Negative behaviour from bi-material elements

Abstract

Most materials we use in our everyday life and which are defined by laymen as ‘normal’ exhibit properties such as getting thinner when stretched and fatter when compressed (positive Poisson’s ratios); expanding when subjected to heat (positive thermal expansion) and / or expanding under reduced pressures (positive compressibility). However materials which defy such common expectations do exist and in fact, materials and systems which exhibit negative Poisson’s ratios (auxetic, i.e. get fatter when stretched and thinner when compressed), negative thermal expansion (get smaller in at least one direction when heated) and / or negative compressibility (i.e. getting smaller under reduced pressures) has been characterized theoretically and / or experimentally.