Composite joints are often plagued by problems from high interlaminar stress concentrations and low through-thickness material strength. For T-joints and L-joints, damage under applied loads is concentrated around geometry changes in the radius bend and delta-fillet. The joints undergo progressive failure through a combination of matrix and delamination cracking, initiated primarily by interlaminar tensile and shear stresses. These local stresses are much higher than the far-field stress due to the geometric stress raiser that exists in the radius and delta-fillet regions where the stiffener and skin sections of the joint are connected.
Design changes based on the principles of biomimicry represent a novel approach for improving the structural properties of composite joints. An important design principle observed within biological structures such as trees or skeletal structures is that they are self-optimized to the condition of uniform strain. Biomimetic engineering aims to understand the connection between the complexity of the hierarchical structure and composition of biological materials and their mechanical properties. Elucidating how nature extracts maximum structural efficiency from the constituent materials through hierarchical design provides engineers with an enormous opportunity to apply these concepts to engineered materials and structures, driving next-generation improvements in mechanical properties.
A bio-inspired optimized T-joint design has a higher failure initiation load and higher absorbed elastic strain energy than the base-line T-joint with a quasi-isotropic ply configuration. FE modeling revealed that the optimized ply stacking pattern was capable of mimicking (in part) the biological principle of uniform stress that exists within structural joints such as the tree branch-trunk joint. Optimization of the ply stacking pattern improved the strength and failure displacement of the T-joint without significantly affecting the global stiffness properties. This optimization methodology works by altering the stress distribution in the radius bend to effectively minimize the interactive stress concentration caused by high interlaminar tensile and interlaminar shear stresses. The optimization was performed with modeFRONTIER, whereas the chosen scheduler was Multi-Objective Simulated Annealing (MOSA) and each program loop involved 100 iterations. Based on this research, bio-inspired optimization has proven an effective technique to improve the strength of composite T-joints without incurring stiffness, weight or cost penalties.