Appendix A 
EFFECT OF IMPACT CRACKS ON ACRYLIC PLASTIC HYDROSPACE WINDOWS 
The performance of flat disc acrylic plastic windows under short- 
term loading has been researched in sufficient detail” to establish 
accurately the implosion pressure of such windows. In these tests, 
considerable pains were taken to insure that no cracks or scratches were 
present in the windows prior to their implosion testing. Under opera- 
tional conditions, however, it is very often impossible to prevent the 
generation of scratches or cracks in the surface of windows. In such 
cases, a real fear exists that the crack introduced initially into the 
high pressure face of the window by impact of an external object may 
serve as the source of catastrophic crack propagation failure at lesser 
hydrostatic pressures than the window is rated. 
For this reason, an exploratory study was conducted. As test 
specimens four flat disc acrylic plastic windows were used of 6-inch 
diameter and approximately 14-inch thickness (Figure A-1). Two of the 
windows were of monolithic construction, having been machined from 1.250 
thick Plexiglas "G'' plate. The other two windows were of laminated 
construction. The inner layer of the laminated window was 31/32 of an 
inch thick Plexiglas "G", the outer layer was 7/32 of an inch thick 
Plexiglas "G'', while the layer bonding together the inner and the outer 
acrylic sheets was cast-in-place Swedlow SS-—3330M of 3/32 of an inch 
thickness. One each of the monolithic and laminated windows were impacted 
in air with a bullet (.22 caliber long rifle Super X), fired from a 
distance 6 feet from the window. The other two windows were left 
untouched for comparison. The laminated window developed a star shaped 
crack that penetrated only the outer 7/32-inch thick layer, (Figure A-2), 
while the monolithic window was penetrated by a family of cracks 22/32 
of an inch deep (Figure A-3). 
All four windows were subjected to hydrostatic pressure in a typical 
flat window flange with a clear opening of 4 inches, and a 0.005-inch 
radial clearance between the edge of the window and the flange. The — 
laminated windows were tested with the thin outer acrylic plastic layer 
serving as the high pressure face, while the fractured monolithic window 
was placed to have the cracked surface serve as the high pressure face. 
In this manner, both cracked windows were tested with the cracked surface 
acting as the high pressure face. Testing of all windows was conducted 
at 650 psi/min pressurization rate in 68-69°F temperature range. 
The windows failed at the following pressures: 
Laminated window, no impact crack = 5500 psi 
Laminated window, with impact crack = 5100 psi 
Monolithic window, no impact crack = 6560 psi 
Monolithic window, with impact crack = 6400 psi 
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