No effort was made to utilize premium grade acrylic plastic materials, 

 or acrylic plastic materials that have been chemically or physically modified 

 (stretched acrylic for example). It was felt that in the exploratory phase of 

 the program having as its objective the utilization of acrylic plastic as struc- 

 tural material for the hull of a submersible the most common available 

 material should be utilized. Only after the exploratory phase of the acrylic 

 plastic pressure hull program was successfully completed could other acrylic 

 plastic materials be considered that, because of their special material proper- 

 ties, would improve either the fabrication process or the performance of 

 acrylic plastic hulls. 



The minimum properties of the commercial grade acrylic plastic plate 

 stock used in the program are shown in Table 1. Since deviations from the 

 material properties published by the material supplier could be expected, 

 extensive tests were conducted on samples cut from plates that were subse- 

 quently used in the fabrication of model and large-scale capsules. No 

 significant deviations were found between the material properties advertised 

 by the manufacturer and the properties determined experimentally from test 

 specimens, except for long-term compression creep (ASTM D621-51 at 

 4,000-psi load), which was found to be 2% in some of the 2.5-inch-thick 

 plates instead of the advertised 0.5%. Plates with exceptionally high (larger 

 than 2%) long-term compression creep were not utilized for the construction 

 of the acrylic plastic capsules. No significant differences were found between 

 the material properties of nominal 0.5-inch-thick acrylic plastic plates used 

 in the fabrication of models and the 2.5-inch plate used in the fabrication 

 of the 66-inch NEMO prototype. The data generated (Figures 14a 

 through 14d) in the previously discussed testing of 0.5-inch and 2.5-inch 

 acrylic plastic plates were used subsequently in the design of acrylic plastic 

 hulls. 



Forming. One of the major problems that had to be resolved 

 successfully before spherical acrylic plastic capsules could be built from the 

 pentagon-shaped structural modules of acrylic plastic plate was how to 

 impart to the commercially available flat plates the desired curvature within 

 tight dimensional tolerances. There were several different forming processes 

 known that could be applied. 



The flat acrylic plastic plates could be ( 1 ) free-blown to desired radius, 

 (2) blown into a spherical mold, (3) sagged into a spherically concave mold, 

 (4) sagged over a spherically convex mold, (5) vacuum sucked into a concave 

 mold, or (6) pressed into a concave mold by a convex male mold. Each of 

 those forming processes possessed advantages and disadvantages that had to 

 be carefully evaluated to arrive at the forming process best suited for the 

 forming of structural hull modules. 



48 



