Deducing the stress-strain response of anisotropic Zr-2.5%Nb pressure tubing by spherical indentation testing

In this paper we developed a method of analysis by which the average stress-plastic strain flow curve of mechanically anisotropic materials can be deduced from spherical indentation test data. Our analysis is based upon spherical indentation tests performed on the extruded and cold-drawn Zr-2.5%Nb CANDU pressure tube material over the range of temperature from 25 °C to 300 °C. The indentation force and depth data were analyzed and σavg and ɛavg were calculated using previously reported equations developed for spherical indentation of isotropic material which were then modified, by incorporating the appropriate Hill's anisotropy coefficients, to characterize the anisotropic yield stress of the indented material. The resulting flow curves were dependent on indentation direction and correspond closely with flow curves obtained from previously reported conventional uniaxial stress tests performed on the Zr-2.5%Nb material. Indentation tests performed with large, 200 μm, and small, 40 μm, diameter spheres indicate that for small diameter indentations, when the indentation depth is less than several micrometers, the calculated σavg is heavily influenced by the depth dependence of the yield strength of the indented material.