KINEMATIC ANALYSIS OF PLASTIC INSTABILITIES OCCURRING IN MICROHARDNESS TESTS

Characteristics of plastic instabilities occurring during dynamic micr ohardness testing of Al-3.3 wt%Mg alloy samples have been investigated . During the experiments the load, F is increased at a constant loadin g rate, mu, and the indentation depth, d is detected. The measurements were carried out at room temperature. It was found that quasi-periodi c 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 investig ated. The dynamic Vickers microhardness, HV similar to(F/d(2)) shows s mall amplitude oscillations around a nearly constant value. These smal l amplitude oscillations were found to change with the penetration rat e of the indenter tip which is related to an N-shaped hardness-indenta tion velocity characteristic. (C) 1998 Acta Metallurgica Inc.