Evidence of ductile (alumina) and brittle (boron carbide) response of ceramics under shock wave loading.

An experimental technique capable of revealing the mode of inelastic, ductile or brittle, behavior of materials under shock-wave compression is presented. The technique utilizes an essential difference between the dependencies of brittle failure threshold and ductile yield stress on the lateral pressure: the brittle failure strength increases rapidly with the pressure whereas the ductile yield stress is much less pressure-sensitive. A controlled lateral pressure (pre-stressing) was produced by installing a shrink-fit steel sleeve on the lateral surface of the disk-like sample. Assuming that the tested ceramic sample obeys either the Von Mises criterion of ductile yielding or the Griffith's criterion of brittle failure, it was expected that the increment of the Hugoniot Elastic Limit caused by the pre-stressing should differ by a factor of about 2.5 for these two cases. The results of the tests performed unambiguously exhibit the ductile response of the alumina ceramic whereas the response of the boron carbide ceramic is certainly brittle.