NiTi SMA Superelastic Micro Cables: Thermomechanical Behavior and Fatigue Life under Dynamic Loadings.

Shape memory alloy (SMA) micro cables have a wide potential for attenuation of vibrations and structural health monitoring due to energy dissipation. This work evaluates the effect of SMA thermomechanical coupling during dynamic cycling and the fatigue life of NiTi SMA micro cables submitted to tensile loadings at frequencies from 0.25 Hz to 10 Hz. The thermomechanical coupling was characterized using a previously developed methodology that identifies the self-heating frequency. When dynamically loaded above this frequency, the micro cable response is dominated by the self-heating, stiffening significantly during cycling. Once above the self-heating frequency, structural and functional fatigues of the micro cable were evaluated as a function of the loading frequency for the failure of each individual wire. All tests were performed on a single wire with equal cross-section area for comparison purposes. We observed that the micro cable's functional properties regarding energy dissipation capacity decreased throughout the cycles with increasing frequency. Due to the additional friction between the filaments of the micro cable, this dissipation capacity is superior to that of the single wire. Although its fatigue life is shorter, its delayed failure compared to a single wire makes it a more reliable sensor for structural health monitoring.

Passive Attenuation of Mechanical Vibrations with a Superelastic SMA Bending Springs: An Experimental Investigation.

This work presents an experimental study related to the mechanical performance of a special design spring fabricated with a superelastic shape memory alloy (SMA-SE). For the experimental testing, the spring was coupled in a rotor machine, aiming to attenuate the mechanical vibration when the system went through a natural frequency without any external power source. It was verified that the reduction in instabilities stemmed from the better distribution of vibration force in the proposed device, as well as the damping capacity of the spring material. These findings showed that the application of the M-Shape device of SMA-SE for three different cases could reduce vibration up to 23 dB when compared to the situations without, and with, 1.5 mm of preload. The M-Shape device was shown to be efficient in reducing the mechanical vibration in a rotor system. This was due to the damping capacity of the SMA-SE material, and because the application did not require any external source of energy to generate phase transformation.

Influence of cyclic loading in NiTi austenitic and R-phase endodontic files from a finite element perspective.

OBJECTIVES: This study aims to evaluate the effects of cyclic loading on the bending moments and the developed stress state of austenitic and R-phase endodontic files through finite element analysis. MATERIALS AND METHODS: The mechanical properties of two groups of NiTi wires, austenite and R-phase, were measured in samples at two different conditions: uncycled and cycled. The cycled condition was achieved by subjecting samples of the two groups to 80% of the corresponding fatigue life under rotating bending efforts. The measured mechanical properties were then used in the finite element analysis, where the boundary and loading conditions were set to replicate a standard bending test. RESULTS: The results showed that mechanical cycling leads to decreasing stress levels and bending moments in the simulated files, especially in the austenitic ones. In comparison with austenite, R-phase presented a more stable mechanical behavior during cycling. CONCLUSIONS: The results show that the moment and stress calculated for an instrument under bending can be considerably decreased after some cyclic work. CLINICAL RELEVANCE: The fatigue related to the clinical use of an endodontic file decreases the moment (as well as the forces) imposed by the instrument during the shaping of a curved root canal. This decrease is directly related to the type of atomic array present in the alloy.

Critical Frequency of Self-Heating in a Superelastic Ni-Ti Belleville Spring: Experimental Characterization and Numerical Simulation.

The mechanical loading frequency affects the functional properties of shape memory alloys (SMA). Thus, it is crucial to study its effect for the successful use of these materials in dynamic applications. Based on the superelastic cyclic behavior, this work presents an experimental methodology for the determination of the critical frequency of the self-heating of a NiTi Belleville conical spring. For this, cyclic compressive tests were carried out using a universal testing machine with loading frequencies ranging from 0.5 Hz to 10 Hz. The temperature variation during the cyclic tests was monitored using a micro thermocouple glued to the NiTi Belleville spring. Numerical simulations of the spring under quasi-static loadings were performed to assist the analysis. From the experimental methodology applied to the Belleville spring, a self-heating frequency of 1.7 Hz was identified. The self-heating is caused by the latent heat accumulation generated by successive cycles of stress-induced phase transformation in the material. At 2.0 Hz, an increase of 1.2 °C in the average temperature of the SMA device was verified between 1st and 128th superelastic cycles. At 10 Hz, the average temperature increase reached 7.9 °C and caused a 10% increase in the stiffness and 25% decrease in the viscous damping factor. Finally, predicted results of the force as a function of the loading frequency were obtained.

Superelasticity and force plateau of nickel-titanium springs: an in vitro study

abstract Objective: This paper analyzed whether nickel-titanium closed coil springs (NTCCS) have a different superelastic (SE) behavior according to activation and whether their force plateau corresponds to that informed by the manufacturer. Methods: A total of 160 springs were divided into 16 subgroups according to their features and activated proportionally to the length of the extensible part (NiTi) of the spring (Y). The force values measured were analyzed to determine SE rates and force plateaus, which were mathematically calculated. These plateaus were compared to those informed by the manufacturer. Analysis of variance was accomplished followed by Tukey post-hoc test to detect and analyze differences between groups. Results: All subgroups were SE at the activation of 400% of Y length, except for: subgroups 4B and 3A, which were SE at 300%; subgroups 4E and 4G, which were SE at 500%; and subgroup 3C, which was SE at 600%. Subgroup 3B did not show a SE behavior. Force plateaus depended on activation and, in some subgroups and some activations, were similar to the force informed. Conclusions: Most of the springs showed SE behavior at 400% of activation. Force plateaus are difficult to compare due to lack of information provided by manufacturers.

resumo Objetivo: o presente artigo analisou se as molas helicoidais fechadas de níquel-titânio apresentam superelasticidade (SE), de acordo com a ativação, e se o platô de força medido corresponde ao informado pelo fabricante. Material e Métodos: 160 molas foram divididas em 16 subgrupos, de acordo com suas características, e foram ativadas proporcionalmente ao comprimento da parte extensível (NiTi) da mola (Y). Os valores de força obtidos foram analisados para determinar as taxas de SE e os platôs de força, os quais foram calculados matematicamente - sendo esses platôs comparados aos informados pelos fabricantes. Uma análise de variância foi realizada, seguida do teste post-hoc de Tukey, para detectar e analisar as diferenças entre os grupos. Resultados: todos os subgrupos apresentaram SE em ativação de 400% do comprimento Y, com exceção dos subgrupos 4B e 3A (que apresentaram SE a 300%), dos subgrupos 4E e 4G (com SE a 500%) e do subgrupo 3C (que apresentou SE na ativação de 600%). O subgrupo 3B não apresentou comportamento superelástico. Os platôs de força dependeram da ativação e em alguns subgrupos, em determinadas ativações, foram semelhantes à força informada pelo fabricante. Conclusões: a maioria das molas apresentou comportamento superelástico na ativação de 400%. Os platôs de força são difíceis de ser comparados, devido à falta de informações por parte dos fabricantes.

Propriedades em flexão de fios de liga de Ni-Ti / Bending properties of Ni-Ti alloys wires

As ligas de niquel-titânio (Ni-Ti) são usadas na fabricação de fios ortodônticos devido principalmente a sua maior resiliência e menor módulo de elasticidade quando comparadas com outras ligas metálicas, especialmente o aço inoxidável. O objetivo do presente trabalho foi comparar as propriedades mecânicas em flexão de fios de liga com memória de forma de diferentes fabricantes e lotes. Dois lotes de três fabricantes foram ensaiados em flexão três pontos de acordo com a norma ISO 15841:2006(E). Os resultados mostraram que os fios designados como termoativados geram tensões menores que os fios designados como superelásticos, observou-se variações de até 28% entre fios designados como superelásticos e 31% entre fios designados como termoativados. Na comparação dos lotes do mesmo fabricante observou-se também a não homogeneidade entre os fios.

The nickel-titanium (Ni-Ti) alloys are used in the manufacture of orthodontic wires mainly due to its greater resilience and low modulus of elasticity when compared to other alloys, particularly stainless steel. The aim of this study was to compare the mechanical properties of shape memory alloys wires in three-point bending in different manufacturers. Two lots of three manufacturers were tested in three-point bending according to ISO 15841:2006 (E). The results showed that wires designated as termoactivated generated lower tensions than the designated as superelastic, variations of up to 28% between wires designated as superelastic and 31% between wires d esignated as termoactivated were found. In the manufacturer lots comparison was also not observed homogeneity between wires.

Nickel-titanium alloys: a systematic review

OBJECTIVE: A systematic review on nickel-titanium wires was performed. The strategy was focused on Entrez-PubMed-OLDMEDLINE, Scopus and BioMed Central from 1963 to 2008. METHODS: Papers in English and French describing the behavior of these wires and laboratorial methods to identify crystalline transformation were considered. A total of 29 papers were selected. RESULTS: Nickel-titanium wires show exceptional features in terms of elasticity and shape memory effects. However, clinical applications request a deeper knowledge of these properties in order to allow the professional to use them in a rational manner. In addition, the necessary information regarding each alloy often does not correspond to the information given by the manufacturer. Many alloys called "superelastic" do not present this effect; they just behave as less stiff alloys, with a larger springback if compared to the stainless steel wires. CONCLUSIONS: Laboratory tests are the only means to observe the real behavior of these materials, including temperature transition range (TTR) and applied tensions. However, it is also possible to determine in which TTR these alloys change the crystalline structure.

Propriedades mecânicas de fios de NiTi e CuNiTi com efeito memória de forma utilizados em tratamentos ortodônticos / Mechanical properties of NiTi and CuNiTi wires used in orthodontic treatment

Objetivou-se nessa pesquisa comparar oito tipos de fios de NiTi superelásticos e termoativos, de seis empresas comerciais (GAC, TP, ORMCO, MASEL, MORELLI e UNITEK) àqueles com adição de cobre (CuNiTi 27 e 35OC, ORMCO), observando se as propriedades mecânicas dos dois últimos justificariam sua escolha clínica. Para tal foram realizados ensaios de tração e microscopia eletrônica de varredura. Os ensaios de tração foram realizados em máquina de ensaios mecânicos da marca EMIC, modelo DL10000, de 10 toneladas de capacidade, no Instituto Militar de Engenharia (IME). A composição química e a topografia superficial dos fios foram determinadas através da microscopia eletrônica de varredura em microscópio da marca JEOL, modelo JSM-5800 LV com sistema de microanálise EDS (energy dispersive spectroscopy). Os resultados mostraram que, de forma geral, os fios de NiTi termoativados apresentaram cargas mais suaves de desativação em relação aos superelásticos. Entre os fios que apresentaram as cargas biologicamente mais adequadas de desativação estão os termoativados da GAC e da UNITEK. Entre os fios de NiTi superelásticos, os de CuNiTi 27ºC da ORMCO foram os que apresentaram as cargas mais suaves de desativação, sendo semelhantes, estatisticamente (ANOVA), às apresentadas pelos fios de NiTi termoativados da UNITEK para a deformação de 4%. Quando comparados os fios de CuNiTi a 27 e a 35ºC, observou-se que os primeiros apresentaram forças de desativação de, aproximadamente, 1/3 das apresentadas pelos últimos, para a deformação de 4%. Quando analisada a microscopia eletrônica de varredura de superfície, os fios de NiTi superelásticos que apresentaram melhores acabamentos foram os da MASEL e MORELLI e os que apresentaram os piores acabamentos foram os de NiTi e CuNiTi 27ºC da ORMCO...

Leveling and aligning orthodontic wires must be able to generate light and continuous forces. Thus need to have high springback and flexibility. For this purpose it was suggested a variety of supereslatic and termoactivated Nickel-Titanium (NiTi) wires that may offer a load-deformation curve, in a constant plataform. Copper NiTi wires are presented as exhibiting better thermoactivating properties for optimum-forces system with better dental movement control. The aim of this study was to compare 8 NiTi superelastic and thermoactivated wires of six different brands (GAC, TP, ORMCO, MASEL, MORELLI and UNITEK) to Copper addicted wires (CuNiTi 270C and 350C, ORMCO) to verify if the mechanical properties of Copper NiTi would support it’s clinical use. Stress-strain tests were done in Engeneering Military Institute (IME-Brazil), through test machine (EMIC- DL 10000 model). Scanning electronic microscope with energy dispersive spectroscopy (JOEL, JSM-5800 LV model) was used to determine chemical composition and superficial topography of the wires. Results showed that, in general, thermoactivated NiTi wires exhibited lower deactivation loads when compared to NiTi superelastics. Among the thermoactivated, the GAC and UNITEK ones are the lighter ones. Among the superelastics, the Copper NiTi 270C (ORMCO) were the lighter ones, statistically similar (ANOVA) to thermoactivated NiTi from UNITEK, for 4% strain. Once Copper NiTi 270C showed deactivated loads 62% lower than Copper NiTi 350C , under 4% strain. As regard to Scanning Electronic Microscopy results for superelastic NiTi wires, better superficial burnishing were found for MASEL and MORELLI ones...