An important failure mode in sandwich structures is the debonding between the core and facesheet, which can destroy the load capacity of the structure. This work addressed the critical interfacial modes and studied the effects of thickness variation of the core material. The single cantilever beam geometry is utilized for conducting experiments after optimizing the thicknesses of the core and facesheet by minimizing the difference in the bending stiffness matrix between the upper facesheet and the lower facesheet/core combination. Two different core material thicknesses were tested. The experimental results showed that the critical energy release rate could be influenced by core thickness variations. Furthermore, the cohesive zone method and elastic-plastic core material model in conjunction with fracture criteria were used to model the entire structure failure response. The validation results predicted load-extension curves in agreement with actual tests for both single cantilever beam geometry specimens. The model also had the ability to predict the crack initiation in the core materials which occurred under the interface zone as in the actual test. In addition, the mixed-mode ratios through the interface area were analyzed as function of crack length to assess its influence on both single cantilever beam thickness specimens.

Composite sandwich structural joints, such as T-joints, are used in many different composite applications to transfers the load orthogonally between two sandwich elements. However, these joints connecting the sections can represent the weakest link in sandwich composite structures due to the lack of reinforcement in the out-of-plane direction. Therefore, this paper presents a new methodology for the design and analysis of composite sandwich T-joints using new biomimetic fabrication methods. The fabricated idea comes from biological fixed joints as an evolutionary alteration processes of trunk-branches of trees. It offers unique attributes to optimize the continuous fiber paths for minimum stress concentrations and multi-sandwich layers to increase the bending stiffness and strength. The focus is on how the biomimetic technique can improve sandwich T-joint structures by increasing their strength and load carrying capability without adding a significant weight penalty. The major attention is to investigate the comprehensive failure modes in the joint numerically and verified by experiments. Investigations were conducted on three different designs of biomimetic composite sandwich T-joints under tension and bending loads. The results show significant improvements to the ultimate load up to 68% in the case of bending load and 40% in the case of pull-off load in the biomimetic sandwich T-joints compared to the reference conventional T-joint design. The final failure was significantly deferred in both load status. The FE models provided important insights into the core failure and delamination of multi-interface biomimetic T-joints.