Shape Memory Alloys (SMA) are smart materials that have attracted increasing attention due to their superior damping properties when compared to conventional structural materials. These functional materials exhibit high damping capacity during phase transformation as well as in the low temperature martensitic state. In this work NiTi SMA, commercial aluminum, stainless steel and brass were submitted to dynamic mechanical analysis (DMA) in a single cantilever mode. Small beam specimens were manufactured to accomplish the DMA tests. The studied NiTi presented a damping capacity peak during phase transformation, being much higher than damping of conventional materials. NiTi SMA also showed an increase of storage modulus after conversion of low temperature phase to high temperature phase while an almost linear decrease is observed for the conventional materials studied.

Inside the Clausius-Clapeyron regime, transformation stresses during superelastic tensile tests of polycrystalline shape memory alloys are linearly dependent on temperature, with coefficients being the slopes of the forward and reverse transformation lines. In this work, experiments are performed to investigate the anisotropy of the slopes of the forward and reverse transformation stress-temperature lines in a NiTi superelastic thin walled tube. The classical Clausius-Clapeyron relation is widely used to model these slopes, although, in a strict sense, this relation is defined at thermodynamic equilibrium. Experimen

Abstract. Shape Memory Alloys are susceptible to annealing heat treatments, which are capable of partially or fully recover atomic mobility and, therefore, affect the overall thermomechanical response of the material. In this work, NiTi SMA orthodontic minicoil springs in superelastic state, widely commercialized, were submitted to annealing treatments as a way to modify their thermomechanical response and adapt it to the use in mechanical systems in other fields besides orthodontics. The main objective is to study the influence of temperature and time of annealing on the thermomechanical behaviour of the coil springs, originally superelastic at room temperature. Using a factorial design, three mechanical properties of interest were studied: spring constant, shear modulus and energy dissipation capacity. It was demonstrated that annealing in the range of 500°C-600°C is capable of converting superelastic springs to an apparently actuator state, as residual strain after loading/unloading at room temperature was observed, when a maximum 7% shear strain was attained in the cross section of the spring's wire.

This study analyses the thermomechanical tensile behaviour of a cold drawn Ti-50.9at.%Ni wire submitted to heat treatment at 598 K for 30 min, which is below the recrystallization temperature (623 K). Such low temperature heat treatment induces a superelastic loop without a stress "plateau". However, the absence or weakness of peaks on its differential scanning calorimetry prevents the determination of specific latent heat. This is a common effect of nanostructured materials such as superelastic wires. A method using strain and temperature field measurements was developed and used to determine thermal power and thermal energy during superelastic tensile tests through a heat balance. From these results and using a thermodynamic approach, forward and reverse specific latent heat and the martensite fraction are estimated as a function of strain and stress.

Active flexible composites can be developed through the coupling of silicone matrices and smart materials such as NiTi shape memory alloys (SMA). This work evaluates the mechanical behavior and adhesion of NiTi SMA ribbons embedded in a silicone matrix for the development of active flexible composites. Ribbons with cross-section of 0.15 × 0.8 mm2 were cold-rolled from superelastic NiTi wires with 0.4 mm in diameter. These ribbons were heat treated to either obtain characteristics of superelasticity (SE) or shape memory effect (SME) at room temperature. Silicone, NiTi wire, and ribbons were characterized by Differential Scanning Calorimetry (DSC), Dynamic Mechanical Analysis (DMA), and tensile tests. Adhesion between SMA ribbons and silicone matrix was evaluated with pull-out tests. The use of an adhesion promoter proved to be indispensable to ensure a good pull-out strength and shear stress at the NiTi ribbon and silicone interface. A silicone rubber cylinder with an embedded SME ribbon was built to evaluate the functionality of the NiTi ribbon in the development of flexible structures, which was demonstrated by the resistive heating and deformation of the SMA/silicone rubber composite.

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.

RESUMOAs ligas com memória de forma (LMF) são materiais ativos que têm atraído atenção devido às suas superiores propriedades de amortecimento quando comparadas aos materiais estruturais convencionais. Esses materiais apresentam uma alta capacidade de amortecimento, tanto durante a transformação de fase quanto na fase martensítica, em baixas temperaturas. Neste trabalho a LMF NiTi, alumínio comercial, aço inoxidável e latão foram submetidos à análise dinâmico-mecânica (DMA) em modo de viga simplesmente engastada. Pequenas amostras na forma de lâminas foram fabricadas para a realização dos testes. A LMF NiTi estudada apresentou um pico de capacidade de amortecimento durante a transformação de fase, levando o amortecimento a valores muito superiores àqueles apresentados pelos materiais convencionais. Além disso, foi observado que a liga NiTi apresenta um aumento do módulo de armazenamento durante a transição da fase de baixa temperatura para a fase de alta temperatura, o que abre uma extensa diversidade de opções para aplicações tecnológicas para essas ligas, enquanto um decréscimo quase linear da rigidez foi verificado para os materiais convencionais estudados. Comparative study of dynamic properties a NiTi alloy with shape memory and classical structural materials ABSTRACT Shape Memory Alloys (SMA) are active materials that have been attracting attention due to their superior damping properties when compared to the usual structural materials. Those materials present a high damping capacity, during the phase transformation as well in martensitic phase, on low temperatures. In this work a NiTi SMA, commercial aluminum, stainless steel and brass were submitted to dynamic mechanical analysis (DMA) in single cantilever mode. Small samples in beam form were manufactured for the accomplishment of the tests. NiTi SMA studied presented a peak of damping capacity during the phase transformation, taking the damping superior values to that presented by the conventional materials. Furthermore, it was observed that the NiTi alloy presents an increase of the storage modulus during the phase transition from low temperature phase to high temperature phase, what provides a great diversity of options for technological applications for those alloys, while an almost linear decrease of stiffness was verified for the studied conventional materials.

In this work, NiTi superelastic meshes are manufactured through plasma melting followed by an injection into a mesh ceramic coating mold obtained by the lost wax technique. This is the first time such a manufacturing sequence is reported for NiTi SMA. Focus is given to the feasibility and efficiency of the manufacturing process. The as-cast NiTi meshes were afterward heat-treated and hot-rolled for thickness reduction and to increase malleability. After manufacturing, the meshes' thermomechanical properties were characterized. Differential scanning calorimetry showed that at room temperature the NiTi meshes can either be used as thermal actuators (using the shape memory effect) or as superelastic devices, depending on whether heating or cooling is performed prior to the application. This behavior leaves great potential for applications in engineering systems and biomedical uses. The superelastic behavior of the NiTi meshes was characterized through tensile tests at room temperature, showing very good cycling stability and strain recovering up to 4%. In the future, microstructural and biocompatibility investigations of the product will be evaluated to optimize the manufacturing process.