| Advanced lightweight structures are continually being developed for the purpose of reducing weight and increasing the payload of aircrafts. Carbon fiber composite sandwich panels with lattice core structures are particularly promising, because they can be ultra-lightweight while also providing multifunctional characteristics. In this dissertation, we present research into the design, preparation and mechanical properties of carbon fiber composite sandwich panels with various cores(3D grid, pyramidal truss, improved lattice truss) as well as lightweight curved shells, and cylindrical shells with pyramidal truss cores.Published research into these configurations was reviewed and discussed, and the key scientific problems requiring solutions were extracted.Carbon fiber composite sandwich panels with 3D grid cores have been designed and manufactured. Analytical models of the panels(with both egg and pyramidal grid cores) have been developed to analyze out-of-plane compression, in-plane compression, and three point bending response. Analytically based failure mechanism maps for in-plane compression and three-point bending were provided. Six groups of specimens were designed based on the predicted failure mechanism maps for in-plane compression, and tested. Face wrinkling, local buckling between grid cores, face crushing, core shear Euler buckling, core debonding, and core crushing have all been observed. In addition, four groups of specimens were designed based on the failure mechanism maps for three point bending, and tested. Core debonding was the dominant failure mode under three point bending. Face wrinkling was also found. The bearing capacity was not determined although face wrinkling occurred under three point bending, the slope of the response load growth will be decrease. Out-of-plane compressive tests of 3D grid-core sandwich panels with three different relative densities have been conducted. Compressive stiffness, compressive strength and quasi-static energy absorption of sandwich panels with egg and pyramidal grid cores have been obtained. The mechanical properties of sandwich panels with pyramidal grid cores were found to be much better than that of the sandwich panels with egg grid cores.The hot press method were explored for fabricating carbon fiber composite truss cores, a group of assembly molds(intellectual property owned by ourselves) was designed for fabricating pyramidal truss cores where continuous fibers are aligned in the direction of struts, for greatest strength. The microstructure and organizations of fibers were examined using a scanning electron microscope. Crushing responses of truss cores with three different relative densities were investigated, where strut failure modes including Euler buckling, compressive fracturing, and delamination. Core debonding was also observed. These failure modes were complemented with an analytic model of the core crushing response. Our results show that the fabricated low-density truss cores have superior compressive strength compared to other lattice truss cores. Then, four different core designs were tested in shear. Euler buckling of struts, delamination of struts and core debonding were observed. When core density is higher than 1.25%, the shear response of specimens was dominated by the face sheet debonding from the pyramidal truss core. Analytical expressions for the failure load of composite sandwich columns under in-plane compression were derived. Three typical specimens with different facesheets and core densities were designed, core shear macro-buckling, face wrinkling and face crushing were founded during the experiments. Analytical models of deflection in the center point and critical load under three point bending were developed. Failure mechanisms of the panels under each loading condition were studied, including face sheet crushing, face sheet wrinkling, core member buckling, core member crushing and core debonding for panels subjected to bending. Failure maps were constructed to predict the response of panels under three-point bending. Finally, eight representative sandwich panels with carbon fiber pyramidal truss cores were fabricated and tested to probe different failure modes. In general, measured failure loads showed good agreement with the analytical predictions and failure mechanism map, although actual strengths are naturally subject to the variable characteristics of typical composites. Quasi-static uniform compression tests and low-velocity concentrated impact tests were also conducted to reveal the failure mechanisms and energy absorption capacity of two-layer carbon fiber composite sandwich panels with pyramidal truss cores.Both electrical discharge machining and laser beam cutting methods were introduced to remove material from corrugated core sandwich panels, leading to lattice cores with much stronger core-to-face-sheet bonding. Both vertical and oblique strut morphologies were created based on corrugated core with two different fiber-orientation architectures. Out of plane crushing and shear responses of fabricated truss cores were measured and analytical models were presented to predict the stiffness, strength and dominant failure modes under each loading condition. The shear strength of the lattice cores was improved by successfully preventing core-to-face-sheet bond failures. The specific stiffness and specific strength of these fabricated core constructions under both core compression and core shear loading were discussed. A comparison to the shear strength of composite pyramidal truss cores made by other methods is presented. We have demonstrated that laser cut cores can substantially increase the upper limit of shear strength for higher relative density pyramidal truss core.To conclude the investigation, curved and cylindrical composite shells with pyramidal truss cores were designed and manufactured. Transverse and longitudinal reinforcements were made by wire-electrode cutting from 7075 aluminum alloy plates. Pyramidal truss cores were fabricated by an interlocking method. The curved facesheets were made of carbon fiber composites. Analytical models for the deflection in the center point and critical load of curved shells under three point bending were derived using the principle of virtual work. Failure mechanism maps of curved shell under three point bending were developed to show the effect of face-sheet thickness. Two typical curved shells with pyramidal truss cores were tested by changing the facesheet thickness. Face wrinkling, face crushing, core debonding and core crushing were investigated and a unique “M” type failure mode was studied. Analytical models for the critical load of cylindrical shell with pyramidal truss cores under axial compression were derived. Failure mechanism maps for cylindrical shell under axial compression were conducted by changing the ply orientation angle of the facesheets. Euler macro-buckling, local buckling between reinforcements and face sheet crushing were observed in the experiments. The finite element method was used to investigate the mechanical behaviors of curved shells with different thickness of face sheet under bending. Three typical cylindrical shells with different height and facesheet thickness were tested in order to obtain the strain values and critical loads under axial compression. Local buckling between reinforcements and face sheet crushing were studied both experimentally and analytically. Our results showed that the fabricated cylindrical shells with pyramidal truss cores have superior bearing capacity. |
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