Characterization of indentation size effects in epoxy

Nanoindentation tests were conducted at different indentation depths, ranging from 3000 to 30 nm, to study the characteristics of indentation size effects in epoxy. Taking the effects of tip imperfection/bluntness into account, nanoindentation tests performed with a three-sided Berkovich tip exhibited strong increases in the hardness with decreasing indentation depth. However, comparative testing with a spherical tip with a radius of 250 microns, showed no changes in the material properties with respect to the distance from the surface. Consequently, different deformation mechanisms must be present in the indentation tests with Berkovich and spherical tips, suggesting effects related to second order gradients of the displacement (such as strain or rotation gradients), which increase with a Berkovich tip but essentially stay constant with a spherical tip when indentation depth decreases.