Additively manufactured aluminium auxetic architecture with targeted mechanical and energy absorption characteristics

Abstract

Auxetic materials offer unconventional properties owing to their negative Poisson’s ratio (−𝜐) leading to deformation modes and mechanical characteristics different to traditional porous architecture. This leads to favourable outcomes for lightweight applications where precise control of the mechanical and crashworthiness responses is required. In this regard, the thesis puts forward an open innovation framework for the selective laser melting (SLM) of auxetic architecture that offers stiffness (E), strength (𝜎𝑡) and energy absorption characteristics suitable for a targeted scenario. The primary objective is to create a framework that integrates numerical modelling, multi-criteria decision-making, and optimisation tools to generate scenario-based auxetic architectures that offer targeted performances. The selection of the five-unit cells were informed by the density and auxeticity criteria. A lower density is required to accommodate large deformation during loading, leading to a relative density range of 0.17-0.26 as suitable to achieve the required porosities. When it comes to unit-cell shape, all fundamental architectures that can lead to auxetic performance were considered. Experimental and numerical analysis is used to reveal the range of −𝜐, E, 𝜎𝑡, specific energy absorption (SEA), peak crush force (PCF), and crush force efficiency (CFE) of the auxetic architectures. The surrogate model developed in this thesis enables the manufacturing of auxetic structures with tailored stiffness (E), (𝜎𝑡) strength, and energy absorption characteristics (SEA, PCF, CFE) to meet specific requirements of the target scenario. The analysis of variance (ANOVA) found the central composite design (CCD) to be suitable for developing the surrogate model and capturing the influence of all design variables on–𝜐, 𝜎𝑦, E, SEA, PCF, CFE for AlSi10Mg metamaterial architecture informed by the sinusoidal ligament architecture (AUX5). When optimising the selected auxetic architecture AUX5 for lightweight application (Scenario 1) a stiffness and strength of 991-1023 MPa and 5.95-5.68 MPa can be expected at a strut thickness and length of 0.371 and 0.632 mm respectively. For crashworthiness performance (Scenario 2), CFE, SEA and PCF can be expected in the range of 69.10-71.62%, 14.48-14.14 kJ/kg and 1762-1850 kN respectively at a strut thickness and length of 0.304 and 1.268 mm. When the scenario changes to a balanced performance (Scenario 3) between targeted mechanical and crashworthiness behaviour can be obtained at 𝑡𝑠 and 𝑙𝑠 of 0.229 mm and 1.268 mm respectively. The resulting characteristics for −𝜐, E, 𝜎𝑦, CFE, SEA and PCF can be expected in the range of -0.21-0.125, 761-771 MPa, 5.53-5.69 MPa, 73.23-69.98%, 17.23-16.89 kJ/kg, 983-960 kN. The error percentage of three scenarios (S1-S3) was less than 5% which justifies the accuracy of the predicted model. The errors were minimised using a validated finite element model to predict the performance characteristics of the auxetic architectures considered. Furthermore, a mesh sensitivity analysis was carried out to ensure results were independent of the meshing strategies used. The results of this study provide a solid foundation for future research and applications in the field of auxetic Material. The thesis demonstrates the use of the analytical hierarchy process (AHP) and Technique for Order Preference by Similarity to Ideal Solution (TOPSIS) methodology to select the best-performing architecture based on five criteria. Overall, this thesis offers a new direction in the development of scenario-based tuneable auxetic architectures.