Strain Hardening around Spherical Indentations

Abstract

Plastic strain distribution around spherical indentations in annealed polycrystalline copper has been determined by Vickers microhardness mapping of carefully prepared median plane sections through the indentations and comparing the microhardness values with those of small cylinders of the annealed copper which had been uniaxially compressed to different plastic natural strains (i.e. true strains, defined as ln(l0/l)) in the range of 0.01 to 0.97. The spherical indentations were of dimensionless sizes a/R in the range of 0.08 to 0.85, where a and R are the radii of contact and indenter, respectively. It is shown that the magnitude of the equivalent maximum plastic natural strain εmax increases nonlinearly with increasing a/R and reaches a value of 0.72 for an indentation of a/R = 0.85. The measured values of the εmax are considerably higher than the `representative' strain εr values predicted by the empirical formula εr = 0.2a/R. The implications of these findings have been discussed in relation to determining the uniaxial flow stress versus strain curve of a power-law hardening solid using the spherical indentation test.