Shape memory polymers (SMPs) are polymers that can recover a large pre-deformed shape in response to environmental
stimuli, such as temperatures, light, etc. For a thermally induced amorphous shape memory polymer, the pre-deformation
and recovery of the shape require the material to traverse the glassy transition temperature Tg under constrained or free
conditions. In this paper, effects of thermal rates to mechanical behaviors of SMP under constrained condition were
investigated. The stress-temperature behavior demonstrates a faster stress decrease than thermal contraction during
cooling and a characteristic stress overshoot during constrained reheating. These observations were explained by a one
dimensional (1D) model that considers the non-equilibrium structure relaxation and viscoelastic behavior of the material.
Traditional fastening systems exhibit various limitations that a next-generation shape memory polymer (SMP) system
can overcome. Bolts and screws provide high-strength attachment but require permanent modification to the system and
are typically visible, depending on the configuration. Adhesive bonding can provide high-strength attachment and low
visibility, but it is irreversible. Hook and loop fasteners offer reversibility, but the fastened strength and the removal
force are similar, limiting the applications. The unique properties of SMP enable a fastening system that offers
advantages not currently available in any one fastening system, including reversibility, low visibility, and high-strength
attachment. Cornerstone Research Group (CRG) designed a fastener system that consists of an array of SMP heads and
stems that interlock. The high modulus of the SMP at room temperature provides rigid attachment, keeping the system
interlocked. When activated above the glass transition temperature (Tg), the heads and stems become soft and flexible,
reducing the force required during attachment and detachment of the system. The shape memory property of the SMP
ensures all heads and stems return to their original position to allow proper alignment. The developed system provides
shear and tensile strength in excess of 300 psi with tensile detachment requiring only 2 psi. The material selection,
design, testing, and optimization of the SMP fastening system are discussed.
Seamless skins for morphing vehicles have been demonstrated as feasible but establishing robust fastening methods for
morphing skins is one of the next key challenges. Skin materials previously developed by Cornerstone Research Group
and others include high-performance, reinforced elastomeric and shape memory polymer (SMP)-based composites.
Recent focus has shifted to improving performance and increasing the technology readiness level of these materials.
Cycling of recently demonstrated morphing skins has determined that an abrupt interface between rigid and soft
materials leads to localized failure at the interface over time. In this paper, a fundamental understanding between skin
material properties and transition zone design are combined with advanced modeling techniques. A thermal gradient
methodology is simulated to predict performance benefits. Experimental testing and simulations demonstrated
improvement in morphing component performance for a uniaxial case. This work continues to advance development to
eliminate fastening as the weak link in morphing skin technology and provides tools for use in morphing structure
Deployable optics comprised of an electroformed, replicated nickel optical surface supported by a reinforced shape memory resin composite substrate have the potential to meet the requirements for rapid fabrication of lightweight, monolithic, deployable, large optics. Evaluation has been completed for various composite constructions including shape memory resin, carbon fiber reinforcement and syntactic fillers bonded to the electroformed nickel surface. Results from optical and structural performance tests on the 0.5 meter aperture deployable test items are also applicable to non-deployable replicated composite optics.
Shape memory composite materials (SMC materials) are being developed by our program to make deployable space optics. The basic procedure involves electroforming an approximately 20 micron thin Ni surface onto a convex master and then casting the shape memory composite material onto the plated master. When good adhesion between the Ni and the SMC material is obtained, the Ni and SMC material come off the master in one piece. The result is a shiny mirror whose metallic surface remains intact after stowing and deploying of the mirror. Achieving the requisite adhesion requires treating the Ni prior to the application of the SMC material. The techniques we use to treat the Ni and the results of making mirrors are described.