Preparation And Mechanical Properties Of X-cor Sandwich Composites

With unique structure, the X-cor sandwich makes up the deficiencies of the foam sandwich in thickness direction. The novel structural composites well satisfy the requirements for light weight and high strength of the materials for high technology, such as aeronautics and astronautics, etc. The existent researches were mostly focus on the prediction of the mechanical properties of the X-cor sandwich through diversified models while the research of its auto-manufacture, micro-structures and failure mechanism were little. Based on the analysis of micro-structures of the X-cor sandwich the manufacture, stiffness, strength and failure mechanism under compression, shear and tension were investigated respectively with theoretical analysis and experiment in this dissertation.According to the photomicrographs of micro-structures in the X-cor sandwich, the basic hypothesis of the elliptic configuration of the resin regions around Z-pin's ends were proposed. Then the relationships between the fiber misalignment angle, major axis length, minor axis length of the resin region and the Z-pin's diameter were established. Based on the parametric representation of the X-cor sandwich, its geometric model was built up. Introducing the modified coefficient respectively the computational models of the compression and shear modulus for the X-cor sandwich based on the homogenization theory were modified; the ANSYS software was utilized to analyse its elastic properties. The theoretical analysis and experimental results were consistent well so the modified theoretical model and the finite element model could be utilized to compute the elastic properties of the X-cor sandwich.Based on the elastic stability theory, using the Winkle elastic foundations, introducing constraint modified coefficients with different angle, density and diameter, considering the affects of residual stresses during manufacturing process the original theoretical model of the compression strength was modified; the constraint modified coefficients had a linear relation with inserting angles. Through the finite element model utilizing the combination of the Tsai-Wu failure criterion, elastic buckling failure criterion, Von-Mises failure criterion and the maximum stress failure criterion and corresponding materials stiffness degradation rules, the effects of the parameters of the X-cor sandwich to its compression, shear and tension strength were studied respectively. The theoretical predictions and experimental results were coincident well that indicated the methods were fit for the computation of the strength of the X-cor sandwich.By the combination of the failure criterion and materials stiffness degradation rules, using the location and distribution of failure elements to simulate the failure process and failure mode, the compression, shear and tension failure mechanism of the X-cor sandwich were studied. The finite element analysis showed: 1) under compression, the resin region and Z-pins failed orderly and its main failure mode were Z-pin's elastic buckling; 2) under shear, the resin region and Z-pins failed in turn and many failure modes appeared simultaneously; 3) under tension, the results indicated the resin region controlled the tensile failure of the X-cor sandwich and its failure mode was Z-pin's pulling out of the facesheets. These result showed the resin region had a significant effect on the failure mechanism of the X-cor sandwich and the initial location and propagation path of failure elements had some dispersion; the Z-pin's existence improved the mechanical properties of the X-cor sandwich and made the X-cor sandwich behave different deformation process and failure modes compared with the foam sandwich; the finite element research correlated with experimental results that proved the validity of the failure mechanism analysis.