Failure zones in polycrystalline aluminas

We have recently demonstrated that glasses show damage after the arrival of a plane, compressive shock wave. The failure travels, in this case, at a u niform velocity, increasing at higher stresses. The purpose of this paper i s to show that a range of polycrystalline aluminas also show failure in a s imilar manner to glasses. The difference is that the wave in aluminas is sl owed and eventually stopped leaving a surface failed zone, whereas the proc ess continues in glasses without reducing speed. These surface failure zone s have a marked effect. One of the consequences is that the Hugoniot elasti c limit for these materials may have to be taken at a higher point in the s tress history than is conventional. A second is that speculation about deca ys with distance of elastic precursors may refer to the stress at which the failure zone is formed rather than to the stress at which yielding takes p lace.