Shape memory alloy based morphing for underwater vehicles

Abstract

During the last fifty years, researchers developed an understanding of shape memory alloys leading to their implementation in high-strain and shape change structures. Since the 1990's oceanographers have been using undersea gliders to study environmental variables in the oceanic water column. By identifying and designing shape memory alloy based adaptive structures for undersea gliders, the operational envelope of these vehicles can be extended. The methods and results described herein explore and evaluate how shape memory alloys can be used to extend the endurance of extant ocean gliders and scribe a path forward to integrate shape memory alloys more broadly in ocean engineering.

A shape memory alloy-based buoyancy engine is proposed in part one of this dissertation. The structural design of this concept was analyzed and a benchtop prototype was produced. Experimental testing of the prototype's displacement-temperature behavior agreed well with simulation predictions. The vertical dynamics of a scaled-up protoype in oceanic operation were also investigated. A coupled dynamic and heat-transfer simulation showed operation of the buoyancy engine to 770m. The fatigue life was estimated from the strain of the alloys and shown to effectively eliminate the constraint of the buoyancy engine on endurance. A semi-active system was proposed and the simulation indicated that the semi-active system would enable surfacing of the glider. Cu-Zn-Al shape memory alloys were identified for oceanic operation.

In part two, shape memory alloys were integrated into the wing of an undersea glider to create change camber and control glider pitch. The hydrodynamic impact of variable camber was also addressed. Steady-state gliding performance analysis showed that the internal pitch control mechanism can be eliminated by implementing variable camber and relocating the wing. The elimination of the internal pitch control mechanism would increase glider payload and permit carrying additional batteries. Variable camber was modeled as a flap deflection, and the hydrodynamic analysis indicated that deflection of a 30% flap past 7° was near separation on the glider wing at relevant angles of attack. Variable camber was achieved by using finite element analysis to model both surface-mounted shape memory alloy wires and integrated shape memory alloy sheet actuators.