hydrostatic loading than the acrylic plastic hull, at equal loading, or at lesser 

 loading. Designing it to fail at lower pressure than the hull would lower the 

 overall safety of the capsule while designing it to fail at pressures above that 

 of the acrylic plastic hull did not raise its overall safety, so a working stress 

 level was chosen for metallic hull components that would cause them to fail 

 at the same depth as the acrylic plastic capsule under short-term hydrostatic 

 loading. 



Specification of Materials. Although the decision had been made 

 early in the research program to utilize acrylic plastic as the transparent hull 

 material and metals for hatches and bottom penetration plate, a set of spec- 

 ifications had to be drawn up for their procurement. 



Several grades of acrylic plastic are commercially available in the 

 form of sheets and plates. The acrylic plastic is available in heat-resistant, 

 unshrunk, utility grade (MIL-P-21 105C); heat-resistant, preshrunk, optical 

 grade (MIL-P-5425); modified cross-linked, preshrunk, optical grade 

 (MIL-P-8184); and modified cross-linked, crack-propagation-resistant, 

 prestretched optical grade (MIL-P-25690). 



Since for the safe performance of the acrylic plastic capsule the 

 most important material properties are compressive strength, shear strength, 

 tensile strength, impact resistance and time-dependent viscoelastic or visco- 

 plastic deformation, all of the existing acrylic plastic grades were carefully 

 examined to determine which one of them best satisfied the material 

 property requirements. A review of the material properties applicable to 

 the NEMO capsule showed, however, that there was very little significant 

 difference between the different grades of acrylic plastic materials with the 

 possible exception of the stretched acrylic plastic that possesses significantly 

 higher impact strength than all the other grades. A survey of acrylic plastic 

 sheet producers revealed, however, that only MIL-P-21005C is available as 

 a standard stock item in thicknesses to 4 inches; Ml L-P-5425 could be 

 obtained as a special order stock item at premium price from the suppliers, 

 while the modified and prestretched acrylic plastic was simply not produced 

 in the thicknesses above 1 inch. Further examination of standard commercial 

 Ml L-P-21 105C plates of up to 4-inch thicknesses revealed that their optical 

 properties are more than adequate for visual observation of hydrospace. 

 Since MIL-P-21 105C satisfied the operationally required optical criteria, 

 was readily available in thicknesses to 4 inches, was the least expensive of 

 all acrylic plastic material grades, and at the same time could satisfy the 

 structural requirements of the capsule, it was chosen as the capsule material. 

 The mechanical and physical properties of the heat-resistant, unshrunk, 

 utility grade acrylic plastic (MIL-P-21 105C) and the ASTM tests used in 

 determining them are shown in Table 1 . 



27 



