Daewon Kim

PHASED ARRAY BEAMSTEERING IN COMPOSITE LAMINATES FOR GUIDED WAVE STRUCTURAL HEALTH MONITORING

In this study, a guided wave phased array beamsteering approach is applied to composite laminates. Current beamsteering algorithms derived for isotropic materials assume omnidirectional wave propagation. Due to inherent anisotropy in composites, guided wave propagation varies with direction and wavefronts no longer have perfect circular shapes.By examining slowness, velocity and wave curves for a given composite laminate, the wavefront from a single source can be described as a function of the angle of propagation and distance from origin. Using this approach, a generic delay and sum beamforming algorithm for composite laminates is developed for any desired wave mode.It is shown that anisotropic wave mode regions can be effectively used for beamsteering in certain directions with a linear array and performance similar or even better than isotropic case. However, the useful range of angles with a 1d linear array for anisotropic wave modes is quite small and other directions exhibit undesired grating lobes and large sidelobes.Copyright

Hybrid Magneto-Active Propellant Management Device for Active Slosh Damping in Spacecraft

This disclosure includes a hybrid magneto-active membrane, which can be used as part of a Magneto-active Propellant Management Device (MAPMD), to actively control free surface effects of liquid materials, such as fuels, and to reduce fuel slosh. The disclosed MAPMD merges aspects of a diaphragm membrane with a magneto-active inlay to control the membrane during in-flight conditions.

Design, characterization, and testing of macro-fiber composite actuators for integration on a fixed-wing UAV

Smart materials offer several potential advantages for UAV flight control applications compared to traditional servo actuators. One important benefit is that smart materials are lightweight and can be embedded directly into the structure of a wing or control surface. Therefore, they can reduce the overall weight of the vehicle and eliminate the need for mechanical appendages that may compromise the form factor of the wing, benefits that become more significant as the size of the vehicle decreases. In addition, smart materials can be used to realize continuous camber change of aerodynamic surfaces. Such designs offer improved aerodynamic efficiency compared to the discontinuous deflections of traditional hinged control surfaces driven by servo actuators. In the research discussed in this paper, macro-fiber composite (MFC) aileron actuators are designed for implementation on a medium-scale, fixed-wing UAV in order to achieve roll control. Macro-fiber composites, which consist of piezoceramic fibers and electrodes embedded in an epoxy matrix, are an attractive choice for UAV actuation because they are manufactured as lightweight, thin sheets and, when implemented as bending actuators, can provide both large structural deflections and high bandwidth. In this study, several MFC aileron actuator designs were evaluated through a combination of theoretical and experimental analysis. The current design consists of glass fiber composite ailerons with two unimorph MFC actuators embedded in each aileron to produce upward deflection. Wind tunnel test results are presented to assess the changes in lift and drag coefficients for different levels of MFC aileron actuation. Preparations for open-loop flight testing using a Skywalker UAV with MFC ailerons are also discussed. In addition, the development of a closed-loop, autonomous flight control system for the Skywalker is overviewed in preparation for conducting simulations and flight testing of an autonomous Skywalker with MFC aileron actuators.

Optimization of helical dielectric elastomer actuator with additive manufacturing

Fabrication of dielectric elastomer actuator (DEA) using additive manufacturing techniques can provide an alternative solution for current manufacturing processes of DEAs that are generally inconsistent and time consuming. In addition, additive manufacturing can allow DEAs with complex geometric configurations to be realized. This study investigates analytical approaches to optimize the performance of helical dielectric elastomer actuator (HDEA) based on additive manufacturing technologies. Optimized geometric configurations tailored to additive manufacturing and proper material selection for elastomer and electrode can improve the overall performance of HDEA. Due to the absence of pre-stretch in the elastomer membranes with additive manufacturing, associated drawbacks, such as electromechanical instability, high external voltage requirement, and their alternate solutions are analyzed and discussed. The performance of HDEA are evaluated by displacement, block force, and weight-to-force ratio by varying multiple geometric parameters including membrane thickness, pitch angle, inner-toouter electrode ratio, and actuation voltage. Since the selection of materials is as important as the geometric parameters of the actuator, printable elastomer and electrode materials with dielectric and mechanical properties for HDEA are evaluated. By optimizing geometric parameters and selecting appropriate materials based on its properties, appropriate manufacturing techniques are discussed to print both dielectric elastomer and electrode layers.

Preliminary Wing Study of General Aviation Aircraft with Stitched Composite Panels

A new approach in the design of aerospace vehicles was recently introduced by NASA and Boeing researchers to meet the new challenges in aviation. This innovative configuration is called pultruded rod stitched efficient unitized structure, a stitched carbon–epoxy material system that offers the opportunity for designing stiffer, lower weight, and more cost-efficient aircraft by eliminating fasteners and incorporating damage-tolerance concepts. Aside from superior structural performance and low-cost manufacturing methods, this configuration must also demonstrate advanced aeroelastic behavior to be fully implemented in commercial aircraft. A preliminary wing study of a general aviation aircraft that embodies pultruded rod stitched efficient unitized structure technology is presented in this paper, which will be compared with the structural, dynamic, and aeroelastic behavior of the original metallic wing model. The study showed that the presence of pultruded rod stitched efficient unitized structure panels le...

Numerical and experimental investigation of matrix effect on sensing behavior of piezoresistive hybrid nanocomposites

Nanocomposites exhibit remarkable electromechanical properties and have potential applications in sensing and actuation. In this work, carbon nanotubes (CNTs) - epoxy nanocomposites are fabricated with the addition of graphite nanoplatelets (GNPs). An improvement in piezoresistivity is observed with the combination of CNTs and GNPs, compared to the use of only CNTs, which indicates the formation of an efficient hybrid conductive networks for strain and electrical transfer in the materials. We investigate the effect of static mechanical loading on the electrical sensing performance of the nanocomposites. The inter-particle distances between the fillers change in the event of applied loading, which leads to a modification of the CNT-GNP hybrid percolated network and hence results in a change of the electrical conductivity. This phenomenon is exploited to use the hybrid composites as strain sensors. Specifically, different matrix materials are tested to investigate their effects on the mechanical and sensing performance of the nanocomposites. In addition, numerical simulations are performed to model the strain sensing performance of the nanocomposites. The effect of the type of matrix on the sensing performance of the nanocomposites is predicted and compared with the experimental results.

Correction to: Design Optimization of Hybrid FRP/RC Bridge

The original version of this article unfortunately contained an error.

Structural health monitoring of inflatable structures for MMOD impacts

Inflatable structures for space habitat are highly prone to damage caused by micrometeoroid and orbital debris impacts. Although the structures are effectively shielded against these impacts through multiple layers of impact resistant materials, there is a necessity for a health monitoring system to monitor the structural integrity and damage state within the structures. Assessment of damage is critical for the safety of personnel in the space habitat, as well as predicting the repair needs and the remaining useful life of the habitat. In this paper, we propose a unique impact detection and health monitoring system based on hybrid nanocomposite sensors. The sensors are composed of two fillers, carbon nanotubes and coarse graphene platelets with an epoxy matrix material. The electrical conductivity of these flexible nanocomposite sensors is highly sensitive to strains as well as presence of any holes and damage in the structure. The sensitivity of the sensors to the presence of 3mm holes due to an event of impact is evaluated using four point probe electrical resistivity measurements. An array of these sensors when sandwiched between soft good layers in a space habitat can act as a damage detection layer for inflatable structures. An algorithm is developed to determine the event of impact, its severity and location on the sensing layer for active health monitoring.

Dynamic piezoresistive response of hybrid nanocomposites

Hybrid nanocomposites with carbon nanotubes and graphitic platelets as fillers are known to exhibit remarkable electrical and mechanical properties with many potential strain and damage sensing applications. In this work, we fabricate hybrid nanocomposites with carbon nanotube sheet and coarse graphite platelets as fillers with epoxy matrix. We then examine the electromechanical behavior of these nanocomposites under dynamic loading. The electrical resistivity responses of the nanocomposites are measured in frequency range of 1 Hz to 50 Hz with different levels of induced strains. Axial cycling loading is applied using a uniaxial electrodynamic shaker, and transverse loading is applied on end-clamped specimen using modified speakers. In addition, a dynamic mechanical analysis of nanocomposite specimen is performed to characterize the thermal and dynamic behavior of the nanocomposite. Our results indicate that these hybrid nanocomposites exhibit a distinct piezoresistive response under a wide range of dynamic loading conditions, which can be beneficial for potential sensing applications.

Numerical analysis of helical dielectric elastomer actuator

Dielectric elastomer actuators (DEA) are known for its capability of experiencing extreme strains, as it can expand and contract based on specific actuation voltage applied. On contrary, helical DEA (HDEA) with its unique configuration does not only provide the contractile and extendable capabilities, but also can aid in attaining results for bending and torsion. The concept of HDEA embraces many new techniques and can be applied in multiple disciplines. Thus, this paper focuses on the simulation of HDEA with helical compliant electrodes that is a major factor prior to its application. The attributes of the material used to build the structure plays a vital role in the behavior of the system. For numerical analysis of HDEA, the material characteristics are input into a commercial grade software, and then the appropriate analysis is performed to retrieve its outcome. Applying the material characteristics into numerical analysis modeling, the functionality of HDEA for various activations can be achieved, which is used to test and comply with the fabricated final product.