Shape memory alloys (SMAs) are a unique class of materials with the ability to recover their shape. Nitinol (Nickel-titanium, NiTi) alloys are the most used SMAs. NiTi alloys are widely used in numerous medical applications such as stents, blood clot filters, arch wires for orthodontic applications, orthopedic implants, and the development of surgical instruments. They exhibit two different behaviors: shape memory and superelastic effects. The shape recovery ability of the NiTi is due to a phase transformation between the austenite and the martensitic phases in a reversible way due to temperature and/or load changes. NiTi alloys can be found in three phases: austenite phase (A), twinned martensite phase (Mt), and detwinned martensite phase (Mdt). The austenite phase is called parent phase, which means that the material is in a high-temperature environment with respect to the martensite phases. The twinned and detwinned martensite phases occur in a low-temperature environment or due to loading. Martensitic start temperature Ms, martensitic finish temperature Mf, austenitic start temperature As and austenitic finish temperature Af are four material characteristics of the NiTi alloys that define the conditions for phase transformations between martensite and austenite. Shape memory and superelastic effect of NiTi are also highly dependent to thermomechanical loading level and rate, and this dependency must be considered in the design of components.
One of the outcomes of the course entitled: “Experimental Methods in Orthopaedic Biomechanics” is for mechanical and bioengineering undergraduate and graduate students to learn the design and implementation of NiTi alloys. This is achieved by conducting two experiments to observe the shape memory and superelastic effects of NiTi alloys. NiTi wires were tested in both experiments.
The superelastic behavior is exhibited when the material is in the Austenite phase and the environment temperature is higher than Af. This superelastic effect exhibits itself when a large enough mechanical load is applied to the material. A 0.025’’ diameter wire manufactured by Fort Wayne Metals, Inc. (Fort Wayne, IN) was used to demonstrate the superelastic effects of the NiTi alloys. The wire was tested under a displacement control mode using a BOSE ElectroForce® 3300 machine by applying a tension force at two different strain rates: an isothermal loading at a rate of 0.005 mm/s and a dynamic loading at a rate of 0.5 mm/s. In the beginning, the material is in the austenite phase. The external load causes a stress-induced transformation from Austenite to detwinned Martensite. The recoverable induced strain is much higher (about 7%) than that of conventional materials. By unloading the material, it transforms back to Austenite and recovers its shape. An infrared camera was used to record the temperature changes of the wire during loading and unloading. The loading rate was set using the Wintest®7 software. The wire was trained at the high strain rate to stabilize the cyclic response. Load-displacement curves, stress-strain curves, and temperature changes in the wires were plotted and analyzed. Students were asked to identify one example of the application of superelastic wires and describe how it works and its advantages compared to traditional materials.
The shape memory effect is the material behavior in response to temperature changes. A Flexinol actuator wire with a 250 µm diameter, manufactured by DYNALLOY, Inc. (Irvine, CA) is used for this demonstration. In this experiment, the NiTi wire is initially in its twinned martensitic phase. It is then loaded and transformed into the detwinned martensite phase. The load is removed, and the material remains in its detwinned martensite phase. Large residual strains (about 8%) can also be observed after removing the load. The wire is then heated which causes the material to transform back to the austenite phase, recovering all of the strains. When the heat is removed, the material returns to its original twinned martensite phase without any change in length. The experiment was repeated for three different loading levels. An Arduino board was used to command the power supply switch to turn on and off depending on the temperature, and to record the the real time temperature and displacement of the NiTi wire. The Arduino board utilized the Parallax Data Acquisition Excel add-on for data acquisition. Stress-strain curves were plotted. Students were also asked to identify one example of the application of the shape memory alloy (SMA) where the design is based on using the shape memory effect of the wire and describe briefly how the SMA wire works.
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