The Mechanical Response of Shape Memory Alloys Under a Rapid Heating Pulse

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 oneoccasional actuation.

Ferromagnetic shape memory flapper for remotely actuated propulsion systems

“…The key characteristics for powerful propulsion are large and quick strokes [3,4]; however, conventional active materials are limited in at least one of these characteristics. For example, piezoelectric materials provide small amounts of strain while shape memory alloys (SMA) have slow response times [5,6]. Ferromagnetic shape memory alloys [7][8][9] (FSMA) show great potential for propulsion devices due to their large strain capability and fast response to external magnetic fields [10][11][12][13][14].…”

The kinetic relation for twin wall motion in NiMnGa

“…Unlike regular shape memory alloys, whose actuation speed is limited by the slow kinetics of a thermally induced phase transformation (Otsuka and Wayman, 1998;Vollach and Shilo, 2010), magnetic actuation of FSMAs presents the possibility for a rapid response at operating frequencies as high as several kHz . Thus, FSMAs are a promising material for applications that require large strains at wide bandwidths.…”

Strain Recovery by TiNi Element Under Fast Heating

“…In the experiments carried out in [8] several actuators -Ti-40.8wt%Ni-9.9wt%Cu SMA cylinders with diameter 5 mm and length 50 mm were pre-compressed to the residual strain 4%. During the direct Joule heating produced by an electric condenser discharge the shortest response time was 4.6 ms and this time for constrained actuators was 6.5 ms. Evidently, this time can be related to the characteristic time of the electric current impulse, since the condensers used in this experiment had a rather big capacitance from 6 to 3 F. A similar experiment has been carried out in work [9]. A Ti-55wt%-Ni SMA wire with diameter 0.2 mm was heated by an impulse of electric current produced by a discharge of an electric capacitor.…”

“…x e x e t x t  Here the boundary condition for the elastic strain at x = 0 is obtained from the boundary condition for the displacement u(0,t) = 0, wave equation 9, and Hooke's law. After applying the Laplace transform to the equation (9) with respect to time t (with the transform variable p) we obtain an ordinary differential equation for the image ) , ( p x e , of the elastic strain e(x,t):…”