Bio-inspired design of aerospace composite joints
Creators: name: Burns, Lauren
Publisher: name: RMIT University
Year: 2024

description: This PhD thesis utilises deep principles in nature governing the architecture of structural joints formed from biological materials, focusing on tree branch joints, in order to create novel prototype designs for carbon fibre reinforced polymer (CFRP) T-joints resulting in improved structural performance. <br><br>The aim of biomimetic engineering is to understand the connection between the organisation of biological materials and their extraordinary mechanical properties. Conversely, a major risk of biomimetic engineering is synergistic effects of complex hierarchical structures found in nature cannot be replicated using production-scale manufacturing processes.<br><br>Deep (defined as common and recurrent) principles in nature are elucidated. The first deep principle in nature is uniform strain under the critical load case, which is achieved by manipulating orthotropic material properties to compensate for geometric stress concentrations. The second deep principle is hierarchical design in which specific architectures at varying length scales work together in synergy to improve overall structural properties. <br><br>The primary outcomes of this thesis are; the development of an understanding of the relationship between the architecture and the mechanical properties and failure modes of tree branch-trunk joints; the development of a validated optimisation methodology based on the principle of uniform strain to improve the failure strength of composite T-joints by adapting the orthotropic material properties to the prevailing loading conditions; improvement in the damage tolerance of composite T-joints by introducing bio-inspired structural design changes that promote extrinsic toughening; and the effects of bio-inspired material optimisation and structural design changes on the internal stress distribution in the T-joint radius bend and delta-fillet regions are quantified through finite element analysis.<br><br>An optimisation methodology based on the uniform strain principle was developed to minimising the peak interlaminar tensile stress in the T-joint radius bend while maintaining similar global laminate stiffness properties to a baseline design. This was achieved by altering the laminate stacking sequence. The effect of integrating the T-joint flange into the skin on the damage tolerance of T-joints was investigated. Experimental bending, tensile and compressive test results are presented in conjunction with finite element analysis (FEA) into the failure modes of the bio-inspired and baseline T-joint designs. The results of a preliminary study into low energy impact damage tolerance of bio-inspired T-joints are also shown. Manufacturing and certification issues are briefly discussed.<br><br>Based on the findings of the FEA, optimisation and experimental results, this PhD research proves that biomimetics and specifically the bio-inspired design principles of uniform strain and hierarchical design are a useful approach in creating novel prototype composite T-joints resulting in improved structural performance. Recommendations for future research are provided as advanced manufacturing techniques expand the possibilities of incorporating greater hierarchy into bio-inspired designs. <br>