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One-dimensional impulse loading experiments have been carried out on Lucite and aluminum to determine if the spall process 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 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 with 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 was varied from 2 x 107 to 50 x 107 sec for Lucite, and from 107to 25 x 107 sec for aluminum. The threshold velocity for Lucite spall remained constant (60 m per sec) indicating that, over this time scale, the spall mechanism for Lucite is independent of time and that a critical tensile stress exists (̃1 kilobar). The threshold driver velocity for aluminum spall increased with decreasing driver thickness indicating a time-dependent spall mechanism for this material-that is, a critical tensile stress does not exist.
Blincow, Donald W.
The Boeing Co.Giannini Controls Corp., SeattleDuarte, Wash.Calif.
Keller, Donald V.
The Boeing Co.Northrop Ventura, SeattleNewbury Park, Wash.Calif.