Public defence in Mechanical Engineering M.Sc. (Tech) Milad Omidi

Opponent Professor Lorenzo Valdevit, University of California. Irvine, USA

Custos Professor Luc St-Pierre, Aalto University, School of Engineering, Department of Mechanical Engineering

Lattice Materials are made from an interconnected network of struts or cell walls. They are among the stiffest, strongest, toughest and lightest materials available. On the other hand, lightweight structural materials are crucial to the transport industry as they offer an efficient option to reduce CO2 emissions. For example, European Commission in 2021 estimated that increasing the use of lightweight materials in the car industry could reduce their CO2 emissions by as much as 20%. Therefore, lattice materials can fulfill this industrial demand for lightweight materials. Planar lattices, as the main focus of this thesis, can be classified as: regular, semi-, and demi-regular lattices. While the properties of regular tessellations, such as hexagonal, square, and triangular lattices, have been studied extensively, those of other promising architectures, like semi- and demi-regular lattices, have remained unexplored. Therefore, the aim of this thesis is to quantify the properties of semi- and demi-regular lattices, and compare them to existing regular architectures. The in-plane mechanical properties of seven semi-regular lattices were presented. A bending-dominated semi-regular lattice is 85% stiffer and 11% stronger than a hexagonal lattice. This topology would be ideal for applications requiring high stiffness and high energy absorption. The fracture toughness of three semi-regular, and three demi-regular lattices was investigated. The numerical simulations revealed that the mode I fracture toughness of two semi-regular lattices scales with exponent 1.5 of relative density, which is unique among planar lattices. Furthermore, two demi-regular lattices scales linearly with relative density, with one outperforming a triangular lattice by 15% under mode I and 30% mode II. The third demi-regular lattice has a fracture toughness that scales with the square root of relative density and matches the remarkable toughness of the kagome lattice. The fracture toughness predicted by FE simulations was in excellent agreement with experiments performed on CT samples produced by additive manufacturing. This demonstrates that it is possible to accurately measure the fracture toughness of lattice materials even though experiments are done with significantly fewer unit cells than what is typically used in FE simulations. The results will be beneficial for the design of specimens in future experimental studies, and the development of guidelines to measure the fracture toughness.

Contact information  milad.omidi@aalto.fi  tel +358 50 475 7366