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Trusses are open, three-dimensional structures with repeatable unit cells useful for high-specific strength, stiffness, and energy-dissipating applications. By infilling Ti-6Al-4V trusses with an incompressible elastomer (polydimethylsiloxane) or compressible foam (polyurethane), we demonstrate non-porous composite materials with controlled mechanical properties. The advantages of infilled composite trusses include corrosion resistance, density- or strength-matching, and the creation of a continuous external surface that can enable surface properties such as resistance to aerodynamic pressure and biomedical compatibility. Flexural and compressive testing of composite trusses show reproducible behavior dominated by the underlying truss in elastic and plastic regimes. Compared to unfilled trusses, those filled with an incompressible elastomer or compressible foam delay initial compressive failure to higher strains. For all investigated energy dissipation metrics, foam-infilled trusses demonstrated equivalent or improved performance compared to unfilled ones. This included plateau stress, volumetric energy dissipation (+19%), and energy absorption efficiency (+12%) and ideality (+20%). For the highest tested truss relative density (ρ∗ = 0.43), 150% improvement in volumetric energy absorption was observed. Compressive loading of the incompressible siloxane-filled trusses yielded higher peak failure stresses but at reduced energy dissipation. Composite trusses can therefore serve as structural solutions to improving energy absorption in higher stress applications with minimal increases to density.

In contrast to morphing wings, which have been intensively studied and demonstrated, relatively little research has focused on morphing fuselages, which are capable of improving agility and range by using the articulating section as an aerodynamic control surface. Herein, we investigate several spring-reinforced cylindrical geometries representational of an articulating fuselage that operate in large-scale bending without buckling. Finite element models analyze design performance under loading and in bending and are compared to experimental prototypes. Axial compression tests are conducted to compute bending work as a function of articulation angle. Strong agreement between model predictions and experimental measurements is observed. Using mechanics-based graphical materials selection, carbon fiber-reinforced polymer composites or lightweight metal alloys are determined to be the most promising materials for successful designs. For the spring-reinforced design space, machined spring-reinforced designs are superior at lower values of articulation work (25 N⋅m) for 25° of articulation. Keywords: Morphing structures, Digital image correlation, Graphical materials selection, Kinematic design, Additive manufacturing

Shape-changing aerospace structures explore the trade-offs between aerodynamic performance and actuation efficiency. Herein, a morphing cylinder is optimized to minimize both its work to actuate in bending as well as radial deviations. Based on derived linear elastic mechanics of a cylindrical shell in bending, it was determined that favorable designs have high circumferential and low axial stiffnesses. We investigate circumferentially reinforced composites that limit radial displacement and allow for out-of-plane bending. Designs of flexible skins with embedded or underlying reinforcement were modeled, fabricated, and compared to unreinforced skins. Finite element analyses predicted the bending kinematics and structural responses. Digital image correlation used during the bending of physical prototypes generated strain contour plots of cylinder surfaces. Strong agreement between model predictions and experimental measurements was observed. The dimensions of each archetype were systematically varied via finite element modeling to obtain a set of nondominated designs capable of 25° of bending without buckling or large radial displacements. For the examined designs, it was determined that flexible skins reinforced with embedded rings were optimal at low (8–15 N·m) and high (>250 N·m) values for the work to articulate while flexible skins reinforced with underlying helical springs were optimal at moderate values (15–250 N·m). Keywords: Morphing structures, Digital image correlation, Finite element analysis, Kinematic design

This work presents the design and fabrication of two multi-element structurally embedded vascular antennas (SEVAs). These are achieved through advances in additively manufactured sacrificial materials and demonstrate the ability to embed vascular microchannels in both planar and complex-curved epoxy-filled quartz fiber structural composite panels. Frequency-reconfigurable antennas are formed by these structures through the pressure-driven transport of liquid metal through the embedded microchannels. The planar multi-layer topology examines the ability to fabricate two co-located radiating structures separated by a single ply of quartz fabric within the composite layup. The multi-element linear array topology composed of microchannels embedded on to a single-layer are used to demonstrate the ability to conformally-integrate these channels into a complex curved surface that mimics an array of antennas on the leading edge of an Unmanned Aerial Vehicle (UAV). A parallel-strip antipodal dipole feed structure provides excitation and serves as the interface for fluid displacement within the microchannels to facilitate reconfiguration. The nominal design of the SEVAs achieve over a decade of frequency reconfiguration with respect to the fundamental dipole mode of the antenna. Experimental and predicted results demonstrate the operation for canonical states of the antennas. Additional results for the array topology demonstrate beam steering and contiguous operation of interconnected elements in the multi-element structure.

Artificial hair flow sensors were fabricated using piezoresistive, radially grown carbon nanotube arrays on glass fibers and investigated for their dynamic aerodynamic response as measured within an instrumented plane-wave tube. The sensors were experimentally observed to provide both a large bandwidth of operation below first resonance and a strong resonance response at selected frequencies above first resonance. The frequency of first resonance was easily tunable by adjusting the length of the exposed hair and could be made to vary from a few hundred hertz to over 13 kHz. Higher frequency bands were accessible for a given hair length using higher-order resonance modes, up to five of which were observed. All of the responses were understood and modeled using a vibrating Euler-Bernoulli beam analysis.

The majority of aerospace structural composites use thermoset resins for their processing flexibility, temperature capability, and environmental durability. In this study, the recovery behavior of Veriflex-E, an epoxy-based, thermosetting, thermally-triggered shape memory polymer (SMP) resin, is investigated in glassy (25 °C) and rubbery (130 °C) states, as a function of shape memory cycles, and as a means to evaluate its potential as a resin for a shape memory composite. At 25 °C, Veriflex-E exhibits a promising high elastic modulus and a positive, nonlinear strain rate sensitivity in monotonic loading. At 130 °C, the strain rate sensitivity in monotonic loading decreases. Stress relaxation after storage in the deformed temporary state and subsequent shape recovery is of particular interest, a challenge to measure, and has not been widely reported for SMPs. The current experimental program measures the influence of strain rate changes in the 10−4–10−2 s−1 range on the stress relaxation response of the material, as well as on the strain recovery behavior at both 25 °C and 130 °C. As expected, the post-relaxation strain is larger with faster loading. Unexpectedly, the total strain recovered after shape memory cycling is more similar to the low temperature deformation. Overall, the results suggest that, while being influenced by both the loading rate and the test temperature, Veriflex-E is a promising candidate for a shape memory composite which could enable adaptive structures.

Shape memory composites (SMCs) based on shape memory alloys (SMAs) and shape memory polymers (SMPs) allow many design possibilities due to their controllable temperature-dependent mechanical properties. The complementary characteristics of SMAs and SMPs can be utilized in systems with shape recovery created by the SMA and shape fixity provided by the SMP. In this research, three SMC operating regimes are identified and the behavior of SMC structures is analyzed by focusing on composite shape fixity and interfacial stresses. Analytical models show that SMPs can be used to adequately fix the shape of SMA actuators and springs. COMSOL finite element simulations are in agreement with analytical expressions for shape fixity and interfacial stresses. Analytical models are developed for an end-coupled linear SMP-SMA two-way actuator and the predicted strain is shown to be in good agreement with experimental test results.