The fracture patterns which develop during the ballistic impact of two series of three layer glass/polycarbonate/glass impact resistant glass panels laminates were studied for both annealed glass and heat strengthened glass plates in the glass panels. The cracking patterns are qualitatively described. The numbers of radial cracks were related to the projectile kinetic energy and the stored elastic strain energy in the heat strengthened glass. An energy balance revealed that the elastic strain energy and the residual stresses in the heat strengthened glass are only partially relieved by the impact cracking. Results suggest that for higher velocity projectile impacts a greater amount of residual stress relief occurs from a greater fragmentation of the heat strengthened glass plates in the panels.
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