page_title: Shape-memory alloy
section_title: Response time and response symmetry
hierarchical_section_title: Shape-memory alloy / Practical limitations / Response time and response symmetry
caption_attribution_description: English: This work shows the comparative force-time response of bare and lagged Ni-Ti shape memory alloy. It is an original work by Leary, Schiavone and Subic. 2010.
context_page_description: A shape-memory alloy is an alloy that can be deformed when cold but returns to its pre-deformed shape when heated. It may also be called memory metal, memory alloy, smart metal, smart alloy, or muscle wire.
  Parts made of shape-memory alloys can be lightweight, solid-state alternatives to conventional actuators such as hydraulic, pneumatic, and motor-based systems. They can also be used to make hermetic joints in metal tubing.
context_section_description: SMA actuators are typically actuated electrically, where an electric current results in Joule heating. Deactivation typically occurs by free convective heat transfer to the ambient environment. Consequently, SMA actuation is typically asymmetric, with a relatively fast actuation time and a slow deactuation time. A number of methods have been proposed to reduce SMA deactivation time, including forced convection, and lagging the SMA with a conductive material in order to manipulate the heat transfer rate.
  Novel methods to enhance the feasibility of SMA actuators include the use of a conductive "lagging". this method uses a thermal paste to rapidly transfer heat from the SMA by conduction. This heat is then more readily transferred to the environment by convection as the outer radii (and heat transfer area) are significantly greater than for the bare wire. This method results in a significant reduction in deactivation time and a symmetric activation profile. As a consequence of the increased heat transfer rate, the required current to achieve a given actuation force is increased.
caption: Comparative force-time response of bare and lagged Ni-Ti shape memory alloy.[29]
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