Releasing systems for aerospace industry based upon Shape Memory Alloys: characterization of materials for actuators

Resumo

Releasing and deployment maneuvers carried out during space satellites launching are usually performed by
utilizing pyrotechnics loads. However, it is considered convenient to replace this technique by others not
requiring explosives (Non Explosive Actuators-NEA). This is mainly due to the necessity of reducing highshock
and vibrations induced levels, also avoiding the contamination of sensible instruments because of dust
and gas release during explosion. In addition, the avoidance of risks associated with storage and manipulation
of explosives and the possibility of performing device retesting prior to final mounting are desirable qualities.
Among NEA devices, those exploiting the singular mechanical behavior of Shape Memory Alloys (SMA)
have reached commercial maturity. In this study, the performance of a NEA device that uses the mechanical
stress generated upon reverse transformation of a mechanically constrained SMA actuator (constrained
recovery effect) to generate the controlled fracture of a notched bolt is analyzed. Firstly, the mechanical
components of the system are described, and the main problems associated with its design are introduced.
Then, the results of the experimental characterization performed on a NiTi SMA cylindrical tube actuator
with 12.7 and 7.8 mm outer and inner diameter respectively, are presented. After an activation stage in which
the cylinder is compressed to induce the martensitic phase (or re-orient the existing martensític phase), the
temperature is raised while a constant displacement condition is imposed. For temperatures near 120 ºC, a
loads increment of 35 kN (440 MPa) is obtained. The repetition of this loading-unloading-heating-cooling
cycle does not generate any important deterioration in the material response.
Keywords: Constrained Recovery, SMA actuators, NiTi, Release systems.