NiTi Sheets Research Articles

High strain and long duration cycling behavior of laser welded NiTi sheets

The use of NiTi in complex shaped components for structural applications is limited by the material cost and machinability and adequate joining techniques have been investigated to minimize the thermal cycle effect on the superelastic and shape memory effects exhibited by NiTi. Laser welding is the most used joining process for this material. However, existing studies mainly address the functional properties of laser welded NiTi wires, and the superelastic cycling tests are limited to either a low number of cycles (maximum 100) or to low strains (below 6%). This paper discusses the results of the cycling behavior exhibited by laser butt welded 1mm thick NiTi plates, when tested to high strains (up to 10%) and for a large number of cycles (600). The superelastic effect was observed and the microstructural changes induced by the laser welding procedure, namely the extension of the thermal affected regions, were seen to influence the evolution of the accumulated irrecoverable strain. Thus, it is possible, by controlling the heat input introduced during welding, to tune the maximum superelastic recovery presented by NiTi laser welds.

Wavelets Image Analysis for Friction Stir Processed TiNi Functional Behavior Characterization

A key topic regarding Ti Ni Shape Memory materials concerns the possibility to attain welded junctions that preserves the shape memory properties of material. Other research topic for SMAs regards the retention of the shape memory effect cyclic stability; in fact, good shape memory properties frequently decrease during SME cycling of material. A method able to improve the cyclic stability of TiNi shape memory effect is the grain refinement. Considering these above mentioned research topics, a solid state welding process, as the Friction Stir Welding, is thus attractive for SMA joining and it exhibits potentials for achieving welded joints affected by microstructural changes that preserve the shape memory properties and retain, furthermore, the cyclic stability of SME. The basic objective of this study was to investigate the feasibility of friction stir welding process to join TiNi shape memory alloy sheets flat memorized, preserving the TiNi shape memory behaviour. This aim has been pursued determining the influence of the thermomechanical modifications induced by Friction Stir Processing of TiNi sheets on the functional properties of material. Optical microscopic investigations of Friction Stir Processed material cross sections have been used to highlight the modified microstructure of processed zone. A proper image processing procedure has been performed in order to quantify the amount of martensitic phase and to detect its morphology modification along the processed region. Particularly each micrographic image at first has been denoised using the 2D Wavelet transform technique and successively a texture segmentation procedure allows evaluating the amount of the martensite and austenite phases and classifying the morphological changes of martensitic regions. The austenitic and martensitic transformation temperatures of material were investigated using a stress applied characterization method suitably set up to perform the whole stress-temperature material characterization.

Study on Measurement of Group Velocity of Lamb Waves in Shape Memory Alloy Sheets

This paper describes the nondestructive evaluation of microstructure using laser-excited Lamb waves to detect the phase transformation in NiTi shape memory alloy sheets. Lamb waves were applied in the NiTi sheet using a pulse laser beam. Piezoelectricity transducers were used to receive the Lamb waves, the group velocities of which were measured using a time-frequency analysis method at different temperatures. Results show that a marked variation in the group velocity occurs during the phase transformation in the NiTi alloy. The dependence of group velocity on temperature provides a effective means of inspecting microstructure transformation in NiTi alloys.

프로그레시브 전단 공정에서 박판 핀 비틀림 최소화를 위한 스프링 배치 최적화

Progressive shearing with blanking dies is commonly employed to produce large quantities of tiny sheet metal electronic parts. Sheet metal pins, which are narrow and long, that are sheared with a progressive die set are often twisted. The twist in the sheet metal pins, which usually occurs in the final shearing operation, generally decreases with increasing blank holding force. The blank holding forces in all shearing operations are not the same because of different shearing positions and areas. In the current study, the optimal layout of the springs in a progressive die set to minimize the twist of the sheet metal pin is proposed. In order to find the holding force acting on the tiny narrow blanks produced with the proposed springs during the shearing process, the equivalent area method is used in the structural analysis. The shearing of the sheet-metal pin was simulated to compute the twist angle associated with the blank holding force. The constraint condition satisfying the pre-set blank holding force from the previous shearing operations was imposed. A design of experiments (DOE) was numerically implemented by analyzing the progressive die structure and by simulating the shearing process. From the meta-model created from the experimental results and by using a quadratic response surface method (PQRSM), the optimal layout of the springs was determined. The twist of sheet metal pin associated with the optimal layout of the springs found in the current study was compared with that of an existing progressive die to obtain a minimal amount of twist.

Stress induced martensite at the crack tip in NiTi alloys during fatigue loading

Crack tip stress-induced phase transformation mechanisms in nickel-titanium alloys (NiTi) were analyzed by Digital Image Correlation (DIC), under fatigue loads. In particular, Single Edge Crack (SEC) specimens, obtained from a commercial pseudoelastic NiTi sheet, and an ad-hoc experimental setup were used, for direct measurements of the near crack tip displacement field by the DIC technique. Furthermore, a fitting procedure was developed to calculate the mode I Stress Intensity Factor (SIF), starting from the measured displacement field. Finally, cyclic tensile tests were performed at different operating temperature, in the range 298-338 K, and the evolution of the SIF was studied, which revealed a marked temperature dependence.

Fatigue properties of a pseudoelastic NiTi alloy: Strain ratcheting and hysteresis under cyclic tensile loading

Low cycle fatigue properties of a commercial pseudoelastic Ni-rich Nickel–Titanium alloy have been analyzed. In particular, strain controlled pull–pull fatigue tests have been carried out on dog-bone shaped specimens, obtained from commercial NiTi sheets. Large ratcheting strains have been observed from the first mechanical cycles, up to a complete stabilization of the stress–strain response of the material. These mechanisms, mainly attributed to the formation of stabilized martensite and dislocation networks, cause a marked evolution of the functional properties of the alloy in terms of pseudoelastic recovery capabilities. This evolution has been systematically analyzed by identifying some functional damage parameters, involving strain, energy, transformation stress, Young’s modulus and phase transition temperatures. Furthermore, the data on structural fatigue, i.e. cycles to failure as a function of strain amplitude, are illustrated and analyzed according to a modified Coffin–Manson approach.

Mechanical and Functional Properties of Nickel Titanium Adhesively Bonded Joints

In this study, adhesive joints made up of commercial NiTi sheets with shape memory capabilities are analyzed. Suitable surface pre-treatments, i.e., degreasing, sandblasting, and chemical etching, are preliminary compared in terms of surface roughness, surface energy, and substrate thinning. Results indicate that chemical etching induces marked substrate thinning without substantial gains in terms of surface roughness and free energy. Therefore, adhesive joints with degreased and sandblasted substrates are prepared and tested under both static and cyclic conditions, and damage development within the adhesive layer is monitored in situ using a CCD camera. Sandblasted specimens have a significantly higher mechanical static strength with respect to degreased ones, although they essentially fail in similar fashion, i.e., formation of microcracks followed by decohesion along the adhesive/substrate interface. In addition, the joints show a good functional response with almost complete shape memory recovery after thermo-mechanical cycling, i.e., a small accumulation of residual deformations occurs. The present results show that adhesive bonding is a viable joining technique for NiTi alloys.

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.

The tribocorrosion behaviour of NiTi alloy

In biomedical applications, NiTi alloys are used mainly because of their favourable shape memory and superelastic properties. However, in many applications the tribocorrosion properties of these alloys can be of critical concern. For this reason the electrochemical and tribocorrosion properties of superelastic NiTi sheet and orthodontic archwire were studied, taking into account their microstructures and the effect of different surface finishes. In the case of the electrochemical tests, samples were tested in artificial saliva, whereas in the tribocorrosion tests the experiments were performed in ambient air, distilled water, and artificial saliva, the latter as a corrosive medium. In these tests, the total wear rate of the alloy samples was determined, together with the corresponding chemical and tribological contributions. It was confirmed that the microstructure of the investigated alloys had a significant effect on the measured electrochemical and tribocorrosion properties.

Crack tip stress distribution and stress intensity factor in shape memory alloys

ABSTRACTThe crack tip stress‐induced martensitic transformation and the resulting stress distribution in a nickel–titanium (NiTi)‐based shape memory alloy have been analysed. In particular, the effects of temperature, within the stress‐induced transformation regime, in single edge crack specimens have been studied by a recent analytical model, finite element simulations and experimental measurements. The results of the analytical model have been compared with those obtained from finite element simulations, carried out by using a special constitutive model for shape memory alloys, and good agreements have been observed. Furthermore, full field numerical results have been used to better understand the evolution of the volume fraction of martensite in the crack tip region. Finally, experimental measurements have been carried out, by using single edge crack specimens obtained from commercially available NiTi sheets by electro‐discharge machining. The results have been analysed by linear elastic fracture mechanics theory, and the analytical model has been used to calculate modified stress‐intensity factors for shape memory alloys. A slightly increase of the critical stress‐intensity factor has been observed with increasing the testing temperature, and this result is in accordance with the model calculations, which indicate a toughening effect, i.e. a reduction of the crack tip stress intensity factor.

Adaptable Orthopedic Shape Memory Implants

Apart from biological factors, the bone healing process strongly depends on the mechanical properties of the implant applied. The stiffness of the implant influences the micro-motion, one of the most important stimuli of the healing process within the fracture gap. Both, the “ideal” stiffness of an implant as well as the alteration of the mechanical properties over the course of healing are still a topic of research. Apart from biodegradable implants or “fixateur externe”, it is not possible to alter the mechanical properties of the implants without second surgery. A “stiffness alterable” implant might have a positive impact on the healing process, e.g. a shorter healing time, an early ability to bear weight or the avoidance of a follow-on operation.In order to adapt the implant to the actual healing situation, nickel-titanium (NiTi) shape memory alloy (SMA) based implants that exhibit a variable middle piece were developed. Due to contactless induction heating, the transition temperature of the SMA is reached and triggers the one-way shape memory effect (SME). This leads to a slight modification of the second moment of inertia and to an adaption of the bending stiffness of the implant, respectively. Based on laser cutting and welding, NiTi-SMA based implants were manufactured by using thin, commercially available NiTi (thickness: 0.5 and 1.0mm, transition temperature Af between 55-65°C) sheets. Depending on the implant design, 4-Point-Bending measurements showed feasible bending stiffness alterations around a range of 50%, with regard to the original stiffness. Thereby, two approaches, increasing the bending stiffness and decreasing the bending stiffness, have been realized.Currently, the implant design is being adapted in order to meet the requirements for human applications. Custom made NiTi-SMA with a transition temperature around 45-50°C could continue to reduce the impact of the heating process and expand this concept to further medical applications, e.g. compression, anchoring or expansion of implants or devices.

Fatigue of pseudoelastic NiTi within the stress-induced transformation regime: a modified Coffin–Manson approach

Strain controlled fatigue tests of a pseudoelastic nickel–titanium (NiTi) shape memory alloy (SMA) have been carried out in this investigation. In particular, flat dog-bone shaped specimens, obtained from commercial NiTi sheets, have been analyzed, under pull–pull loading conditions, in two subsequent steps: (i) material stabilization and (ii) fatigue life estimation. The first step was carried out to obtain a stable pseudoelastic response of the SMA, i.e. with no residual deformations upon unloading, and it can be regarded as a preliminary processing condition of the alloy. Results on functional fatigue, i.e. in terms of stabilized pseudoelastic response, and on structural fatigue, in terms of cycles to failure, are reported and discussed. Furthermore, a modified Coffin–Manson approach for fatigue life estimation of SMAs is proposed, which takes into account the strain mechanisms involved during repeated stress-induced martensitic transformations.

Achieving Small Structures in Thin NiTi Sheets for Medical Applications with Water Jet and Micro Machining: A Comparison

NiTi shape memory alloys (SMA) are used for a variety of applications including medical implants and tools as well as actuators, making use of their unique properties. However, due to the hardness and strength, in combination with the high elasticity of the material, the machining of components can be challenging. The most common machining techniques used today are laser cutting and electrical discharge machining (EDM). In this study, we report on the machining of small structures into binary NiTi sheets, applying alternative processing methods being well-established for other metallic materials. Our results indicate that water jet machining and micro milling can be used to machine delicate structures, even in very thin NiTi sheets. Further work is required to optimize the cut quality and the machining speed in order to increase the cost-effectiveness and to make both methods more competitive.

Microstructures and properties of laser cladding NiTi alloy with W for biomedical applications

The feasibility of improving the radiopacity of NiTi by means of Nd:YAG laser cladding is investigated in the present study. Fine elementary tungsten powder was pasted on NiTi sheets and then melted using laser. The resulting microstructure was analyzed by a scanning electron microscope (SEM) equipped with energy-dispersive X-ray (EDX) spectrometry. The results show that the solubility of tungsten in the NiTi-matrix is low and the excessive tungsten forms fine precipitates and W-rich compounds, which result in the average W content in the fusion zone up to 8 at.%. The laser-clad sample failed near the end of the plateau with a tensile strength of about 410 MPa.

Culture of human epidermal melanocytes with human amniotic membrane as a scaffold and their biological ability identification

Objective To establish a primary culture of human melanocytes from tiny skin sheets harvested by using a suction blister method, to carry out a serial subcultivation of the melanocytes with human amniotic membrane （AM） as a scaffold, and to observe the influence of AM on the adhesion, proliferation and dendrite development of melanocytes. Methods Tiny skin sheets were collected from the flexual forearm or lower abdomen of a healthy male volunteer by a suction blister method and melanocytes in the skin sheet were counted following Dopa staining under a microscope. The trypsinized skin sheets were scraped with a scalpel to harvest melanocytes which were subjected to a primary culture. Then, the melanocytes were inoculated onto fresh or cryopreserved AM followed by a culture for various durations （4, 8 and 12 days）. The morphology and dendrite development of melanocytes were visualized under an inverted microscope after dopa-staining, cell viability evaluated by MTT assay, the adhesion to AM examined by hematoxylin and eosin （HE） staining protocol. Results The density of melanocytes was 1543.1±13.3 cells per mm2 and 857.4±101.7 cells per mm2 in skin sheets obtained from the forearm flexure and lower abdomen of the volunteer, respectively. A skin sheet of about 25.1 mm2 from approximately two blister roof was required to ensure the success of primary culture of melanocytes within 1 month. After culture on fresh or cryopreserved AM for 4, 8, and 12 days, most melanocytes were bi-polar with extended slender dendrites compared with those cultured in common cell culture medium. HE staining showed that melancytes adhered and were evenly distributed on the basement membrane of AM. MTT assay showed that the AM inhibited the proliferation of melanocytes, and no statistical difference was observed in the inhibitory effect between fresh AM and cryopreserved AM （P＞ 0.05）. Conclusions Enriched with melanocyes, flexural forearm is a preferable donor site to offer skin sheets for primary culture of melanocytes. Human AM could improve the adhesive growth and dendrite development of melanocytes, and may serve as a promising bioscaffold for in vitro expansion of melanocytes. Key words: Melanocytes; Cell culture techniques; Vitiligo

Pulsed Nd:YAG laser cutting of NiTi shape memory alloys—Influence of process parameters

Shape memory alloys (SMAs), in particular Nitinol (NiTi), are of increasing interest in research and industry due to their outstanding properties, e.g. the shape memory effect (SME) and high biocompatibility. Obviously, it is necessary to machine these elements from NiTi sheet materials using suitable processing methods that provide high precision and retain the shape memory effect. Pulsed Nd:YAG laser cutting of 1 mm thick NiTi shape memory alloys for medical applications (SMA-implants) has been investigated. Due to the local energy input only small heat-affected zones (HAZ) occur and the shape memory properties remain. The influence of key parameters like pulse energy, pulse width, and spot overlap on the cut geometry, roughness and HAZ is shown.

Wafer-Scale Manufacturing of Bulk Shape-Memory-Alloy Microactuators Based on Adhesive Bonding of Titanium–Nickel Sheets to Structured Silicon Wafers

This paper presents a concept for the wafer-scale manufacturing of microactuators based on the adhesive bonding of bulk shape-memory-alloy (SMA) sheets to silicon microstructures. Wafer-scale integration of a cold-state deformation mechanism is provided by the deposition of stressed films onto the SMA sheet. A concept for heating of the SMA by Joule heating through a resistive heater layer is presented. Critical fabrication issues were investigated, including the cold-state deformation, the bonding scheme and related stresses, and the titanium-nickel (TiNi) sheet patterning. Novel methods for the transfer stamping of adhesive and for the handling of the thin TiNi sheets were developed, based on the use of standard dicing blue tape. First demonstrator TiNi cantilevers, wafer-level adhesively bonded on a microstructured silicon substrate, were successfully fabricated and evaluated. Intrinsically stressed silicon dioxide and silicon nitride were deposited using plasma-enhanced chemical vapor deposition to deform the cantilevers in the cold state. Tip deflections for 2.5-mm-long cantilevers in cold/hot state of 250/70 and 125/28 mum were obtained using silicon dioxide and silicon nitride, respectively. The bond strength proved to be stronger than the force created by the 2.5-mm-long TiNi cantilever and showed no degradation after more than 700 temperature cycles. The shape-memory behavior of the TiNi is maintained during the integration process.

Phenomenological description of thermomechanical behavior of shape memory alloy

On the basis of a thermomechanical phenomenological model, we analyze the thermomechanical behavior of polycrystalline NiTi. Pseudoelastic response and strain-temperature response under fixed stress are studied by using finite element simulation. Calculated mechanical and thermal hysteresis behaviors of NiTi sheet are in good agreement with those observed experimentally. Hardening in stress–strain hysteresis loop and sharp change of strain in strain-temperature hysteresis loop are described by numerical simulation. The result from thermomechanically coupled calculation shows the phenomenon that phase transition occurs by nucleation and propagation of transformation fronts.

Crystallographic mechanisms of fracture in a textured polycrystalline TiNi shape memory alloy

The crystallographic mechanisms of the crack initiation and propagation in a textured polycrystalline TiNi shape memory alloy were studied using scanning electron microscope and ex situ electron backscattered diffraction techniques. The results indicated that stress-induced martensitic transformation was triggered and formed a sandwich structure at the front of the crack tip prior to the crack initiation in the polycrystalline TiNi alloy. The crack propagated intragranularly and occasionally across the grain boundaries. The closed packed planes of the B2 parent phase such as {110} and {100} were revealed to favor the crack propagation. The stress-induced B2+B19′ sandwich structure determined the crack propagated by a microscopic sawtooth pattern and therefore a ductile feature in this textured polycrystalline TiNi sheet specimen.

Survey of the in situ configuration of cold-rolled, nickel-rich NiTi sheets to create variable component functions

An outstanding feature of shape memory alloys is their potential to produce different functional effects, like thermal shape memory or superelasticity in one component. The cold-rolled NiTi sheet strips presented in this study offer various options, since they do not feature perceptible thermal shape memory or superelasticity due to a high dislocation density. Instead, to achieve a specific local function, the sheet strips are subjected to in situ heat treatment carried out by a laser or a local resistive heating element. There is a need to adjust the duration and intensity of the heat input in order to obtain different functional properties.