SHOCK FRACTURE AND RECOMPACTION OF CERAMICS (WAVES, SPALL, IMPACT, COMPACTION, HUGONIOT)

The main objective of this study was to develop a comprehensive model for shock fracture of ceramics. Plate impact experiments were conducted. The dynamic target response was measured with VISAR and manganin gauges. Three commercial ceramics were employed, alumina AL-300, alumina AD-85 and to a lesser extent, beryllia Ceralloy 418.; A double impact technique for studying the properties of shock-damaged materials was developed. The first impact causes spall in the target while the second impact recompacts the target. Analysis of the transmitted waves gives the properties of the damaged region. A "self trap" recovery technique was developed to examine spalled targets.; Examination of the recovered targets, as well as the timing of spall signals has led to a "spall zone" model, in which spall occurs not at a plane but in a finite region. The model applies to highly elastic solids and assumes that spall fracture propagates away from the initial spall plane at 20 to 60 percent of the longitudinal wave speed until quenched by incoming compression waves. A theoretical model that treats the spall zone homogeneously was used in a modified SWAP code.; The Hugoniot Elastic Limit (HEL) of 6.5 mm thick AL-300 is 8 GPa. The HEL of AD-85 decays from 5.9 GPa in 6.4 mm thick samples to 4.1 GPa in 20.4 mm. This decay partially explains the discrepancy between static and dynamic strength measurements. A dispersive plastic-like wave observed in alumina was explained by crack-dislocation interactions. The HEL of 4 mm thick beryllia is 9.4 GPa from which a zero porosity HEL of 9.8 GPa was deduced. Transverse stress measurement provided a direct measure of target strength at the HEL. Bar impact tests determined strength at a different strain rate.; The spall strength of alumina is 0.3 to 0.4 GPa when shocked to 3 GPa, diminishing for stronger shocks because of stable growth of cracks under compression. Beryllia has a small spall strength when shocked to 90 percent of its HEL. Above the HEL all three materials fractured in compression. Spall toughness in brittle materials was defined and evaluated.