All failed windows exhibited a cone shaped failure surface, with the apex 
of the cone being located just below the center of the high pressure face 
of the window. Very little difference was observed between the fracture 
patterns in the windows with impact cracks and those without (Figure A-4). 
The comparison of implosion pressures shows that no significant decrease 
in the window's critical pressure occurred due to the presence of cracks 
generated prior to pressurization by impact of rifle bullets on the high 
pressure face. Also the implosion pressures of laminated windows were 
somewhat lower than those of monolithic windows. 
Several tentative conclusions can be drawn from this data. First, 
a crack on the high pressure face of an acrylic window does not necessarily 
lead to a catastrophic failure by rapid crack propagation at lesser pres-— 
sures than the critical pressure of a window without such a crack. Such 
a crack, however, must not penetrate more than 50 percent of the window 
thickness and must be located in the center of the window. Second, in view 
of the fact that the operational pressure rating of an acrylic window 
generally is only about 1/10 to 1/12 of its critical pressure under short- 
term loading, no danger exists if the window with cracked high pressure 
face is inadvertedly subjected only once to its operational depth. Third, 
a laminated window with a soft bonding layer does not possess as high a 
critical pressure as a monolithic window of identical diameter and thick- 
ness. Fourth, a laminated window with an impact crack on the high pressure 
face does not possess a higher critical pressure than a monolithic window 
with an impact crack. 
Although it is understood that those conclusions apply directly only 
to specimens tested under short-term loading, they also apply, in all 
probability, to flat disc windows of different proportions, as well as to 
conical windows. It must be emphasized, however, that the above conclu- 
sions apply only to cracks on the high pressure face of the window. What 
the behavior of windows with impact cracks on the low pressure face is has 
not yet been explored in any detail. 
Still, regardless of the encouraging results from this very brief 
study all impact cracks should be avoided on either the high or the low 
pressure faces of the window. If cracks do occur, the window should be 
replaced immediately. 
28 
