A pathway towards the inverse design of all-composite honeycomb core sandwich panels

All-composite honeycomb cellular core sandwich panels are gaining wide popularity in lightweight structure applications due to their high specific stiffness and strength and multi-functional benefits. The honeycomb cellular core sandwich panels consist of a honeycomb core sandwiched between two face sheets. The performance of such sandwich panel is related to multiple geometric and material parameters of the core and face sheets. Due to a large number of parameters and their complex interactions affecting the performance of the honeycomb cellular core sandwich panels, the optimal design of sandwich panels is difficult and demands a systematic approach by the designer. This thesis focuses on developing the necessary design tools required for the accurate and efficient inverse design of all-composite honeycomb core sandwich panels considering the key geometric and material parameters of the core and face sheets. First, a strain energy-based homogenisation model is developed to calculate the in-plane and out-of-plane effective stiffnesses of the laminated composite honeycomb core. Unlike the other existing models, the proposed model is applicable for all types of honeycomb cellular core geometries and both single lamella or laminated walls of different materials. Therefore, the proposed model contributes towards significantly enhancing the state of knowledge on the design of honeycomb cellular core sandwich panels. The proposed homogenisation model was validated using the finite element (FE) analysis results of different honeycomb core geometry and material combinations. The results from the proposed model and FE analysis showed a good agreement for all the different honeycomb core configurations considered in the study. Next, the sandwich panels with honeycomb cores were analysed for the global responses using the equivalent models based on the first-order shear deformation theory (FSDT). The honeycomb cores in the sandwich panels were represented as a homogeneous continuum with the effective stiffness matrix obtained from the proposed homogenisation model. The sandwich panels were analysed for the deflections and in-plane normal stresses of the face sheets under static bending and the global critical buckling load under uniaxial compression using the equivalent models. The predictions were compared against results from the FE models of the sandwich panels with the actual core structure. A good agreement was found between the predictions from the proposed models and the FE results. Since the proposed equivalent model for the sandwich panels cannot capture the possible local failures which are essential part of the sandwich panel design, new simplified semi-analytical models were developed to explicitly consider the local failures. A semi-analytical approach was developed for predicting the critical shear buckling load of the laminated composite honeycomb cores of different shapes. In the proposed model, two different boundary conditions were considered for the edges of the core walls. While using simply-supported boundaries for all the edges of the core wall gave conservative predictions of the critical shear buckling load, boundary conditions of rotationally restrained longer edges of the wall gave very close predictions of the critical shear buckling strain to the results from the FE analysis. The effect of different fibre lay-ups and shear loading conditions on the shear buckling strength were investigated for honeycomb cores with different shapes. A semi-analytical model was also proposed to predict the intracellular buckling of laminated composite face sheets with non-rectangular cells. The proposed approach was formulated to be as general as possible to take into account different geometric shapes of the cell, rotational restraints at the boundaries of the cell, and different loading conditions which had not been considered in the existing analytical solutions. Using the proposed approach, first, intracellular buckling of laminated composite face sheets with hexagonal cell was studied under various compressive loadings. While the proposed approach with simply-supported boundaries for the cell gave conservative results, predictions with rotationally restrained boundaries for cell gave very close predictions to the FE results considering various conditions such as different cell sizes, core density, face sheet’s fibre lay-up and loadings. The effect of all these different parameters on the intracellular cellular buckling load of the laminated composite face sheets were also studied