Kinematic analysis of plastic instabilities occurring in microhardness tests

Characteristics of plastic instabilities occurring during dynamic microhardness testing of Al-3.3 wt%Mg alloy samples have been investigated. During the experiments the load, F is increased at a constant loading rate, mu, and the indentation depth, d is detected. The measurements were carried out at room temperature. It was found that quasi-periodic motion of the indenter head sets in after a critical load, F(c), has been attained. The time period, Delta T(p), Of the oscillations and the depth increase in one period, Delta D(p), have been investigated as a function of time while the loading rate was varied between 1.4 and 70 mN/s. The dependence of F(c) on the loading rate has also been investigated. The dynamic Vickers microhardness, HV similar to(F/d(2)) shows small amplitude oscillations around a nearly constant value. These small amplitude oscillations were found to change with the penetration rate of the indenter tip which is related to an N-shaped hardness-indentation velocity characteristic. (C) 1998 Acta Metallurgica Inc.