Mechanism of Spall in Lucite

One-dimensional impulsive loading experiments have been carried out on Lucite to determine whether the spall occurs instantaneously upon the attainment of a critical tensile stress, or if there is a time delay between the attainment of a given tensile stress and spallation. The experiments involve the collision of two flat Lucite plates: a driver and a target. The velocity of the driver is adjusted until the threshold velocity for spall of the target is obtained. The experiment is repeated but the thicknesses of the two plates scaled by some factor (α). The maximum tension is then unchanged, but the time over which it acts is changed by the factor (α). In this manner the duration of tension required to spall Lucite was varied from 2×10−7 to 5×10−6 sec. The threshold velocity for spall remained constant (60 m/sec) indicating that, over this time scale, the spall mechanism for Lucite is time-independent and that a critical tensile stress exists (∼1 kbar).