Thermo-Mechanical Methodology for Stabilizing Shape Memory Alloy Response
- Sunday, 01 December 2013
- John H. Glenn Research Center, Cleveland, Ohio
This innovation is directly applicable to actuator applications employing shape memory alloys.
This innovation is capable of significantly reducing the amount of time required to stabilize the strain-temperature response of a shape memory alloy (SMA). Unlike traditional stabilization processes that take days to weeks to achieve stabilized response, this innovation accomplishes stabilization in a matter of minutes, thus making it highly useful for the successful and practical implementation of SMA-based technologies in real-world applications. The innovation can also be applied to complex geometry components, not just simple geometries like wires or rods.
SMAs are being developed for use as actuators, switches, and other devices in aerospace, automotive, and many other industries. An important aspect of developing a useful SMA technology is the ability to achieve a stabilized material response. Most SMAs exhibit dimensional instability when thermally cycled in the presence of applied stress. In order to mitigate the need for the design to deal with such issues, “training” or stabilization of the materials response must be achieved prior to utilizing the material under service conditions.
The process of stabilizing an SMA for actuator response is generally thought of as a stabilization of the strain during thermal cycling under conditions of fixed stress (the so-called isobaric response). Although this formulation is entirely appropriate, the underlying reason for the strain stabilization is governed by the internal states that the combination of stress and temperature produce (in this case, one of the drivers being transient and one being fixed). Hence, any combination of stress and temperature that would produce the same strain state could also stabilize the material for the intended service condition. In general, thermal cycling under fixed stress is commonly used to achieve stabilized behavior, but this process is not only time-consuming but costly.
The current innovation replaces this process with an alternate method utilizing mechanical cycles under conditions of fixed temperature (the so-called isothermal response), since mechanical cycling takes far less time than thermal cycling. The current innovation describes a process for determining this link, followed by achieving stabilization by a rapid and efficient mechanical cycling treatment. To begin, the stabilization point for the material (the absolute strain levels achieved after stabilization) is established by performing an isobaric experiment under conditions identical to those that will be used during service. Once known, a set of isothermal mechanical cycling experiments is performed using different levels of applied stress. Each of these mechanical cycling experiments is left to run until the strain response has stabilized. Once the stress levels required to achieve stabilization under isothermal conditions are known, they can be utilized to train the material in a fraction of the time that would be required to train the material isobarically. Once the strain state is achieved isothermally, the material can be switched back under isobaric conditions, and will remain stabilized for the service conditions.
The advantage of approaching the problem via this technique is that it is now possible to reduce the amount of time required to achieve a stabilized material response from days to weeks, down to a matter of minutes. The significant reduction in time translates into a more cost-effective solution for SMA-based technologies that in turn improves the viability of SMA device utilization.
This work was done by Santo Padula of Glenn Research Center.
Inquiries concerning rights for the commercial use of this invention should be addressed to NASA Glenn Research Center, Innovative Partnerships Office, Attn: Steven Fedor, Mail Stop 4–8, 21000 Brookpark Road, Cleveland, Ohio 44135. LEW-18594-1
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