Determining yield stress via measurement of nanoindentation plastic zone radii

Nanoindentation is a method of measuring the mechanical properties of materials that cannot be tested by conventional techniques. A method for determining yield stress from indentation has been developed by Huang and Gerberich based on Johnson's spherical cavity model. The method requires only a knowledge of the peak load and corresponding plastic zone radius to determine yield stress. This has previously been demonstrated for large scale indentations, but not for nanoindentation where assumptions such as isotropy and homogeneity may no longer be valid. Nanoindentation experiments have been performed on a series of cold rolled and annealed OFHC copper samples. Different annealing temperatures resulted in a range of initial yield stresses which were measured by conventional tensile testing. Electropolished samples were then tested using the Hysitron Triboscope (an add-on device to an atomic force microscope) which allowed for collection of both indentation and image information from which load and plastic zone radius could be measured. Comparison of yield stress measurements taken from nanoindentation and tensile tests shows an agreement to within 10% for initial yield stresses as low as 190 MPa and as high as 360 Mpa.