Meso-Characteristics Analysis And Stiffness Prediction Of Stitched Foam-Cored Sandwich Structure

Foam-cored sandwich composites has been widely used in aeronautics field due to its higher flexural stiffness, structural efficiency and lower cost. However, the low interface strength between face sheets and cores would delaminate, which lead to the failure of whole structures. So, the applications of sandwich composites in aircraft structures were limited.Stitching can reinforce the interface strength and improve out-plane performances with less weight increment. Currently researches are mainly focused on the manufacturing process and mechanical tests of the new-type structure, and researches on mechanical model and numerical analysis are few. To expand the application of stitched foam-cored sandwich structure in aircraft structure, the prediction of stiffness and strength were important.The meso-characteristics of foam-cored sandwich structure were analyzed firstly. The characteristics of stitched laminates include five parameters which are angle, length, width of fiber waves, diameter of stitches and gap between resin and fiber. After measurements and analysis of the characteristics of stitched laminates with various stitch densities and stitch angles, the meso-characteristics parameters of the face sheets were decided. These parameters were basis of the unit cell, and the representative volume element method was used. Then the finite element modeling (FEM) of three-dimensional (3-D) unit-cell was conducted.Through proper examples, the model was verified to predict the stiffness of structures as follows: stitched laminar, stitched laminates and stitched foam-cored sandwich structures which have 0°,±45°, 90°plies. Then the influences of disturbed fiber, resin-rich region, diameter of stitches, materials of stitches, thickness and materials of foam-core on the stiffness of sandwich structure were estimated.