455 transition temperatures higher than 120°C . In addition,  shape- memory strains up to 8- 10% can be observed  with these SMA alloys representing a significant advantage over standard NiTi alloy s limited to shapememory strains up to 6- 7%. On the other hand NiTi alloys are electrically resistive and can therefore be  directly heated by Joule effect using through current as energizing source. Due to their high c opper content,  CuAl -based alloy s have the main drawback of being electrically conductive such that CuAl -based actuators   shall be equipped with a dedicated heating system  that also needs to withsta nd deformations as large as  10% while keeping its full heating functionality . Higher transition temperatures and larger deformations  make in general the CuAlX more suitable for space applications , however at the cost of a more complex  SMA heating design.  

Mechanically , the challenge was to establish a bi-stable design (no electrical current needed to maintain  both end positions ) that is highly reliable and able to sustain the laun ch loads with out state change. Friction,  wear, fretting and cold welding are issues that have been considered and worked around  during the actuator  development phase, but the true challenge turned out to be the mechanical stability  of the bi -stable  mechanism during launch, which required a particular effort . 

SMA Wire Actuator Main Features  

SMA wires work in traction in their straight configuration to provide  their full capability while being  accommodated in parallel in the actuator to allow redundancy.  

The main challenge in the heating system design was to su stain the large actuation deformation (8-10%)   produced by the CuAlXx alloy. Indeed t o take full advantage of the capabilities of the selected SMA material,  the wire was  equipped with the following elements:   • Wire Ends, pro viding the mechanical interface  • An electrical crimp for electrical back loop through the SMA   • A heating system to provide the nec essary thermal power to the SMA  

   Figure 3.  Equipped SMA Wire  

A performance test highlighted the capacity of the naked wire (wire without heating system) to perform  several thousands of cycles, making the required 1000 cycles life test easily achievable for the SMA itself.    The dimensional change leading to actuation is intrinsic to the microstructure of the material and therefore heavily dependent on the production processes. High reliability of the SMA wire can therefore be insured by a thorough control of the production process es and characterization of production batches . The heating  system was identified as the main design challenge very early in the project and a particular effort was  dedicated to improve its design and reliability. Therefore, each wire features  a redundant heating system .  In addition to this heating system redundancy,  the device was designed to be fully operative and delivering  the full nominal force with a single  disabled  SMA wire , providing a double redundancy in the system .   The SMA wire heating system is based on the use of a geometrically adaptive external heating element  running along the full length of the actuating wires. The SMA wires being conductive, they are used as return lines to allow the electrical power ing from a single side. This avoid s the use of movable electrical  connections following the extended and retracted states of the actuating wires on the opposite side.    Mechanical interface Mechanical interface (fixed end) Heating system