Adelaide Nespoli

Design and thermo-mechanical analysis of a new NiTi shape memory alloy fixing clip

In this work, a new NiTi shape memory alloy (SMA) bone fixator is proposed. Thanks to the shape memory effect, this device does not need any external tool for the fixation, as the anchorage is obtained only by the self-accommodation of the clip during the parent transformation. Calorimetry and thermo-mechanical tests were used to evaluate the phase transformation temperatures and to estimate the forces generated both during the fixing surgical procedure and after the surgical operation. An application on animal anatomical sample was also performed; an appropriate mechanical tightness as well as a good handiness has been found.

DSC and three-point bending test for the study of the thermo-mechanical history of NiTi and NiTi-based orthodontic archwires

It is a known fact that the NiTi orthodontic archwire is one of the first and most diffuse biomedical applications of shape memory alloys. In the last years, none deep study about orthodontic archwires has been conducted from the material point of view. In general, the clinical response is the principal aspect that has been investigated for this application. Nonetheless, the accurate mechanical and physical characterization of the archwires can be very important to add new developments to this biomedical product and to give a substantial contribution to the indispensable evolution that is crucial for better clinic results. In fact, the principal aspect that it is needed for further improvements is the study of the optimal force that does not cause damage to the surrounding tissues. According to this statement, a deep study about the thermo-mechanical characterization of several pseudoelastic commercial archwires used in the straight-wire low-friction techniques is presented. In detail, flexural mechanical tests in the three-point-bending configuration were conducted to assess the archwires unloading force, while differential scanning calorimetry was used to study the phase transition temperatures, and the thermo-mechanical history of each specimen. Both NiTi and NiTiCu commercial archwires were tested, and different geometries were considered. For all the archwires, an excellent repeatability of the results has been found. This series of characterizations provides a complete view of the thermo-mechanical properties of the material, and therefore it shows the possibility to modulate the functional properties developed by the device as a function of the biological field.

Design and experimental characterization of a NiTi-based, high-frequency, centripetal peristaltic actuator

Development and experimental testing of a peristaltic device actuated by a single shape-memory NiTi wire are described. The actuator is designed to radially shrink a compliant silicone pipe, and must work on a sustained basis at an actuation frequency that is higher than those typical of NiTi actuators. Four rigid, aluminum-made circular sectors are sitting along the pipe circumference and provide the required NiTi wire housing. The aluminum assembly acts as geometrical amplifier of the wire contraction and as heat sink required to dissipate the thermal energy of the wire during the cooling phase. We present and discuss the full experimental investigation of the actuator performance, measured in terms of its ability to reduce the pipe diameter, at a sustained frequency of 1.5 Hz. Moreover, we investigate how the diameter contraction is affected by various design parameters as well as actuation frequencies up to 4 Hz. We manage to make the NiTi wire work at 3% in strain, cyclically providing the designed pipe wall displacement. The actuator performance is found to decay approximately linearly with actuation frequencies up to 4 Hz. Also, the interface between the wire and the aluminum parts is found to be essential in defining the functional performance of the actuator.

Laser and Surface Processes of NiTi Shape Memory Elements for Micro-actuation

In the current microtechnology for actuation field, shape memory alloys (SMA) are considered one of the best candidates for the production of mini/micro devices thanks to their high power-to-weight ratio as function of the actuator weight and hence for their capability of generating high mechanical performance in very limited spaces. In the microscale the most suitable conformation of a SMA actuator is given by a planar wavy formed arrangement, i.e., the snake-like shape, which allows high strokes, considerable forces, and devices with very low sizes. This uncommon and complex geometry becomes more difficult to be realized when the actuator dimensions are scaled down to micrometric values. In this work, micro-snake-like actuators are laser machined using a nanosecond pulsed fiber laser, starting from a 120-μm-thick NiTi sheet. Chemical and electrochemical surface polishes are also investigated for the removal of the thermal damages of the laser process. Calorimetric and thermo-mechanical tests are accomplished to assess the NiTi microdevice performance after each step of the working process. It is shown that laser machining has to be followed by some post-processes in order to obtain a micro-actuator with good thermo-mechanical properties.

A new design of a Nitinol ring-like wire for suturing in deep surgical field.

The present work proposes a new suturing procedure based on self-accommodating suture points. Each suture point is made of a commercial NiTi wire hot-shaped in a single loop ring; a standard suture needle is then fixed at one end of the NiTi suture. According to this simple geometry, several NiTi suture stitches have been prepared and tested by tensile test to verify the closing force in comparison to that of commercial sutures. Further experimental tests have also been performed on anatomic samples from animals to verify the handiness of the NiTi suture. Moreover, surface quality of sutures has been carefully investigated via microscopy. Results show that the NiTi suture expresses high stiffness and a good surface quality. In addition, the absence of manual knotting allows for a simple, fast and safe procedure.

Temperature-modulated differential scanning calorimetry for the study of reversing and nonreversing heat flow of shape memory alloys

The measurement of the thermophysical parameters associated with the thermoelastic martensitic transformation (TMT) of shape memory alloys (SMAs) is fundamental for the correct simulation of the process describing the outcome of SMA actuation. In particular, there are two important energy components during a TMT: the first one is a reversing form related to the nucleation and accommodation of martensite twin boundaries upon cooling, and it is restored when the martensite is heated up to austenite finishing transformation temperature. The second one is represented by a nonreversing form associated with the movement of the grain interfaces, with the defect forming and with the work raised when a plastic transformation occurs. This nonreversing component is dissipated in the form of heat and internal frictional work. Standard differential scanning calorimetry (DSC) measures the sum of these two contributions; therefore, DSC is not able to identify the reverse component that is the one useful in simulation and mathematical modeling of the SMA actuation. Temperature-modulated DSC (MDSC) is really able to separate the two energy forms by the superimposition of a sinusoidal temperature signal to the standard DSC linear temperature ramp. In this paper, MDSC is used to investigate the evolution of reversing and nonreversing heat flows of several NiTi and NiTi-based wires and sheets depending on the microcrystalline state. Observations on the reversing specific heat capacity trend are also drawn.

New Developments on Mini/Micro Shape Memory Actuators

In the actuation field, the most common shape memory alloy is quasi-equiatomic NiTi system, commercially known as Nitinol®. Ti-rich NiTi compounds show characteristic transformation temperatures higher than the room one and they can recover high values of deformation. Moreover, these intermetallic compounds are widely used since they exhibit high thermal and mechanical cycling stability. Other NiTi-based alloys are also employed for this kind of applications. In particular, NiTiCu, with Cu substituting Ni in the 3-10at% range, or NiTiCo system are used respectively when the application requires narrow thermal hysteresis or high stiffness.

Processing and surface treatments for pseudoelastic wires and strands

ABSTRACT The aim of this work is a preliminary investigation of the manufacturing process of Ni-Ti and Ni-Ti-Cr wires and strands to be applied in mechanical applications. The diamond wires for stone cutting were used as a case study. A prototype of multiwire Ni-Ti strand has been prepared, characterized and thermally treated in this research. The pseudoelastic strands have not been widely investigated in the literature to date, and they are not available in the market with this size (final diameter in the 1–2 mm range). A mechanical characterization (tensile and three-point bending tests), as well as surface analysis (scanning electron microscopy [SEM], X-ray photoelectron spectroscopy [XPS], Energy Dispersive X-ray Spectrometry [EDS], dynamic wettability), of a single wire and multiwire strand was performed. Moreover, different surface treatments to be used during the manufacturing process have been tested for improving the adhesion of a polymeric coating (polyurethane) as protection against abrasion and as the binding element of the diamond beads assembled on the strand. The surfaces were treated by mechanical roughening and different chemical modifications (acid and peroxide etching) and characterized.

Study of pseudoelastic systems for the design of complex passive dampers: Static analysis and modeling

This work presents an experimental and numerical analysis of several parallel systems of NiTi pseudoelastic wires. Standard tensile tests were accomplished to evaluate the global damping capacity, the energy dissipated per cycle and the maximum attenuated force in a static condition. Besides, a numerical model was implemented to predict the damping response of more complex pseudoelastic arrangements. It was found a damping capacity upper limit of 0.09 regardless the number and the length of the NiTi components. In addition, it was found that the energy dissipated per cycle is related to the strain and to the number of the NiTi components; furthermore, the system composed of NiTi wires with different length allows for an elastic region that is related to the numbers of wires and that presents a modulation of the stiffness. Finally, the proposed numerical model allows a precise design of complex pseudoelastic combinations as it is able to represent the rhombohedral characteristic.

Functional characterization of NiTi Shape Memory elements for smart micro-actuation

Shape Memory Alloys (SMAs) are smart and functional materials, which are considered good candidates for the activation of devices for the automotive, aerospace, biomedical and mechanical systems, thanks to the shape memory effect. In this work, a study on the mechanical response of NiTi SMA snake like elements has been proposed. The production route of these elements from thin sheets, was given by laser machining followed by chemical etching. The micro-elements were characterized by means of calorimetric analysis for the definition of the theoretical operating temperatures and by means of thermo-mechanical testing for the evaluation of their functional performances. Mechanical tests has been carried out to assess the tensile behavior of martensite and austenite separately, and to evaluate the thermal hysteresis under different constant loads. Moreover, Finite Element Modeling (FEM) has been also accomplished to study the numerical evaluation of the stress field that origins by the application of the different loads in both the martensitic and austenitic phases.Copyright