Cornerstone Research Group Inc. (CRG) has developed and demonstrated a composite structural solution called
reflexive composites for aerospace applications featuring CRG's healable shape memory polymer (SMP) matrix. In
reflexive composites, an integrated structural health monitoring (SHM) system autonomously monitors the structural
health of composite aerospace structures, while integrated intelligent controls monitor data from the SHM system to
characterize damage and initiate healing when damage is detected. Development of next generation intelligent
controls for reflexive composites were initiated for the purpose of integrating prognostic health monitoring
capabilities into the reflexive composite structural solution.
Initial efforts involved data generation through physical inspections and mechanical testing. Compression after
impact (CAI) testing was conducted on composite-reinforced shape memory polymer samples to induce damage and
investigate the effectiveness of matrix healing on mechanical performance. Non-destructive evaluation (NDE)
techniques were employed to observe and characterize material damage. Restoration of mechanical performance
was demonstrated through healing, while NDE data showed location and size of damage and verified mitigation of
damage post-healing. Data generated was used in the development of next generation reflexive controls software.
Data output from the intelligent controls could serve as input to Integrated Vehicle Health Management (IVHM)
systems and Integrated Resilient Aircraft Controls (IRAC). Reflexive composite technology has the ability to
reduce maintenance required on composite structures through healing, offering potential to significantly extend
service life of aerospace vehicles and reduce operating and lifecycle costs.
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
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 (T<sub>g</sub>), 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.