A different type of indentation size effect

Pop-in during nanoindentation, which indicates the onset of dislocation plasticity, was systematically investigated in annealed and pre-strained single crystals of nickel using spherical indenters with different tip radii. As the indenter radius and pre-strain decrease, the maximum shear stresses determined from the pop-in loads increase. This represents a new type of indentation size effect (ISE), based not on the measured hardness as in conventional ISE, but on the stress needed to initiate dislocation plasticity.