Measurements of the dynamic spall strength in aluminum, copper, and Metglas shocked by a high-power laser to hundreds of kilobars pressure are reported. The strain rates in these experiments are of the order of 10 7 s Ϫ1 , which cannot be reached in impact experiments. The free-surface velocity behavior associated with spallation is characterized by oscillations caused by the reverberations of the spall layer. An optically recording velocity interferometer system was developed to measure the free-surface velocity time history. This diagnostic method has the advantages of being a noninterfering system and produces a highly accurate continuous measurement in time. The spall strength was calculated from the free-surface velocity as a function of the strain rate. The results show a rapid increase in the spall strength, suggesting that a critical phenomenon occurs at strain rates ϳ10 7 s Ϫ1 , expressed by the sudden approach to the theoretical value of the spall strength.

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Measurements of the dynamic spall strength in aluminum and copper shocked by a high power laser to pressures of hundreds of kbars show a rapid increase in the spall strength with the strain rate at values of about 107 s−1. We suggest that this behavior is a result of a change in the spall mechanism. At low strain rates the spall is caused by the motion and coalescence of material’s initial flaws. At high strain rates there is not enough time for the flaws to move and the spall is produced by the formation and coalescence of additional cavities where the interatomic forces become dominant. Material under tensile stress is in a metastable condition and cavities of a critical radius are formed in it due to thermal fluctuations. These cavities grow due to the tension. The total volume of the voids grow until the material disintegrates at the spall plane. Simplified calculations based on this model, describing the metal as a viscous liquid, give results in fairly good agreement with the experimental data and predict the increase in spall strength at high strain rates.

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A simple device for the production and study of cylindrical shock waves, based on the sliding discharge mechanism, is described. Initial gas pressures of up to several hundred torr and line source energies from below 1 J cm-1 up to above 10 J cm-1 can be applied. Interferometric measurements carried out on shock waves produced by line source energies of 1.8 J cm-1 have yielded information on the initial ionisation of the gas ( approximately 5%), on the compression of the shock front ( approximately 3.5 rho 0) during the early stages and on its velocity as a function of the radius. Good agreement was found with numerical calculations of cylindrical shock waves.