Rate-dependent spallation properties of tantalum

Spallation experiments are conducted on high-purity tantalum using VISAR instrumentation for impact stresses of 9.5 GPa and 6.0 GPa. The high-amplitude experiment exhibits very rapid initial spall separation, while the low-amplitude shot is only slightly above the threshold for void growth and thus exhibits distinct rate-dependent spallation behavior. These experiments are analyzed in terms of simple tensile fracture criteria, a standard rate-dependent void-growth model, and a rate-dependent void growth model in which the expected plastic volume strain makes no contribution to the relaxation of the mean stress. Recovery tests and VISAR measurements suggest an additional resistance to spallation that follows the rapid coalescence of voids; this effect is termed the secondary spall resistance and is due to the convoluted nature of the spall plane and the resulting interlocking fracture pattern that is developed and for which the stress remains unrelieved until the spall planes have separated several hundred microns