Christodoulou, I; (2017) Mechanical properties of micro-architectured lattices: Edge effects, fatigue and fracture. Doctoral thesis , UCL (University College London).

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Abstract

Micro-architectured lattices offer unique combinations of stiffness, strength and toughness at low density that makes them ideal for lightweighting applications. This thesis quantifies, compare and contrast various aspects of the mechanical performance of micro-architectured lattices with special emphasis on the role of microstructure upon their effective macroscopic properties. The differences in mechanical performance between lattices with a stochastic and various regular-periodic micro-architectures (Square, Hexagonal, Triangular and Kagome micro-structure), with each exhibiting either a stretch or bending-dominated deformation at the cell-scale, are systematically quantified. First, the elastic and yield properties of infinite-sized lattices are obtained by finiteelement modelling of representative unit-cells, applying the appropriate boundary conditions, where there is an excellent agreement with analytical predictions from existing literature. By relaxing the ‘infinite-size’ assumption, the effects of finite specimen size on the effective macroscopic stiffness and strength are quantified for uniaxial and shear loadings. The predicted size effects were found to be a direct consequence of the strong and weak boundary layers that emanate from the specimen boundaries. The influence of these edge effects on the macroscopic stiffness and strength are quantified for the Square, Triangular and Kagome lattices, and the results compared to existing ones for the Diamond, Hexagonal and stochastic Voronoi lattices in the literature. Second, in addition to the monotonic loading studied above, the cyclic stress-life response is also investigated for the regular lattices. A non-linear continuous fatigue damage model for high cycle fatigue is implemented which allows the simulation of strain accumulation in lattices until failure. The proposed model is able to predict with reasonable accuracy the S-N curves for Diamond lattices to shear fatigue where experimental data is available in the literature. The numerical model is then used to elucidate the shear and uniaxial fatigue response of other periodic lattice micro-architectures. Fatigue damage is found to originate in locations that are also affected by the boundary layers. A major difference in the response between bending- and stretching-dominated lattices is revealed; the bending-dominated micro-architectures accumulate damage within a larger lattice area (volume) and in a more progressive manner compared to their stretch-dominated counterparts. Last, the fracture toughness of stochastic Voronoi lattices is studied using an idealised LEFM (Linear Elastic Fracture Mechanics) approach and the results compared to those of periodic lattices. The role of relative density, micro-structure regularity and loading mode are also explored. It will be shown that the toughness predicted by numerical simulations of the CT (compact tension) and SENB-3PB (single-edge notched in threepoint bending) test specimens reveal a specimen-size dependency and a disparity with the corresponding predictions by the idealised LEFM approach: the origins of these are also clarified.

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