aggravating the local imperfection. Ultimately the curvature decreases locally 

 to such an extent that the flexure stress dominates the stress field. When the 

 tensile component of the flexure stress overcomes the compressive membrane 

 stress at the center of the flat spot on the interior surface of the capsule, failure 

 of the acrylic plastic occurs (Figure 89). That the local failure occurs in flexure 

 is substantiated by (1) the similarity of the failure surface on the interior of the 

 hull (Figure 90) to the fracture on interior of flat disc acrylic plastic windows 

 that fail in flexure (Figure 91) and (2) by the presence of the circular crack on 

 the exterior surface of the hull at the location where the flexure hinge encir- 

 cling the flat spot was located. 



5. Crazing and cracks were found on the acrylic plastic bearing surfaces 

 in contact with the steel end plates after conclusion of long-term sustained 

 pressure loadings at 500-, 750-, and 900-psi pressure levels (Figures 92 through 

 95). No cracks or crazing were found in capsules subjected to long-term sus- 

 tained loading at pressures less than 500 psi or higher than 900 psi. It is 

 hypothesized that the formation of cracks on the acrylic plastic bearing 

 surface in contact with the end plate depends on the magnitude of time- 

 dependent strain (elastic and plastic components of strain) rather than stress 

 in the bearing surface. The model capsule under 250-psi long-term sustained 

 hydrostatic loading did not exhibit cracks because the magnitude of the time- 

 dependent strain at that stress level is small even after 1 year of sustained 

 loading. The models under pressure in excess of 900 psi also exhibited no 

 cracks because although the time-dependent strain rate at these stress levels 

 is very high, the magnitude of time-dependent strain is low at the moment 

 of failure, as the failure of the model occurs within a relatively short time. 

 Only in the 500-to-900-psi range of hydrostatic loading does the magnitude 

 of time-dependent strain in the acrylic plastic bearing surface become suffi- 

 ciently large to cause cracks in the acrylic plastic surface at the steel end 

 plate, as the stress level is fairly high and the duration of sustained loading 

 without implosion is long. 



The cracks were not uniformly distributed across the thickness of the 

 hull at the bearing surface but were primarily concentrated on the inner half 

 of the wall. The deepest cracks were at the location on the bearing surface 

 that corresponded to the bottom edge of the O-ring groove in the steel end 

 plate. The cracks were circumferential with respect to the steel end plate and 

 propagated themselves at right angles to the bearing surface. The depth of 

 these cracks was greater in the model that was subjected to 750 psi for 

 4,500 hours (Figures 92 and 93) than in the model at 900 psi loaded for 

 72 hours. This could be explained by the fact that at the completion of 

 the test the magnitude of time-dependent strain in the model loaded at 

 900 psi, although the stress level was higher on the latter. 



124 



