Edge and size effects in micro-architectured lattices

Prof. PJ Tan, University College London, UK

10:00-11:00, September 18, 2023

Room 211, Zhongshi Building, Xuhui Campus

Biography

Dr Tan is a full Professor of Applied Mechanics at UCL where he heads the High Strain Rate Laboratory. His research deals with the structural and functional (mechanical performance) characterisation of engineering materials and structures across a broad range of scales. He is particularly interested in understanding how light-weight materials (foams, micro-architectured lattices, composites, etc.) and structures (sandwich panels, bonded structures etc.) respond to unconventional loadings, and how they may be designed, or used in combination, to withstand more arduous operating conditions; including dynamic loading, thermal and mechanical stresses, enhanced resistance to spall and shock without structural degradation. A major driver of his research concerns the application of engineering solutions to improving the physical resilience of built systems to extreme load cases arising from malevolent actions, accidents or natural hazards.

Abstract

An important consideration when designing with micro-architectured lattices is to understand how the presence of an additional length-scale (associated with its characteristic cell size) influence their effective mechanical properties. In general, a lattice specimen must be ‘sufficiently’ large to achieve its bulk properties. However, the bulk mechanical properties predicted by existing scaling-laws (derived from representative unit-cell with periodic boundary conditions) do not account for such size-dependent effects. Two examples will be presented to illustrate how size and edge effects influence the modulus and strength of ‘under-sized’ specimens – they are often encountered when lattices are used as core material in sandwich construction – and, also, its mode-I fracture toughness (KIC).

The first example will cover how the effective mechanical properties of a finite-sized Kagome lattice – this typically deforms by stretch under remote loading ¬- are affected by specimen size (number of complete cells along their width and height) for two simple boundary-value problems, viz. simple shear and uniaxial compression; the underlying mechanism(s) responsible for this will be elucidated. In the second example, an attempt will be made to quantify, and identify the physical origins behind, why the toughness of nominally isotropic (elastic-brittle) stochastic lattices are affected by the number of cells in the uncracked ligament and, also, by the fracture test configuration (CT and SEN(B)-3PB). Some issues related to the toughness measurement of lattice materials will be highlighted.