From these findings it appears that almost all residual stress concentrations 

 can be eliminated from the joints if the bonding process is modified to permit 

 complete filling of all joints in the capsule with adhesive prior to initiation of 

 the polymerization process. In this manner polymerization of the adhesive 

 in the joints could proceed uniformly. This will not eliminate some of the 

 typical residual stresses in the joint; shrinking of the adhesive in the joint space, 

 whose width is held constant by mechanical spacers, will generate tensile 

 stresses in the filler material. These residual stresses, however, will be fairly 

 uniform, with only a small increase in magnitude at the tips of the pentagons. 

 Because of this uniformity the maximum typical stress will be less than if the 

 joint was cast in increments. 



Besides residual stresses, active stresses were also investigated with 

 the photoelastic technique. The tests were performed on a small-scale acrylic 

 plastic capsule subjected to hydrostatic loading inside an internal pressure 

 vessel. Since it was known from other tests that the highest stresses were 

 around the polar steel plates, only that location was investigated photoelasti- 

 cally. When the 15-inch-OD x 14-inch-ID acrylic plastic capsule was externally 

 pressurized to 500 psi, the maximum shear stress value (photoelastically inte- 

 grated across the wall thickness) near the edge (approximately 0.050 to 

 0.100 inch from the edge) of the polar penetration in the acrylic plastic was 

 found to be approximately 1 ,300 psi. This experimental value compares 

 favorably with the maximum shear stress value calculated from the extrapo- 

 lated strain data measured with electric resistance strain gages at 100 psi of 

 hydrostatic pressure near the edge of the polar penetration (Figures 82 and 

 83). Shear stresses could not be measured photoelastically at the very edge 

 of the penetration because of the shadow cast by the edge of the steel hatch 

 in the capsule. 



SUMMARY 



From the discussion of photoelastic investigation of residual and 

 active stresses in the acrylic plastic capsules, it appears that this stress investi- 

 gation technique is ideal for the evaluation of residual stresses introduced 

 into the capsule by the fabrication process. In fact, the measurement of 

 residual stresses inside the joints between pentagons probably would not 

 have been possible with any other stress investigation technique. 



The photoelastic technique is also applicable to measurement of 

 active stresses in the capsule under hydrostatic loading. However, because 

 special equipment (pressure vessel windows and lights and a means of remotely 

 orienting the capsule) is required for such testing, the use of electric resistance 

 strain gages is more attractive. 



182 



