Materials mechanical size effects: a review

Abstract

Knowledge of the mechanical behaviour of materials is essential in materials science and applied mechanics. At small sizes, this behaviour shows significant departures from the classical elastic–plastic model. The strength of a material increases either when the structure is small or when only a small volume is under strain: the so called size effect. Intrinsic size effects arise due to microstructural constraints, such as grain size or second phase particle precipitation. Extrinsic size effects are caused by dimensional constraints. These extrinsic constraints might be due to small sample size, where dislocation motion and other physical mechanisms are affected by the presence of a surface or interface, or due to small strained volume, where the dimensional constraint arises from the testing system. Generally, both sample size and strained volume constrain deformation. Interactions between intrinsic and extrinsic size effects are particularly interesting, though current understanding of this topic is limited. The purpose of the present review is to survey small scale strengthening phenomena and to assess the merits of classical and current mechanisms proposed to explain these effects. Practical applications of size effects are briefly reviewed.