At the 310°F temperature of acrylic plastic and 160°F temperature 

 of the dies, the residual maximum shear stress at the edge of the dished disc 

 was measured photoelastically (see Appendix B for discussion of photoelastic 

 effects in acrylic plastic) to be approximately 3,700 psi. The magnitude of 

 residual stresses decreased somewhat towards the center of the formed disc. 

 Thus, for example, the maximum shear stress 1.5 inches away from the edge 

 of the disc was only 1 ,500 psi. 



An attempt was made to eliminate the residual stresses by subsequent 

 annealing of the formed spherical sector. The formed sectors were placed on 

 a flat surface and inserted in a 170 to 180°F oven for 24 hours. After the 

 sector had cooled to 95°F at a rate of approximately 7°F/hour, it was 

 checked dimensionally and photoelastically. The dimensional check dis- 

 closed that the radius of curvature increased by approximately 0.020 inch 

 while the magnitude of residual stresses decreased only very slightly. The 

 basic change in residual stresses consisted of lowering the peak stresses by 

 more evenly redistributing the residual stresses over the whole spherical 

 sector surface. Because of the dimensional change generated in spherical 

 sectors during annealing, it was decided in the future to anneal the spherical 

 sectors only after they had been machined into pentagons and bonded into 

 a sphere, as then they would be constrained by the spherical shape of the 

 hull from changing their spherical radii excessively. An alternative approach 

 was considered in which the spherical segments would be placed into a mold 

 for annealing, so that the mold would give the sector even support and thus 

 prevent changes in curvature due to sagging. Although this approach was 

 considered feasible, it was not utilized as it was felt that the mold and the 

 oven would be involved too long while individually annealing the 300 

 spherical sectors needed for fabrication of 25 acrylic plastic capsule models. 

 However, individual annealing of spherical sectors was recommended for 

 fabrication of the single 66-inch prototype capsule, as the individual annealing 

 of only 12 sectors would not impose an undue time requirement on the avail- 

 able oven and mold. 



The change in thickness of the acrylic plastic plate after thermoforming, 

 but prior to annealing was measured. The thickness of the acrylic plastic disc, 

 for example, changed from the previously uniform 0.494 inch to 0.485 inch in 

 the center, to 0.480 inch along the edge (1/2 inch from the edge), and to about 

 0.470 to 0.475 midway between the center and the edge (Figure 25). These 

 small changes in thickness were considered to be reasonable as the absolute 

 change in thickness was actually less than 5%, and uniformity of thickness 

 change within 3%. 



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