Actuator Applications with Single-Crystals of Cu-Zn-Al Shape Memory Alloy
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ABSTRACT In the present work the use of single-crystals of Cu-Zn-Al Shape Memory Alloys (SMA) in actuator applications is analyzed. The actuator considered here, a device capable of doing work in response to temperature changes, is based on a single-crystal nucleus of a Cu-Zn-Al SMA coupled to a conventional spring that represents the load to be displaced- A special experimental stage was designed for performing controlled thermal cycles under load. In this way the effects of different parameters (cycle number, friction, temperature range, load level) on the actuator behavior can be studied. From the results obtained, the use of a single-crystal of an SMA in a thermostatic device is analyzed and compared with the commercial wax actuator performance. INTRODUCTION Shape Memory Alloys (SMA) are characterized by strong shape changes which are due to diffusionless transformations, the so-called martensitic transitions [11] The characteristic effects shown by SMA (one-way shape memory effect, pseudoelasticity, trained two-way shape memory effect) suggest the possibility of applications in different areas like medicine, electric industry or mechanical couplings {2]. However, few products have reached industrial success. Two common points among different trials can be remarked: "* Most of the intended applications to date are based on the use of polycrystalline material of different alloys (TiNi based alloys, Cu-Al-Ni, Cu-Zn-Al). "* The lack of a design tool for an accurate SMA device. In some cases a procedure by trial and error is adopted. In others, results from beam theory are used to make CAD-programs and design-charts. The main difficulty is due to the complex behavior of SMA elements in the stress-strain-temperature space (3-s-T): non-linearity, hysteresis, path dependence and time evolution of behavior. In addition, loading geometries different from pure uniaxial tension are commonly found. Bending and torsion arrays are used for displacement magnification. The resulting nonhomogeneous distribution of stress diminishes the material usage efficiency. Shape Memory Alloy (SMA) applications are classified according to the function performed by the SMA element: free recovery, constrained recovery, actuator or work production devices, pseudoelastic and damping applications [2-31. In this work the use of Cu-ZnAl single-crystals in actuatorapplications is analyzed. A general conceptual design procedure is discussed- for uniaxial stress loading geometry. This procedure is directly related to basic aspects of Cu-Zn-Al SMA behavior. We call actuator a device in which an SMA element performs mechanical work in response to an external stimulus, i.e. an electric current (electrical actuator) or a temperature change (thermal actuator). In the following only a thermal actuator device will be considered- The conceptual design tool is finally applied to the analysis of a thermostatic device. 369 Mat. Res. Soc. Symp. Proc. Vol. 459 ©1997 Materials Research Society
EXPERIMENT
Single-crystals of Cu-Zn-Al with a composition of Zn= 16
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