This jagged internal crack resulted in a detached spall, whereby
material is ejected out the rear surface, opposite from the point of impact.
Spall is caused by a mismatch between compressive shock waves generated at the impact, and
tensile shock waves reflected at the free rear surface. The impact velocity in this test was about 6.92 km/s.
When a target is impacted by a projectile traveling at hypervelocity,
a compressive (pushing) shock wave is generated. When it reaches
the free unsupported surface at the back of the plate, it is reflected as a
tensile (pulling) wave. The tensile wave is often strong enough to
pull some of the material away from the back of the plate, and produce internal
cracking. In some cases, material is thrown off the back of the plate
(at very high velocity) without generating a complete perforation of the plate. This
is referred to as detached spall, and can be just as lethal as if the
projectile had passed all the way through the plate.
Detached spall is observed in the test shown at right. While
there is no perforation or hole on the back surface of the aluminum slab,
material has been ejected out the rear by the shockwaves induced.
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