Showing papers on "Shape-memory alloy published in 2000"

Optimisation of processing and properties of medical grade Nitinol wire

Abstract: Summary The purpose of this paper is to review the current processing and resultant properties of standard Nitinol wire for guide-wire applications. Optimised Ti-50.8at%Ni wire was manufactured according to industry standards by precise control of the composition, cold work and continuous strain-age annealing. Mechanical properties of this wire are reported from 100°C to 200°C to demonstrate the effects of test temperature. Within the ‘superelastic window’ the plateau stresses are linearly related to test temperature. Additional ageing treatments can be used as a tool to fine-tune transformation temperatures and mechanical properties. A review of the fatigue properties of thermomechanically-treated Nitinol wire shows that they are affected by test temperature, stress and strain.

Nitinol medical devices and implants

Abstract: Superelastic Nitinol is now a common and well-known engineering material in the medical industry. While the greater flexibility of the alloy drives many of its applications, there are also a large number of lesser-known advantages of Nitinol in medical devices. This paper reviews 7 of these less-obvious but very important reasons for Nitinol's success, both past and future. Several new medical applications will be used to exemplify these points, including the quickly-growing and technologically-demanding stent applications. Stents are particularly interesting because they take advantage of the thermoelastic hysteresis of Nitinol.

MICROSTRUCTURE OF NiTi SHAPE MEMORY ALLOY DUE TO TENSION-COMPRESSION CYCLIC

Abstract: Dept. of Metallurgy and Materials Engineering, Catholic University of Leuven, de Croylaan 2, 3001Heverlee, Belgium(Received 16 June 1997; accepted in revised form 20 October 1997)Abstract—Experimental results have shown that, during mechanical cycling under tension–compressionload within 24% strains, the NiTi shape memory alloy is cyclic strain-hardened. The maximum stressesunder both tension and compression increase with increasing number of cycles and tend to stabilize withfurther cycling. The present work is focused on the martensite microstructure developed as a result ofmechanical cycling. TEM observations show that, before cycling, the martensite variants are well self-accommodated to each other with the h011itype II twinning as the main lattice invariant shear. Aftermechanical cycling, the martensite plates are still self-accommodated and the (111) type I twinning is mostfrequently observed. In addition to the stress-induced re-orientation of martensite and twin boundarymovement within the martensite plate, various lattice defects have been developed both in the junctionplane areas of martensite plates and within the martensite twins. # 1998 Acta Metallurgica Inc.1. INTRODUCTION

Control of Wave Propagation in Periodic Composite Rods Using Shape Memory Inserts

Abstract: Longitudinal wave propagation is controlled using shape memory inserts placed periodically along rods. The inserts act as sources of impedance mismatch with tunable characteristics. Such characteristics are attributed to the unique behavior of the shape memory alloy whereby the elastic modulus of the inserts can be varied up to three times as the alloy undergoes a phase transformation from martensite to austenite, With such controllable capability, the inserts can introduce the proper impedance mismatch necessary to impede the wave propagation along the rods. An analytical model is presented to study the attenuation capabilities of the composite rods and to determine the influence of the various design parameters of the inserts that can control the width of the pass and stop-bands. The numerical results demonstrate the potential of shape memory alloys in controlling the dynamics of wave propagation in rods. Furthermore, the obtained results provide a guideline for designing inserts that are capable of filtering out selected excitation frequencies through proper adjustment of the geometry of the inserts as well as their activation strategies.

Drive unit using shape memory alloy

Abstract: Disclosed herein is a drive unit using a shape memory alloy including: a shape memory alloy member made from a shape memory alloy, the shape memory alloy member exhibiting superelasticity when being energized; a drive body connected to the shape memory alloy member, the drive body being moved from a stopping position to a specific operational position when the shape memory alloy member is energized; and a locking mechanism for retaining the drive body at the specific operational position. With this configuration, the drive unit using a shape memory alloy is capable of reducing the power consumption as well as miniaturizing the drive unit.

Occurrence of ferromagnetic shape memory alloys (invited)

Abstract: This article presents an overview of the necessary conditions for the formation of shape memory alloys (SMAs) and how these must overlap with the occurrence of ferromagnetism so that ferromagnetic shape memory alloys (MSMAs) may be formed. The electronic, elastic, and geometric conditions permit an understanding of the occurrence of Cu- and Fe-based SMAs as well as NiMnGa Heusler MSMAs except that the naively determined critical electron concentration for the latter, 7.3, disagrees with the values of 1.5 and 8.6 accepted for the Hume–Rothery phases and certain Fe-based systems.

Experimental determination of thermal and electrical properties of Ni-Ti shape memory wires

Abstract: The thermal and electrical properties of shape memory alloys (SMA) are known to be different in their austenitic and martensitic phases. This paper addresses the determination of the phase-dependent heat capacity, thermal conductivity and electrical resistivity of a SMA wire. While the heat capacity measurements are relatively straightforward using a differential scanning calorimeter, the determination of the thermal conductivity and the electrical resistivity are more difficult in view of the possible non-uniformity in the material state of a SMA wire during a phase transformation. Experimental procedures are developed and used to determine these properties in either phase and, along with a previously developed finite-element code accounting for the non-uniform material states, the austenite and martensite properties are determined from the experimental data. For the SMA wire tested, the thermal conductivities of the austenite and martensite phases are determined to be 2.8×10-2 J (mm S K)-1 and 1.4×10-2 J (mm S K)-1 respectively, a difference of 100%. The electrical resistivities of the austenite and martensite phases are determined to be 8.371×10-4 Ω mm and 9.603×10-4 Ω mm respectively, a difference of about 14.7%.

The fatigue behavior of shape-memory alloys

Abstract: Shape-memory alloys have two unique properties: the shape-memory effect (ability of a material to be deformed at a low temperature and then revert to its prior shape upon heating) and superelasticity (the ability of a material to experience large recoverable strains when deformed). Many applications that take advantage of these properties require cyclic deformation, making fatigue behavior an important consideration.

Shape control of NITINOL-reinforced composite beams☆

Abstract: The shape of composite beams is controlled by sets of flat strips of a shape memory nickel–titanium alloy (NITINOL). The strips are embedded in the composite fabric of these beams inside sleeves, which are placed on the neutral axes. Prior to their insertion inside the beams, the NITINOL strips are thermally trained to provide and memorize controlled transverse deflections. Proper activation of the shape memory effect of the appropriate strips is utilized to produce controlled shapes of the NITINOL-reinforced beams. A mathematical model is developed to describe the behavior of this class of SMART composites. The model describes the interaction between the elastic characteristics of the composite beams and the thermally induced shape memory effect of the NITINOL strips. The effect of various activation strategies of the NITINOL strips on the shape of the composite beams is determined. The theoretical predictions of the model are validated experimentally using a fiberglass composite beam made of 8 plies of unidirectional BASF 5216 prepregs, which are 9.75 cm wide and 21.20 cm long. The beams are provided with four NITINOL-55 strips, which are 1.2 mm thick and 1.25 cm wide. The time response characteristics of the beam are monitored and compared with the corresponding theoretical characteristics. Close agreement is obtained between the theoretical predictions and the experimental results. The obtained results suggest the potential of the NITINOL strips in controlling the shape of composite beams without compromising their structural stiffness.

Training two-way shape memory alloy by reheat treatment

Abstract: Unlike one-way shape memory alloy (OWSMA), which can only remember the high temperature (austenite) shape, two-way shape memory alloy (TWSMA) remembers both high temperature and low temperature shapes. It is believed that training process plays a key role in the introduction of two-way shape memory from an initially one-way shape memory alloy. Various training methods have been proposed [1–6]. In general, either thermal/mechanical cycling or severe (plastic) deformation is essential. From fabrication and stability points of view, they are either non-convenient or have a lack of reliability. In this paper, we take a look at the feasibility of a new training method: by reheat treatment. In most cases, shape memory alloys are supplied as raw materials, i.e. without shape memory. To memorize a certain shape, heat treatment is normally required. This is to say, fixing the shape memory alloy into a required shape and then heating it up for a certain period of time at a high temperature. After this, one-way shape memory alloy is formed. Next, same procedure is repeated again but with different clamped shape of SMA and different heat treatment time. This is training TWSMA by reheat treatment. A 0.5 mm diameter NiTi shape memory alloy wire with one-way shape memory at room temperature was used for experiments followed. First, it was immersed into boiling water to measure its initially remembered high temperature, circular shape. An average diameter of 47.7 mm was noted. The NiTi wire was then cut into several pieces of required length. Wire-holders were fabricated to constrain the NiTi wires at the required diameters of 8 mm and 12 mm. The specimens undergoing different treatment were indicated as: a1 (8 mm), b1 (12 mm), a2 (8 mm), b2 (12 mm), a3 (8 mm) and b3 (12 mm). Reheat treatment was then conducted on the secured specimens at a high temperature of 500 ◦C with varying reheat treatment time of 10, 30, 60 min for a1 and b1, a2 and b2, a3 and b3, respectively. The specimens were rapidly quenched in cold water upon retrieval. To demonstrate the two-way shape memory introduced into the reheat treated specimens, thermal cycling was performed. The cold condition for thermal cycling was set at ice water temperature (Tc:0 ◦C) while hot condition was set at boiling water temperature (Th:100 ◦C). A full thermal cycle will see the specimens subjected to the hot condition first before transferring into the cold condition. 100 thermal cycles were carried out on each specimen. Measurements were done at 1st, 10th, 25th, 50th, 75th and 100th cycles of both hot and cold shapes. The evolution of diameter was then plotted (see Fig. 1, where symbol “o” stands for hot shape and “∗” for cold shape). Figs 1(a1) and (b1) show that at the first thermal cycle, specimens (a1) and (b1) expanded significantly to a larger diameter (a1 : 13.5 mm, b1 : 24.0 mm) in the hot condition from their initially preset shape. Further expansion was observed when exposed to the cold condition. Subsequent thermal cycling showed the specimens remembered a hot shape of a slightly smaller diameter at hot condition while expanding to a cold shape of a larger diameter at the cold condition, i.e. the trace of two-way shape memory. As the number of thermal cycles increases, the change in diameter between both shapes decreases. In Figs 1(a2) and (b2), the first thermal cycle shows that both specimens (a2) and (b2) having their diameter remained relatively close to their reheat treatment shape when subjected to hot condition. This shows that the specimens remember the new trained shape as their hot shape. A significant increase in diameter took place when both specimens were immersed into the cold condition. Thereafter, throughout the 100 thermal cycles, both specimens alternate between the hot and cold shapes, showing the effectiveness of training TWSMA by reheat treatment. Figs 1(a3) and (b3) show minute shape change for specimen (a3) and (b3) which were reheat treated for 1 h at 500 ◦C. Figs 2 and 3 show the importance of reheat treatment time in obtaining the optimum two-way shape memory.

Exploration of TiNi shape memory alloy for potential application in a new area: tribological engineering

Abstract: TiNi alloy is a well known shape memory alloy and has been widely used for bio-medical, mechanical and electrical applications. Recent research has demonstrated that TiNi alloy exhibits high resistance to wear and can be a superior tribo-material. Performance of this alloy benefits from its pseudoelasticity, resulting from a reversible martensitic transformation. Extensive research has been conducted at the University of Alberta to investigate wear behavior of TiNi alloy during various wear processes, including erosion, corrosive erosion, sliding wear and microscopic wear. The mechanism responsible for high wear resistance of TiNi alloy has been clarified to some degree and new phenomena are being continuously discovered. In particular, efforts have been made to develop tribo-composites using TiNi alloy as the matrix, reinforced by hard ceramic particles, including nano-structured particles. The composites obtained possess considerably enhanced wear resistance. This paper briefly reports progress in our studies on the development of this novel tribo-material.

A New Thermoelastic Model for Analysis of Shape Memory Alloy Hybrid Composites

Abstract: A constitutive model and a finite element formulation are developed for predicting the thermomechanical response of SMA hybrid composite structures subjected to combined thermal and mechanical load...

On the temperature dependence of the superelastic strength and the prediction of the theoretical uniaxial transformation strain in Nitinol

Abstract: This paper describes an alternative method for applying the Clausius-Clapeyron equation in a study of the effect of temperature on the superelastic stress in the shape memory alloy Nitinol—a candidate material for many medical devices. including, in particular, endovascular stents. This new analysis will provide some clarification on the controversy regarding estimation of the thermodynamic equilibrium temperature. The theoretical uniaxial transformation strain has been calculated by means of the Clausius–Clapeyron equation. The calculated value of strain, 5.0%, corresponded closely to the experimentally measured value of 4.7%.

Evaluation of the characteristics of a shape memory alloy spring actuator

Abstract: A shape memory alloy (SMA) spring actuator for an active catheter was fabricated by the heat treatment of wound SMA wire on a mandrel. In order to evaluate the characteristics of the fabricated SMA spring actuator, four types of tests were performed, as follows: isothermal loading and unloading, measurement of shape recovery force, temperature follow-up, and load follow-up tests. In this work, it was first developed that the prediction method of the shape recovery force was based on the neural network theory. The prediction results were better than the results from the conventional constitutive equation of SMA. From the experimental results, it was also proved that the following characteristic for loads was better than that for temperature and the following characteristics could be enhanced according to cooling and control methods.

Corrosion Resistance and Biocompatibility of Passivated NiTi

Abstract: Equiatomic nickel-titanium (NiTi) or Nitinol possess a unique combination of properties, including superelasticity and shape memory, which are very attractive for biomedical applications. NiTi has been used in orthopedic and orthodontic implants for several decades and has contributed to significant improvements in these fields [1, 2]. This alloy is rapidly becoming the material of choice for selfexpanding stents, graft support systems, filters, baskets and various other devices for minimally invasive interventional procedures (Fig. 1) [1,3]. While the superior performance of NiTi over conventional engineering materials for implants is well documented [1,4,5], the high nickel content of the alloy (55 weight % Ni) and itspossible influence on biocompatibility continues to be an issue of concern. This concern is further complicated by the conflicting literature on corrosion resistance.

Novel fabrication technique of TiNi shape memory alloy film using separate Ti and Ni targets

Abstract: This paper proposes a novel fabrication technique of TiNi shape memory alloy (SMA) film and verifies the validity by showing the actual fabrication process and the final results. Special attention is paid to the characterization of differential scanning calorimetry (DSC). An essential part of the fabrication process is the co-sputtering from separate pure Ti and Ni targets. Co-sputtering is carried out using a multi-target sputtering system where RF power for each target can be controlled independently. As-sputtered film is vacuum-annealed. The phase transformation behavior of the fabricated film is investigated by DSC and temperature-controlled X-ray diffractometry (XRD). Finally, shape recovery behavior is observed. The proposed novel fabrication technique is validated by these results.

Thermo-mechanical characterization of shape memory alloy torque tube actuators

Abstract: A parametric study was performed on NiTiCu shape memory alloy torque tubes. Four samples with varying wall thickness values were evaluated for their thermo-mechanical response. The torque against the angular displacement was measured for each sample when purely martensitic and purely austenitic. The recovery torque was measured as a function of the pretwist applied during loading in the martensite phase. The trends observed were compared to three models with relatively good agreement. The recovery torque for biaxial tension/compression-torsion tests were measured with values comparable to pure torsional loadings. During the biaxial loading, a time-dependent phenomenon was observed. To evaluate the time dependence, standard creep tests were performed to elucidate the influence of the load on the temporal response of the material.

Finite-element modeling of phase transformation in shape memory alloy wires with variable material properties

Abstract: In this paper, we address the issue of modeling the temperature distribution in a shape memory alloy (SMA) wire with variable thermal and electrical properties. This is done in the context of a one-dimensional (1D) boundary value problem where an initially martensitic SMA wire is electrically heated under zero-stress conditions. The model accounts for an evolution in the thermal conductivity, electrical resistivity and heat capacity during the phase transformation. The evolution in the 1D temperature field is found by implementing a Galerkin-based finite-element method. This is used in combination with a recursive iteration scheme to accurately determine the change in the material properties during a time step. The numerical approach is validated by comparing it with a known analytical solution with variable thermal properties. A parametric study on the SMA phase transformation indicates that, based on the considered values for the material properties, the heat capacity is the least important factor that needs to be accounted for, whereas the electrical resistivity is the most important. It is also demonstrated that the time required to complete a martensite to austenite transformation for a SMA wire subjected to an adiabatic boundary condition is lower if the model accounts for property variations. In fact, when the cyclic response of a SMA wire actuator subjected to an adiabatic boundary condition is the issue at hand, a model that does not account for property variations will predict a lower frequency of actuation than a model that does account for the property variations, as dealt with in this paper.

The control of the post-buckling response in thin composite plates using smart technology

Abstract: Restoration forces, associated with embedded activated pre-strained shape memory alloy wires, have successfully been employed to enhance the post-buckling behaviour of various laminated plate structures. An extensive experimental and numerical programme has been conducted, the results of which will be presented. The manufacturing methodology of the hybrid SMA/carbon/epoxy plates is outlined. Such specimens feature 0.4-mm diameter shape memory alloy wires located within tubing at desired locations. Numerical thermal analysis has been employed to predict the non-uniform temperature profile, attributed to shape memory alloy activation through resistive heating, within the laminates. Structural finite element analysis has been employed to determine the hybrid plates' adaptive response while under the influence of a uniaxial compressive load in excess of its critical buckling value. It is shown that, utilising the considerable control authority generated, even for a small actuator volume fraction, the out-of-plane displacement of the post-buckled laminates can be significantly reduced. Such displacement alleviation allows for load redistribution away from the specimens' unloaded edges. With the increase in use of composite materials within aerospace platforms, it is envisaged that the hybrid adaptive SMA/laminate configuration will extend the operational performance over conventional materials and structures, particularly when the structure is exposed to an elevated temperature.