In-plane deformation of kōwhaiwhai auxetic structure : a thesis presented in partial fulfilment of the requirements for the degree of Masters in Food Technology at Massey University, Palmerston North, Manawatū, Aotearoa New Zealand

Abstract

This thesis explores the development and characterisation of a novel kōwhaiwhai auxetic structure, providing bespoke mechanical functionality. Inspired by traditional Māori art and building on prior work, the research aims to characterise the geometric and mechanical properties of the kōwhaiwhai auxetic structure, with a focus on its Poisson’s ratio, tensile behaviour, and performance under varying humidity conditions to be used as a foundation for future packaging solutions involving the auxetic. The research used a combination of experimental testing, finite element modelling, and statistical analysis to understand the auxetic structure. Tensile tests were conducted to determine the Poisson’s ratio and stiffness, and FEM was used to simulate the tensile tests as a foundational model for understanding the Poisson’s ratio, stiffness, and stress distribution. Uniaxial tensile tests of the structure showed the influence of measurement regions on the apparent Poisson’s ratio of the structure and discussed the discrepancy with literature due to this. The trends described experimentally were shown numerically as well, showing the strength of the underlying model. Testing samples conditioned at high humidities showed a significant impact of relative humidity on the fibre-based material but the magnitude of this effect was not influenced by auxetic geometry on the material. Hinge thickness was the most significant parameter affecting stiffness of the kōwhaiwhai auxetic design. In the context of the effect of humidity on the material, this impact could be offset by a parameter change, showing the high tuneability of the auxetic. This research advances the understanding of auxetic materials properties as a foundation for packaging applications, demonstrating the kōwhaiwhai structure’s potential for use in diverse environmental conditions. The findings provide a framework for optimising auxetic designs to enhance shock absorption and sustainability in the packaging industry.