Dynamic hardness measurements using a dropped ball: with application to 1018 steel

A practical method for measuring the hardness of metals at high strain rates (>103 s−1), using a dropped ball, is developed and demonstrated on cold-rolled plain carbon (1018) steel. The metal is assumed to be one-dimensional and rigid plastic. By formulating the problem in terms of the lateral, rather than the depth, dimensions of the indentation, the dynamic hardness can be obtained at load, obviating the need for rebound energy corrections. In addition, the Tabor strain equation relating to equivalent unidirectional compression is derived, and methods are also derived to include “piling-up” around indentations for plastic flow at constant volume. A previously unrecognized phenomenon that can occur in projectile indentation of metals, “ballistic strain softening”, is described. It is related to the fact that the dynamic hardness obtained by this test is a function of the ball radius times the strain rate divided by the strain. As a result, comparison of the dropped ball test flow stress data with those of constant strain rate tests requires a transform of the Tabor strain and strain rate data to constant strain rate conditions. When this is done, the dynamic yield stress data that are determined as a function of the effective strain rates agree with comparable published data for mild steel obtained using more conventional tests. The results also show that there is greatly enhanced strain hardening in irons and steels above a critical strain rate, which increases with purity. A dislocation model is used to explain this increase in terms of an additional dislocation generation mechanism coming into play.