Recent advances in structurally embedded vascular antennas (SEVAs) have demonstrated the ability to integrate microfluidic-enabled electromagnetically-tunable reconfigurable antennas into planar epoxy-filled quartz fiber composite panels. Reconfiguration of these antennas has been enabled by the pressure-driven flow of a liquid metal alloy within the patterned microchannels. These are embedded into the composite using novel sacrificial materials, additive manufacturing, and postprocessing fabrication techniques. The wide tunability, radiation pattern stability, and repeatability of these structures have been demonstrated, and now the deployment of these fabrication techniques to develop aerodynamically efficient composite shapes makes SEVA attractive for agile aircraft antenna. This paper will highlight the composite fabrication of a multi-element SEVA, or Structurally Embedded Antenna Array (SEVA), within a complex curved article that resembles an aircraft leading-edge. It includes a summary of the design and operation of the structure as a phased array of physically-reconfigurable antennas. These frequency-agile antennas can also be interconnected to form larger contiguous reconfigurable antenna structures that extend the range of electromagnetic tunability and/or provide other operational modalities. Keywords— Computational Design, Composite Manufacturing, Aerospace Structures, Materials, Analytics, Simulation, Automation

The microvascular strategy demonstrates unique advantages in shape-memory polymer applications as it can achieve both rapid thermal activation and deactivation during a typical shape-memory cycle. In addition, localized overheating can be avoided in comparison with other heating methods, such as the magnetic and electric heating. In this paper, a finite-element-based computational analysis was performed to study the rapid thermal response of shape-memory polymer composites with an embedded microvascular system. We show that the polymer shape-fixing speed, the shape fixity after a given cooling time, and the shape-recovery rate are significantly enhanced due to the rapid cooling and heating effect of internal microvasculature. The effect of the composite dimensions and microvascular channel arrangement on the activation and deactivation was studied. Typically, reducing the composite thickness and tube spacing increases the shape-recovery speed of the shape-memory polymer composite by effectively reducing the length of the thermal conduction pathways. Strategies for achieving an optimized tube arrangement were also discussed based on the considerations of the system mass and thermal boundary conditions. The results in this paper provide a guideline for further designs and applications of shape-memory polymer composites with embedded microvasculature.

Crickets, locusts, bats, and many other animals detect changes in their environment with distributed arrays of flow-sensitive hairs. Here we discuss the fabrication and characterization of a relatively new class of pore-based, artificial hair sensors that take advantage of the mechanical properties of structural microfibers and the electromechanical properties of self-aligned carbon nanotube arrays to rapidly transduce changes in low speed air flow. The radially aligned nanotubes are able to be synthesized along the length of the fibers inside the high aspect ratio cavity between the fiber surface and the wall of a microcapillary pore. The growth self-positions the fibers within the capillary and forms a conductive path between detection electrodes. As the hair is deflected, nanotubes are compressed to produce a typical resistance change of 1-5% per m/s of air speed which we believe are the highest sensitivities reported for air velocities less than 10 m/s. The quasi-static response of the sensors to point loads is compared to that from the distributed loads of air flow. A plane wave tube is used to measure their dynamic response when perturbed at acoustic frequencies. Correlation of the nanotube height profile inside the capillary to a diffusion transport model suggests that the nanotube arrays can be controllably tapered along the fiber. Like their biological counterparts, many applications can be envisioned for artificial hair sensors by tailoring their individual response and incorporating them into arrays for detecting spatio-temporal flow patterns over rigid surfaces such as aircraft.

Performance demands of future unmanned air vehicles will require rapid autonomous responses to changes in environment. Towards this goal, we expect that the next generation flight control systems will include advanced sensors beyond the contemporary array. One promising scenario correlates measurements of flow footprints over aircraft surfaces with aerodynamic data to aid navigation and feedback control algorithms. As a sensor for this concept, we construct artificial hair sensors (AHSs) based on glass microfibers enveloped in an annular, radially-aligned piezoresistive carbon nanotube (CNT) forest to measure air flow in boundary layers. This study includes an analysis of the sensitivity based on laboratory scale electromechanical testing. The sensors in this work utilize nine micron diameter S2 glass fibers as the sensing mechanism for coupling to boundary layer air flows. The annular CNT forest resides in a fused silica microcapillary with electrodes at the entrance. The sensor electrical transduction mechanism relies on the resistance change of the CNT forest due to changes in both the bulk and contact resistance as a function of mechanical loading on the fiber. For the electromechanical analysis, the sensors are controllably loaded to measure both the force and moment acting at the base of the hair and the resulting deflection of the CNT forest inside of the microcapillary is measured to estimate the stress on the forest and the pressure between the forest and the electrode. The electrical responses of the sensors are compared to the mechanical state of the CNT forest. This work represents the development of a characterization tool to better understand and control the response of CNT based AHSs.

Artificial hair sensors consisting of a piezoresistive carbon-nanotube-coated glass fiber embedded in a microcapillary are assembled and characterized. Individual sensors resemble a hair plug that may be integrated in a wide range of host materials. The sensors demonstrate an air-flow detection threshold of less than 1 m/s with a piezoresistive sensitivity of 1.3% per m/s air-flow change.

This work investigates the fabrication, experimentation, testing, and modeling of shape memory composites consisting of two-way shape memory alloy (SMA) tubes embedded in a shape memory polymer (SMP) matrix. The hybrid system here investigated is thermally activated via internal transport of thermal fluids through the SMA vascular system. The resulting shape memory composite (SMC) combines the high modulus and high specific actuation force of SMAs with the strong shape fixing and variable stiffness of SMPs to create a light-weight composite capable of controllably and rapidly achieving two shape memory states. Specifically, a 25° thermally induced out-of-plane bending state is achieved with a 2% volume fraction of SMA in the composite after 2 min of being activated by an internal thermal fluid. Here, while the thermal structural design of the SMC was not optimized and the thermal cycling was significantly restricted by the low thermal conduction of the SMP, the deflection of the composite was within 20% of the expected value modeled by the thermal–mechanical finite element analysis (FEA) here performed. The close agreement between the experimental performance and the modeled composite response suggests that morphing composites based on SMAs and SMPs are promising structures for adaptive applications.

The integration of material systems to include thermal activation and deactivation of shape memory polymers represents a key challenge for adaptive systems. Microvascular fluid flow with hot or cold fluid is used as an energy transport mechanism to activate and deactivate shape memory polymers, where the maximum temperature is limited by the hot fluid temperature to prevent overheating. A thin panel was constructed from the Veriflex® shape memory polymer and included an array of 10 parallel microvascular tubes. The panel is strained orthogonally to the tube direction in the activated state. An analytical steady-state surface temperature model is applied to predict the surface temperature range during activation. Thermography is used to measure the steady-state surface temperature for heating and the dynamic surface temperature for both heating and cooling. The decay constants and surface temperature range for heating and cooling are examined as a function of applied strain (0%–20%) and fluid flow rate (0–25 g min−1 of water). The decay constants depended strongly on flow rate and weakly on strain. The observed vascular cooling rate was up to six times faster than the comparable natural convection cooling rate for the flow rates tested. This faster cooling rate can significantly reduce temperature cycle times.

Thermal activation of shape memory polymers requires a heating mechanism that will achieve a temperature range that exceeds a minimum triggering temperature but does not overheat the material. In laboratory practice, ovens are typically used to achieve a uniform temperature during testing. In practical applications, active heating schemes must be utilized that are robust enough to handle changing environmental conditions. In this work, we analyze the intricacies of vascular heating and cooling methodologies for shape memory polymers operating in an open environment. Our methodology is based on analytical modeling of the steady state surface temperature of shape memory polymers that incorporate vascular channels. With the material properties and environmental conditions, the model is used to predict appropriate channel geometry for triggering the shape memory polymer. Thermography is used to verify the model predictions for real systems of shape memory polymers.

Aerospace-grade, quasi-isotropic composite laminate panels of IM7/977-2 carbon fiber/epoxy prepreg are cured in an autoclave with microvascular channels. The channels are created both with a stainless steel tube insert and a removable mandrel to create lined and unlined passages, respectively. The interlaminar fracture toughness is examined as a function of the channel diameter and orientation to the adjacent, unidirectional fiber plies in a double cantilever beam test. The channels allow for a heat exchange fluid to flow through the composites and facilitate thermal transport. Thermography is used to examine the steady state surface temperature profiles of the horizontally oriented panels operating with internal fluid flow and external natural convection. A two-dimensional, analytic model was developed for the surface temperature profiles to understand the thermal transport within the specimens. The framework for the design and operation of multi-channel systems is established and a case study for surface temperature control is examined.

Sandwich composites that exhibit complete mechanical and geometric healing when subjected to three-point bending are realized using a novel, pressure-triggered granular core. The sandwich composites are constructed with fiberglass-reinforced, epoxy face sheets and a granular core consisting of noncohesive, glass microballoons. By controlling the granular core hydrostatic compression in the range of 0-0.083 MPa, the observed shear modulus of the core spanned nearly three orders of magnitude. Core compression is achieved by evacuating the interior of the sandwich structure and allowing the atmospheric pressure acting on the sandwich to compress the core. Although the compressed cores typically begin to yield near 1% shear strain, the degenerate nature of the core allows for complete recovery of the modulus even after a large strain. By exploiting the inherent stiffness of the sandwich skin and the tunable modulus of the granular core, these sandwich composites are able to undergo healing cycles and completely recover both their mechanical and geometric properties.

The primary purposes of a core in a sandwich composite are to keep the face sheets separated by a fixed distance and to transmit shear stresses. Syntactic foam cores consisting of hollow glass microspheres and resin can form strong, lightweight cores. By underfilling the interstitial space in a packed microsphere bed with a binder, a three-phase syntactic foam is created that has a percolated void network. In a sealed sandwich composite, a void network allows for the entire core of the sandwich composite to be evacuated and mechanically compressed by the exterior pressure. By combining this compression with a heating cycle, it is possible to repair core cracking and core/face sheet interface debonding when a reversible binder is used. Upon cooling, the healed sandwich restores its properties. We examine the relation between the mechanical properties of these sandwich composites and the healing methodologies.

Compact actuation that is integrated into a structure's material system has the potential to provide rapid structural reconfiguration while reducing weight. The effect of scale (diameter, overall length and segment length) on the performance of cylindrical fiber-reinforced McKibben-like Rubber Muscle Actuators (RMA) was investigated. An "activation" pressure was observed for all actuators at a value that depended upon the actuation construction. Upon pressurization past the activation threshold, the overall force, stroke, and work capacity increased with increasing actuation length and diameter. The actuation force per unit RMA cross-sectional area was predicted, and experimentally observed, to be roughly constant after activation. By segmenting a longer actuator, a larger contraction and lower actuation force could be achieved. Though actuation forces decreased as actuator diameter and length decreased, the force per unit actuator volume was shown to increase with decreasing diameter including a roughly 4-fold increase in force/volume between the 0.5" and 0.05" actuators. However, due to the small amount of total contraction for the smaller diameter actuators, the relative work per actuation volume was decreased by roughly 35% in comparing those same actuators. Thus, small diameter RMAs have great potential to provide needed linear actuation force within adaptive material systems.