New method for estimating local *strains using nanoindentation and atomic force microscopy
Scope and method of study. A new method has been developed for determining strains in very small regions using a micro-tensile stage and nanoindentation as tools. In this investigation indents formed in nanoindentation are used as probes to measure local deformations in a small region that can hardly be measured with most existing techniques. Strains were introduced in samples via a small in-situ tensile testing system, originally developed for in-situ SEM, and modified and adapted here for use with the AFM. Variation of local strains in very small regions compared to global strains has also been established. Two approaches are used. In the first approach, length changes of a triangular indent are used to determine normal strains and the corresponding normal and shear strain components. In the second approach, displacement changes in any pair of three indents are monitored for computation of normal strains and the associated normal and shear strain components using strain transformation relations. Indents were made on a polycrystalline copper tensile specimen, and the residual indent geometry is monitored by an Atomic Force Microscope (AFM).
Findings and conclusions. The results show that the shape of the residual indents depends on the crystal orientation and the angle which the slip bands formed in individual grains. The technique developed is complementary to existing strain measurement techniques, and is especially suitable at micro- and nano-scales. The techniques are validated at two scales, namely microscale with the use of microindents formed by a Nanoindentation system, and nanoscale using nanoindents made by AFM tips by comparing strains measured using this technique and the deformations determined using digital image correlation (DIC) under relatively small deformations. Further validation of these techniques is made by performing a virtual "digital" tensile test consisting of simultaneous digital stretching and shrinking of an image of the nanoindents by known values and analyzing the strains. Reasonably good agreement has been obtained at both scales. The validated technique is then used to measure relative deformations of multiple grains and deformations within a single grain in the polycrystalline copper. Strains associated with a slip band were determined by this technique. (Copies available exclusively from MIT Libraries, Rm. 14-0551, Cambridge, MA 02139-4307. Ph. 617-253-5668; Fax 617-253-1690.)
0794: Materials science