The indentation size effect in the spherical indentation of iridium: A study via the conventional theory of mechanism-based strain gradient plasticity

The indentation size effect in spherical indentation experiments is studied via the conventional theory of mechanism-based strain gradient plasticity (CMSG) established from the Taylor dislocation model. Two approaches are adopted in the present study. The first, an extension of Johnson’s [Johnson, K.L., 1970. The correlation of indentation experiments. Journal of the Mechanics and Physics of Solids 18, 115–126.] theoretical indentation model based on CMSG, fails to predict the experimental data for iridium. The finite element method for CMSG is used to characterize the indented material in the second approach. The predicted indentation hardness agrees well with the experimental data. A simple, analytic indentation model is established to give the indentation hardness H=H02+141α2μ2bR in terms of the radius R of the spherical indenter, where H0 is the indentation hardness without accounting for the tip radius effect (i.e., given by classical plasticity theories), μ is the shear modulus, b is the magnitude of the Burgers vector, and α is the empirical coefficient around 1/3 in the Taylor dislocation model.