Selective Laser Melting (SLM) based bio-inspired sandwich composite structures
Creators: name: Inam Ullah
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 400101
subject: Aerospace materials
classificationCode: 401408
subject: Manufacturing processes and technologies (excl. textiles)
classificationCode: 401707
subject: Solid mechanics

description: The aim of this PhD project is to develop bio-inspired, impact resistant sandwich composite structures for aerospace applications by utilising Selective Laser Melting (SLM) process. The overall strength of sandwich structures depends upon the compression and shear strength of the core material and the de-bonding strength between laminates and core. This research focuses on the enhancement of these properties by proposing unique geometric features for the core and its interface with the composite skin. Basic design concepts for core geometry have focussed on rod-shaped trabecular bone structures, atomic lattice structures and egg shell structures. Due to the complexity of these structures, additive manufacturing technology is found to be imperative to enable the manufacture and subsequent testing and optimisation of these structures. Selective Laser Melting (SLM) is used to produce these structures in Ti-6Al-4V, which was found to be best suited for this high level aerospace application. Geometric analysis has shown that the manufactured structures display some variation from the designed and optimised geometries, but the variations are found to be minimal for a wide range of design parameters. A key stage of the work includes the simulation of core structures under combined loading conditions of compression and shear and result validation through mechanical testing. The optimal configurations of the truss structures based on geometric parameters are evaluated through numerical simulation and comparison is drawn with existing structures. The performance of these structures shows up to 4 times improved strength for both loading cases and competitive stiffness as compared to conventional honeycomb structures. Advanced 3D core structures are also shown to have better energy absorption capacities in compression as well as shear compared to most commercial core materials. These findings are depicted with the help of Ashby material selection charts. In order to improve the skin-core adhesion, interface features on SLM built structure are also explored. Vertical pins are proposed as a bio-inspired reinforcing interface feature, which when manufactured through SLM can inherently produce a surface with improved mechanical interlocking capacity similar to structures found in the natural design of porcupine quill. Plain (without additional interface features) and pinned specimens with different pin sizes are tested for mode-I fracture toughness. When compared to carbon z-pins of similar length, the titanium pins show significantly higher toughness. The results of these tests are further used to develop a numerical model for cohesive behaviour between pin and composite material and pin pull-out. Finally, the core and sandwich structures with the proposed features are tested for comparison and validation of impact performance; showcasing the superior performance of 3D advanced core structures for elastic impact and crush resistance.