The Mechanical Response of Shape Memory Alloys Under a Rapid Heating Pulse |
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Authors: | S Vollach D Shilo |
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Institution: | (1) Department of Mechanical Engineering, Technion, Haifa, 32000, Israel |
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Abstract: | Shape memory alloy (SMA) actuators are very promising due to their large strain and work-output, but are considered to be
very slow due to their cooling rate. In this article, we explore the capabilities of a fast one-directional actuation mode
based on one-occasional rapid Joule heating of SMA elements. For this purpose, a unique experimental system has been developed
that applies a high-voltage electric pulse to a detwined NiTi wire and measures the resulting displacement due to the martensite
to austenite phase transformation. The electric pulse is tuned to produce variable temperature jump of up to 160°C within
a risetime of few microseconds. One end of the wire is clamped while the displacement of the other end is monitored both by
a laser doppler vibrometer and by an optical encoder that measures the displacement of a grating device. Analysis of experimental
results reveals a dead time delay between the electric pulse and the wire’s response, which is attributed to the austenite
nucleation time and which determines the limit on the fastest possible shape memory actuation. Further analysis reveals relations
between the dead weight used and the average acceleration and maximal velocity obtained. In particular, the maximal velocity
correlates to a constant kinetic energy delivered by the wire, which suggests a constant integrand over the stress-strain
curve regardless of the dead-weight used. A comparison of actuation performances demonstrates that our actuation experiments
are significantly advantageous over other fast actuation methods in almost every actuation aspect reviewed. This demonstrates
the great potential of SMA for applications that require high speeds and large displacements one-occasional actuation. |
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