The decision to select Piccard's concept had a sound engineering basis. 

 First, considerably more is known about the engineering properties of acrylic 

 plastic than those of massive glass. Much of this is data generated during the 

 design of aircraft windows and canopies. 4, 5 Second, acrylic plastic has already 

 been successfully used in the construction of model-scale and full-scale pressure- 

 resistant capsules with predictable collapse pressures. 6 " 10 Third, Commercial 

 catalogs list acrylic plastic plate in sizes up to 4 x 6 feet and 4 inches thick which 

 could be used for fabricating the spherical pentagon modules. F.ourth, a wealth 

 of experience and equipment exists for machining, thermoforming, and bonding 

 acrylic plastic. 



However, even with all this information, there was not sufficient 

 hardware-oriented data to design with confidence a man-rated acrylic plastic 

 capsule. There was, for example, insufficient information on the compressive 

 strength of acrylic plastic in biaxial and triaxial stress fields generated by long- 

 term or cyclic load application. The data from the testing of acrylic plastic 

 windows for aircraft could serve here only as a general guide since the data 

 applied primarily to tensile stresses, rather than compressive stresses. However, 

 because the strength of acrylic plastic is somewhat less in tension than in com- 

 pression, much of the tensile stress data could serve as the first-order conservative 

 approximation for prediction of magnitudes in compressive stresses and strains. 



The available research findings from testing acrylic plastic capsules under 

 external hydrostatic pressure described in detail the response of different acrylic 

 plastic capsule designs to short-term loading. 6 " 9 These findings do not cover 

 long-term or cyclic loadings, which may cause the capsule to buckle through 

 creep deformation or to fail by material fatigue, but they do indicate not only 

 that Piccard's spherical pentagon concept is feasible, 10 but also that pressure 

 capsules based on any sound engineering design can be built successfully from 

 acrylic plastic. 



The experience in fabrication of acrylic plastic components accumulated 

 by the industry pertained primarily to acrylic plastic stock less than 1 inch in 

 thickness. Most of the techniques developed for thin acrylic plastic stock, except 

 possibly for thermoforming and bonding, were applicable also to the thick stock 

 that would be utilized in the full-scale capsule. Thermoforming posed special 

 problems because the transfer of heat through thick acrylic plastic sections is 

 very slow, and thus special procedures are required for heating and cooling. 

 Bonding of thick curved sections also posed special problems because contact- 

 cementing large pressure hulls is not feasible and bonding of pressure hulls 

 with a self-polymerizing cement had not been tried. 



Thus, although some research data and experience in the application 

 of acrylic plastic to the construction of pressure resistant structural compo- 

 nents or whole structures existed, additional data and fabrication experience 

 had to be generated before a man-size, acrylic plastic capsule could be designed 

 and built for use at continental shelf depths. 



