A Preliminary Two-Dimensional Palpation Mechanics for Detecting a Hard Inclusion by Indentation of a Soft Matrix Under Large Strain

A rigid inclusion is embedded at a finite depth in a soft layer resting on a rigid substrate. A spherical indenter presses vertically onto the surface, deforming the matrix and displacing the inclusion. A subsurface inclusion initially near the indentation axis moves primarily downward, until an unstable lateral jump occurs to minimize the energy stored in the elastic medium. Such an instability is unique to soft materials undergoing large deformation. A two-dimensional plane-strain finite element analysis is used to simulate the 3D phenomenon.

Deformation micromechanisms of ZnO single crystals as determined from spherical nanoindentation stress–strain curves

In this work, instrumented nanoindentation experiments with two spherical tips with radii of 13.5 and 1 μm were used to explore the deformation behavior of ZnO single crystals with two orientations, C (basal) and A (prism). By converting the nanoindentation load–displacement data to stress–strain curves, we show that the main reason the hardening rates are higher for the C plane than they are for the A plane is the activation of dislocations—with widely different flow stresses—on different sets of slip planes. For the former, glide occurs on basal planes as well as pyramidal planes; for the latter, glide occurs predominantly on basal planes. The C plane is roughly twice as hard as the A plane, probably due to the orientation of basal planes with respect to the indentation axis. A Weibull statistical analysis of the pop-in stresses indicates that the inherent defect concentration at or near the surface is the main factor for the initiation of plastic deformation. The strain energy released when the pop-ins occur determines their extent. The elastic moduli values, determined by Berkovich nanoindentation, are 135 ± 3 GPa and 144 ± 4 GPa for the C and A planes, respectively. In the C orientation repeated indentations to the same stress result in fully reversible hysteretic loops that are attributed to the formation of incipient kink bands.