Stress-strain curve for aluminium from a continuous indentation test

Abstract

Continuous indentation tests using a 6.35 mm diameter steel ball were carried out on polycrystalline aluminium (99.995%) at forces up to 942 N (96 kg) and a total displacement of 65μm. On loading the results were observed to follow the classical Hertz equation until the elastic limit was reached at 4.6±0.2 N (0.47 kg), 1.02±0.05μm. The unloading results after plastic indentation were found to fit the Hertz solution for an indenter in a spherical hole. Using the Hertz theory it was possible from the unloading results to determine the mean stress and strain under the ball, together with the indentation diameter, plastic strain, Meyer stress and ratio of elastic to total strain, enabling a stress-strain curve for hardness to be drawn. The elastic limit of aluminium occurred at a stress of 4.7±0.2×10⁸Pa (46kg mm⁻²) and a strain of 1.27±.05%. At a total strain of 11.25% the stress was 11.7±0.2×10⁸Pa (115 kg mm⁻²).