Spallation model for the high strain rates range

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 10(7) 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. (C) 1998 American Institute of Physics. [S0021-8979(98)04121-8].