Smooth Spalls and the Polymorphism of Iron

Abstract

When Armco iron is loaded with a transient wave in which the pressure exceeds 0.131 megabar, smooth spalls may be observed. The two distinct compression shocks which are transmitted by the iron are followed by a steep rarefaction which may be called a rarefaction shock. At a free surface, the first compression shock is reflected as a backward-facing rarefaction wave. This wave may also steepen into a rarefaction shock which eventually meets the forward-facing rarefaction shock. The interaction of these waves results in a localized tension of destructive magnitude, which produces a smooth fracture. Location of the plane of this interaction was calculated by applying the theory of hydrodynamics. These calculations are inexact because the speed of sound for relief waves was taken from Hugoniot data since other data are not available. Fair agreement was obtained between experimental and calculated results. The work indicates that, in the case of a double spall, the fracture most remote from the original free surface may sometimes be formed first.