|MadSci Network: Engineering|
These are materials which are stable at two or more temperature states. While in these different temperature states, they have the potential to be different shapes once their 'Transformation temperatures' (Tx) are reached. Shape Memory Alloys (SMA) and Shape Memory Polymers (SMP) are materials with very different shape changing characteristics. While exposed to their Tx, devices made from SMAs have the potential to provide force such as in the case of actuators. Devices made from SMPs in contrast, while exposed to their Tx, provide mechanical property loss as in the case with releasable fasteners. This phenomena whether SMA or SMP is called Shape Memory Effect (SME). Shape memory polymers Typical polymers posses what is called a glass transition temperature (Tg) i.e. the temperature below which they are glassy and above which they are rubbery. The shape memory effect in polymers is a novel physical property exhibited best by amorphous polymers whose glass transition temperature is marginally higher than room temperature and whose transition from glass to rubber is particularly sharp. In this case, strain energy may be stored in the polymer by quick mechanical deformation (e.g. by stretching) at T > Tg, such that t << t, followed by cooling below Tg. Here, t is the deformation time and t is a characteristic relaxation time marking the transition from rubber-like to liquid-like behavior. Recovery of the strain, or shape memory, is exhibited upon reheating the sample above Tg, allowing a return of the stretched polymer chains to more equilibrium, coiled stuctures. Reversal of this process is only possible, by repeating the same cycle or by incorporating the polymer into a device featuring an elastic spring or another shape memory polymer with a distinct glass transition temperature. A major application of shape memory in polymers has been as heat-shrinkable tubing. References: (1) http://www.ims.uconn.edu/~mather/ Shape memory alloys: NiTi shape memory metal alloy can exist in a two different temperature- dependent crystal structures (phases) called martensite (lower temperature) and austenite (higher temperature or parent phase). Several properties of austenite NiTi and martensite NiTi are notably different. When martensite NiTi is heated, it begins to change into austenite. The temperature at which this phenomenon starts is called austenite start temperature (As). The temperature at which this phenomenon is complete is called austenite finish temperature (Af). When austenite NiTi is cooled, it begins to change onto martensite. The temperature at which this phenomenon starts is called martensite start temperature (Ms). The temperature at which martensite is again completely reverted is called martensite finish temperature (Mf). Composition and metallurgical treatments have dramatic impacts on the above transition temperatures. From the point of view of practical applications, NiTi can have three different forms: martensite, stress- induced martensite (superelastic), and austenite. When the material is in its martensite form, it is soft and ductile and can be easily deformed (somewhat like soft pewter). Superelastic NiTi is highly elastic (rubber- like), while austenitic NiTi is quite strong and hard (similar to titanium). The NiTi material has all these properties, their specific expression depending on the temperature in which it is used. References: (1) http://www.elecdesign.com/magazine/1998/sept0198/tbrk/0901bk.shtml (2) http://herkules.oulu.fi/isbn9514252217/html/x317.html
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