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Journal ArticleDOI

Design and fabrication of a bending rotation fatigue test rig for in situ electrochemical analysis during fatigue testing of NiTi shape memory alloy wires

05 Mar 2013-Review of Scientific Instruments (Rev Sci Instrum)-Vol. 84, Iss: 3, pp 035102-035102
TL;DR: The current investigation proposes a novel method for simultaneous assessment of the electrochemical and structural fatigue properties of nickel-titanium shape memory alloy (NiTi SMA) wires using an in situ electrochemical cell in a custom-made bending rotation fatigue (BRF) test rig.
Abstract: The current investigation proposes a novel method for simultaneous assessment of the electrochemical and structural fatigue properties of nickel-titanium shape memory alloy (NiTi SMA) wires. The design and layout of an in situ electrochemical cell in a custom-made bending rotation fatigue (BRF) test rig is presented. This newly designed test rig allows performing a wide spectrum of experiments for studying the influence of fatigue on corrosion and vice versa. This can be achieved by performing ex situ and∕or in situ measurements. The versatility of the combined electrochemical∕mechanical test rig is demonstrated by studying the electrochemical behavior of NiTi SMA wires in 0.9% NaCl electrolyte under load. The ex situ measurements allow addressing various issues, for example, the influence of pre-fatigue on the localized corrosion resistance, or the influence of hydrogen on fatigue life. Ex situ experiments showed that a pre-fatigued wire is more susceptible to localized corrosion. The synergetic effect can be concluded from the polarization studies and specifically from an in situ study of the open circuit potential (OCP) transients, which sensitively react to the elementary repassivation events related to the local failure of the oxide layer. It can also be used as an indicator for identifying the onset of the fatigue failure.
Citations
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Journal ArticleDOI
TL;DR: A review of cable architecture and stress states experienced during testing is followed by an overview of the effects of changes in material composition, microstructure, processing and test conditions on fracture and fatigue behaviour of wire and cable systems used in biomedical applications.
Abstract: Fine wires and cables play a critical role in the design of medical devices and subsequent treatment of a large array of medical diagnoses. Devices such as guide wires, catheters, pacemakers, stents, staples, functional electrical stimulation systems, eyeglass frames and orthodontic braces can be comprised of wires with diameters ranging from 10s to 100s of micrometres. Reliability is paramount as part of either internal or external treatment modalities. While the incidence of verified fractures in many of these devices is quite low, the criticality of these components requires a strong understanding of the factors controlling the fracture and fatigue behaviour.1,2 Additionally, optimisation of the performance and reliability of these devices necessitates characterisation of the fatigue and fracture properties of its constituent wires. A review of cable architecture and stress states experienced during testing is followed by an overview of the effects of changes in material composition, microstructure, pr...

36 citations

Journal ArticleDOI
24 Jun 2015
TL;DR: In this paper, the state of the surface of the fatigued NiTi wire was monitored by following the evolution of the electrochemical open circuit potential (OCP) together with macroscopic stresses and strains.
Abstract: Fatigue of superelastic NiTi wires was investigated by cyclic tension in simulated biofluid. The state of the surface of the fatigued NiTi wire was monitored by following the evolution of the electrochemical open circuit potential (OCP) together with macroscopic stresses and strains. The ceramic TiO2 oxide layer on the NiTi wire surface cannot withstand the large transformation strain and fractures in the first cycle. Based on the analysis of the results of in situ OCP experiments and SEM observation of cracks, it is claimed that the cycled wire surface develops mechanochemical reactions at the NiTi/liquid interface leading to cumulative generation of hydrogen, uptake of the hydrogen by the NiTi matrix, local loss of the matrix strength, crack transfer into the NiTi matrix, accelerated crack growth, and ultimately to the brittle fracture of the wire. Fatigue degradation is thus claimed to originate from the mechanochemical processes occurring at the excessively deforming surface not from the accumulation of defects due to energy dissipative bulk deformation processes. Ironically, combination of the two exciting properties of NiTi—superelasticity due to martensitic transformation and biocompatibility due to the protective TiO2 surface oxide layer—leads to excessive fatigue damage during cyclic mechanical loading in biofluids.

24 citations

Book ChapterDOI
01 Jan 2017
TL;DR: In this paper, shape memory materials such as nitinol are introduced, and the importance of composition, melting methods, and component fabrication and heat treatment is reviewed, with an emphasis on cardiovascular stents, where the materials have been widely utilized.
Abstract: This chapter initially describes some fundamental aspects of shape memory alloy systems, including essential crystallographic features and how shape memory and superelastic characteristics are derived from these crystal structures and their temperature or stress dependence. Practical shape memory materials such as nitinol are introduced, and the importance of composition, melting methods, and component fabrication and heat treatment is reviewed. Microstructural changes during heat treatment and how these influence material transformation temperatures and mechanical properties are described. Methods for measuring transformation temperatures are briefly reviewed. Minimally invasive device applications for nitinol are reviewed in some detail, with an emphasis on cardiovascular stents, where the materials have been widely utilized. The advantages of the unique features of nitinol in stenting applications are presented and these are substantiated with a review of some clinical applications in coronary, peripheral, and aortic vessel applications. Stenting for other nonvascular indications and other devices such as vena cava filters and septal occluders are also presented. The use of the material in orthodontic and orthopedic applications is summarized, with porous nitinol structures offering interesting opportunities in relation to improved tissue integration. Important performance characteristics such as fatigue, corrosion, and imaging compatibility are also addressed. In conclusion, it is predicted that new applications for nitinol will continue to evolve.

17 citations

Journal ArticleDOI
TL;DR: In this article, the self-corrosion current density of the sample in the NaCl solution decreases to −8.4922E-08 A/cm2 after coupling electrical pulse and ultrasonic treatment (CEPUT).
Abstract: The loose oxide layer on the surface of Ti-6Al-4V titanium alloy can be removed and a dense α-phase layer with the thickness of 11 μm can be produced simultaneously by coupling electrical pulse and ultrasonic treatment (CEPUT). In addition, after CEPUT, the surface grain is refined, the surface roughness is reduced to 0.529 μm, and the surface hardness reaches to 627 HV, showing significant gradient change. During the CEPUT, the processability of the α-phase oxygen-rich layer is improved by the electroplasticity of high peak currents, and the α-phase oxygen-rich layer produces large plastic deformation. Most of the α-phase oxygen-rich layer is removed after the CEPUT, and the remaining coarse α-phase grains are refined to form a dense α-phase layer. The corrosion resistance of the dense oxide layer on the surface has been studied. The results show that the self-corrosion current density of the sample in the NaCl solution decreases to −8.4922E-08 A/cm2 after CEPUT. Compared with the untreated sample, the self-corrosion current density is reduced by two orders of magnitude. The reason for the increased corrosion resistance of titanium alloys may be that the electrolyte is blocked by the dense and intact α-phase layer on the surface and cannot penetrate further into the titanium alloy.

17 citations

Journal ArticleDOI
TL;DR: A SMA (shape memory alloy) wire-based separation actuator with high-load capacity and simple structure, which can endure the typical thermal and vibration environment tests without unexpected separation or structure damage, and separate normally after these environment tests.
Abstract: This paper proposes a SMA (shape memory alloy) wire-based separation actuator with high-load capacity and simple structure. The novel actuator is based on a one-stage locking mechanism, which means that the separation is directly driven by the SMA wire. To release a large preload, a group of anti-friction rollers are adopted to reduce the force for triggering. In addition, two SMA wires are used redundantly to ensure a high reliability. After separation, the actuator can be reset automatically without any auxiliary tool or manual operation. Three prototypes of the separation actuator are fabricated and tested. According to the performance test results, the actuator can release a maximum preload of 40 kN. The separation time tends to decrease as the operation current increases and it can be as short as 0.5 s under a 7.5 A (the voltage is 5.8 V) current. Lifetime test indicates that the actuator has a lifetime of more than 50 cycles. The environmental tests demonstrate that the actuator can endure the typical thermal and vibration environment tests without unexpected separation or structure damage, and separate normally after these environment tests.

8 citations

References
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Journal ArticleDOI
TL;DR: In this article, the authors present 10 less-obvious, but very important, reasons for nitinol's success, both past and future, including the quickly growing and technologically demanding stent applications.
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 the applications, there are actually a large number of lesser-known advantages of nitinol in medical devices. This paper reviews 10 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 in that they involve new and complex manufacturing techniques, present a demanding and interesting fatigue environment, and most interestingly, take advantage of the thermoelastic hysteresis of nitinol.

1,446 citations

Journal ArticleDOI
TL;DR: In this paper, structural and functional fatigue of NiTi shape memory alloys is investigated and four cases of fatigue are discussed: (1) the evolution of the stress-strain hysteresis in low cycle pull-pull fatigue of pseudo-elastic NiTi wires.
Abstract: Cyclic loading is one of the generic characteristic features of many of the present and potential future applications of NiTi shape memory alloys, no matter whether they exploit mechanical (pseudo-elasticity) or thermal shape memory (one and two way effect). Cyclic loading may well be associated with structural and functional fatigue, which both limit the service life of shape memory components. By “structural fatigue” we mean the microstructural damage that accumulates during cyclic loading and eventually leads to fatigue failure. There is a need to understand how microstructures can be optimized to provide good fatigue resistance. The term “functional fatigue” indicates that shape memory effects like the working displacement in a one way effect (1WE) actuator or the dissipated energy in a loading–unloading cycle of a pseudo-elastic (PE) damping application decrease with increasing cycle numbers. This is also due to a gradual change in microstructure. In both cases it is important to know how fatigue cycling affects shape memory properties. The present paper considers structural and functional fatigue of NiTi shape memory alloys. It discusses four cases of fatigue in NiTi shape memory alloys: (1) The evolution of the stress–strain hysteresis in low cycle pull–pull fatigue of pseudo-elastic NiTi wires. (2) Bending–rotation fatigue rupture of pseudo-elastic NiTi wires. (3) Strain localization during the stress induced formation of martensite. (4) Generic features of functional fatigue in NiTi shape memory actuator springs. The paper shows that fatigue of shape memory alloys is a fascinating research field and highlights the need for further work in this area.

661 citations

Journal ArticleDOI
TL;DR: In this paper, a rotary bending fatigue tester was used to measure the fatigue life of TiNi and TiNi-10Cu alloys, which were annealed at 673 K for 3.6 ks.
Abstract: Fatigue lives of Ti–Ni and Ti–Ni–10Cu alloys were measured using a rotary bending fatigue tester. The fatigue tests were carried out at 308, 323, 338, 368, and 398 K, respectively, under a constant strain amplitude condition. The Af points of Ti–Ni and Ti–Ni–10Cu alloys, which were annealed at 673 K for 3.6 ks, were 351 and 331 K, respectively. Two types of strain amplitude vs fatigue life curves were observed, one composed of two straight lines with one turning point whereas the other composed of three straight lines with two turning points. The upper turning point coincided with the elastic limit strain and the lower one with the proportional limit strain. The single turning point was observed in a fatigue test condition under which both the limit strains are almost the same. The fatigue life decreased with increasing test temperature in general. However, it became less sensitive to test temperature both in higher and lower temperature regions. Deformation mode and applied stress during fatigue testing are factors affecting fatigue life. However, the fatigue life of the Ti–50.0 at.% Ni is always longer than that of the Ti–40Ni–10Cu (at.%) if the fatigue life is plotted as a function of temperature difference between test temperature and Ms.

204 citations

Journal ArticleDOI
TL;DR: In this paper, the corrosion behavior of NiTi in Hanks' solution at 37°C was assessed by the use of electrochemical methods and the results were compared with Ti-6Al-4V alloy and 316L stainless steel, materials traditionally used as orthopaedic implants.

152 citations

Journal ArticleDOI
TL;DR: In this article, the influence of wire diameter and rotational speed on fatigue rupture behavior in bending rotation fatigue experiments was investigated and it was shown that the dependence of fatigue life on wire diameter was not observed when the experiments were conducted in a silicon oil bath at constant temperature or at low rotational speeds in air.
Abstract: In this study, we consider the influence of wire diameter and rotational speed on fatigue rupture behaviour in bending rotation fatigue experiments. We show that the dependence of fatigue life on wire diameter and rotational speed is no longer observed when the bending rotation fatigue experiments are conducted in a silicon oil bath at constant temperature, or at low rotational speeds in air. Moreover, we qualitatively discuss the stress distribution in the wire during fatigue testing as a starting point for various mechanical investigations, such as the calculation of the dissipated energy in the wire during bending rotation experiments or the bending moment acting during bending rotation.

129 citations