Compatibility and Nickel Suppression of NiTi Shape Memory Alloy

Electro-Fenton process as a modified Fenton’s oxidation method in waster water treatment can provide a stable hydroxyl radical (·OH) source by continuous reaction of electrochemically generated H2O2 with Fe2+ ions for surface oxidation modification of NiTi shape memory alloy (SMA). In this work, effects of electro-Fenton process on blood compatibility and nickel suppression of NiTi SMA were investigated by SEM and XPS, inductively-coupled plasma mass spectrometry (ICPMS), hemolysis analysis and blood platelet adhesion test. It is found that electro-Fenton process is a notably effective way to impede out-diffusion of Ni from NiTi SMA in simulated body fluids during the entire ten week immersion period. It can also improve the hemolysis resistance and thromboresistance of biomedical NiTi SMA. The improvement of blood compatibility and nickel suppression of NiTi SMA can be attributed to the formation of surface titania film with a Ni-free zone near its top surface by electro-Fenton process.

Crack growth resistance of shape memory alloys by means of a cohesive zone model

Crack growth resistance of shape memory alloys (SMAs) is dominated by the transformation zone in the vicinity of the crack tip. In this study, the transformation toughening behavior of a slowly propagating crack in an SMA under plane strain conditions and mode I deformation is numerically investigated. A small-scale transformation zone is assumed. A cohesive zone model is implemented to simulate crack growth within a finite element scheme. Resistance curves are obtained for a range of parameters that specify the cohesive traction–separation constitutive law. It is found that the choice of the cohesive strength t 0 has a great influence on the toughening behavior of the material. Moreover, the reversibility of the transformation can significantly reduce the toughening of the alloy. The shape of the initial transformation zone, as well as that of a growing crack is determined. The effect of the Young's moduli ratio of the martensite and austenite phases is examined.

Theoretical investigation of wear-resistance mechanism of superelastic shape memory alloy NiTi

Recent experimental research indicates that superelastic shape memory alloy nickel–titanium (NiTi) is superior to stainless steel against wear and could be applied in tribological engineering. It is believed that the super wear resistance of shape memory alloys is mainly due to the recovery of the superelastic deformation. Our recent wear study indicates that wear rate is very sensitive to the maximum contact pressure. In the present investigation, which involves applying Hertz contact theory and the finite element method, the wear behaviour of shape memory alloys is examined against that of stainless steels through analyzing the maximum contact pressure and the plastic deformation. Our investigation indicates that the contribution of superelasticity to the high wear resistance of NiTi is directly linked to the low transformation stress and the large recoverable transformation strain. Furthermore, the low Young's modulus of this alloy also plays an important role to reduce the maximum contact pressure and therefore reduce the wear rate. Additionally, the high plastic yield strength of transformed martensite NiTi enhances its wear resistance further.