The mechanics of size-dependent indentation

Indentation tests at scales on the order of one micron have shown that meas ured hardness increases significantly with decreasing indent size, a trend at odds with the size-independence implied by conventional plasticity theor y. In this paper, strain gradient plasticity theory is used to model materi als undergoing small-scale indentations. Finite element implementation of t he theory as it pertains to indentation modeling is briefly reviewed. Resul ts are presented for frictionless conical indentations. A strong effect of including strain gradients in the constitutive description is found with ha rdness increasing by a factor of two or more over the relevant range of beh avior. The results are used to investigate the role of the two primary cons titutive length parameters in the strain gradient theory. The study indicat es that indentation may be the most effective lest for measuring one of the length parameters. (C) 1998 Elsevier Science Ltd. All rights reserved.