| Ideal smart structures should integrate structural strength, functionality, controlling and information processing capabilities among which information transmission is critically important. It functions like eyes and ears of a human being. Therefore, an ideal smart structure should first be able to send and receive electronic signals. In the field of aerospace and astronautics, microstrip antennas have been widely used due to their superior performance as well as their excellent conformability which renders them better stealth property and reliability than traditional antennas.Most smart materials and microstrip antennas are based on fiber reinforced composites. However, composites are generally laminated structures which are easy to delaminate. Once the antenna component delaminates from the base composite, the whole antennas system will malfunction or fail. Therefore, to improve the reliability of the structure, the composite has to be modified. Three-dimensional woven preforms for structural composites have three sets of yarns, namely warp, weft and Z-yarns interlaced to form an integrated structure. Compared with 2D laminates, 3D woven composites have the following advantages. First, 3D woven preform can be used to produce near-net-shape composites. Secondly, through-thickness properties of the composite can be adjusted by controlling the amount of Z yarns which could arrest the cracks formed during impact loading, leading to high resistance to both ballistic impact damage low velocity impact damage. The 3D woven preforms usually exhibit good dimensional stability in the warp and weft directions and a low in-plane shear modulus resulting in good formability.Due to the improved resistance to delamination for 3D woven composites, the reliability of the antennas based on these materials can be greatly improved when the antennas are integrated into the composites. In addition, the microstrip antennas based on the three dimensional fabrics can be fabricated as soft structures for conformal antennas integrated into a garment which may be used as battle field uniforms for the soldiers and detectives.In order to design and simulate this type of antennas, the mechanical and dielectric properties of the substrate materials have to be determined and predicted.The objectives of the current research are as follows:1. To determine the relationship between the structural characteristics of 3D orthogonal woven hybrid composites and their tensile, impact and dielectric properties;2. To establish a theoretical model to predict the dielectric constants of 3D orthogonal woven single fiber type and hybrid composites and verify the model using experimental results;3. To design and fabricate a conformal load bearing microstrip antenna structure based on computer simulation results.4. To test the electrical and impact performance of the antenna and compare the results with the computer simulation.The following are the findings of this research:To widen the range of electrical and mechanical properties available to the designers of the antennas based on 3D woven composites, hybrid composites which contain two or more types of fibers may be used. Therefore, it is necessary to understand how these properties are associated with the arrangement or structure and amount of yarns of different fibers. To understand the hybrid effect of the 3D hybrid composites, five types of hybrid structures of glass/aramid composites were investigated. Aramid/glass hybrid composites with three different stacking sequences and their corresponding single fiber type composites have been fabricated and their tensile, impact and dielectric properties were investigated. The trend of tensile strength and modulus of the composites followed the rule of mixtures (ROM) closely and a small but positive hybrid effect for tensile strength of the hybrid composites was observed. The hybrid composites in general had a higher impact resistance than the single fiber type composites and the hybrid composite in which fiber volume fractions for glass and aramid fiber were the most balanced showed the highest impact ductility. The aramid fiber composite showed a lower dielectric constant and a higher dielectric loss than the glass fiber composites. However, the dielectric constant of the hybrid composites decreased first and then increased as the volume fraction of aramid fiber increased, which did not follow the mixing rule for dielectric constants of compounds. The dielectric loss of the composites increased monotonically as the volume fraction of aramid fiber increased which agreed well with the mixing rule. It is also found that the mechanical and dielectric properties of the hybrid composites are also dependent on the stacking sequence or the arrangement of the two types of yarns. Therefore, in predicting the electrical properties of a 3D woven perform based antenna, the structural parameters of the composite have to be taken into account.In order to predict the dielectric properties of a 3D woven composite, a theoretical model was proposed based on the binary mixture rule. This model adopts a representative volume containing n×m×l subunits, each of which is composed of either unidirectional composite or net resin. The dielectric constants of these subunits were determined using the binary mixture rule and that for the representative volume was calculated by integration of the dielectric constants of all subunits over the whole representative volume. The model shows that with the same fiber volume fraction, a component with a larger cross-sectional area perpendicular to the electric field has a greater contribution to the composite dielectric constant. For experimental verification, single fiber type basalt/epoxy and aramid/epoxy as well as interplay and intraply basalt/aramid/epoxy 3D orthogonal woven hybrid composites were fabricated and their dielectric properties were measured using waveguide method at a frequency range of 8-12GHz. At 10GHz, the experimental results agreed well with the calculated results from the model for the single fiber type composites, while a positive hybrid effect on dielectric constant was observed for the two hybrid composites.Based on the above results, a series of 3D conformal load bearing microstrip antennas were designed and simulated using Ansoft HFSS, the most advanced antenna design and simulation software. Based on the simulation results, an optimum design of the 3D conformal load bearing microstrip antenna structure was selected and a prototype antenna was fabricated. In this antenna, the radiation element and the ground plane were woven using conductive yarns interlaced into the 3D composites by the Z yarns. The voltage standing wave ration (VSWR) and the radiation pattern of the antenna were measured. It was found that the results of the experimental measurement matched those from the computer simulation. To verify that the 3D conformal load bearing microstrip antenna had superior structural integrity and impact resistance, a series of low velocity drop tower impact tests were carried out with impact energy levels changing from 1J, 2J, 3J, 4J, 5J, and 15J. The radiation pattern of the antenna was almost unchanged when the antenna was impacted repeatedly until the impact energy reached 15J. This shows a greatly enhanced reliability of the 3D conformal load bearing microstrip antenna.The results of the current study proved that it is feasible to design and fabricate the microstrip antenna based on the 3D orthogonal woven structure. In addition to the comparable electric properties to the traditional antennas, the 3D conformal load bearing microstrip antenna has advantages of good conformability and excellent impact resistance. Therefore it is likely that this type of antenna will be used as a platform for smart structures that can transmit signals in telecommunications for both civilian and military purposes. |
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