The process of cratering in materials is described in terms of four phases: transient, primary, secondary, and recovery. Rod projectiles were used in an experimental study of the relative importance of each phase and its influence on final crater dimensions. Aluminum and steel rods with aspect ratios of 1/6 to 25 were impacted into metallic and nonmetallic target materials at impact velocities of 0.3 to 6.7 km/sec. During the primary penetration phase, rod deformation was found to be comparable to that observed for shaped-charge liner collapse, and measured rod penetration rates showed good agreement with rates calculated on the basis of incompressible fluid flow considerations. Empirically developed equations are presented for predicting crater penetration and volume in metal targets.
Tests were conducted on high-purity beryllium plate to determine the Hugoniot equation of state and shock wave profile up to 50 kbar, and the spall threshold for elastic pulse widths of 0.2, 0.4, and 0.8 jusec, using flat-plate impact techniques. Tests were carried out at material temperatures of 22°C and 260°C. This beryllium was found to have a two-wave elasticplastic structure with a highly ramped transition from the elastic precursor to the plastic wave. No change in the material equation of state was observed for a temperature increase of 22°C to 260°C. The spall threshold (as defined by the onset of microcracking) was found to be time dependent, with impact velocity required for spall increasing with decreasing pulse width, and temperature dependent, with velocity increasing with increasing temperature.
The use of high-resolution, time-resolved measurements of rear surface velocity in shock loaded 6061-T6 aluminum is discussed as a means of studying spall fracture. The influence of stress pulse shape, material temperature, maximum compressive stress and degree of fracture on free surface motion is presented and correlated with maximum tensile stress at the spall plane. A summary of dynamic properties data (equation of state, elastic wave velocity, stress-strainstrain rate behavior, spall threshold) and a brief discussion on fracture in metals are also presented.iii
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