Appendix B 

 PHOTOELASTIC INVESTIGATION OF CAPSULES 



Since acrylic plastic is a photoelastically active material, the acrylic 

 plastic capsule lends itself to photoelastic techniques for investigation of stress 

 distribution in the acrylic plastic parts of the hull. Photoelastic investigations 

 may be used during fabrication for measurement of residual stresses both in 

 the formed spherical pentagons, as well as in the bonded joints. I n addition 

 photoelastic investigations may be used for measurement of stresses during 

 actual application of external loads to the capsule. Because the photoelastic 

 effect functions at the molecular level, stresses can be determined quite accu- 

 rately in the acrylic plastic capsule around any stress concentration, a feat 

 that is difficult to accomplish with other stress measurement techniques. 



Before the photoelastic technique, however, was applied to the 

 acrylic plastic capsule, the photoelastic sensitivity of acrylic plastic had to 

 be accurately determined. Since a literature search failed to uncover refer- 

 ences to the photoelastic constants of grade G Plexiglas, the material used 

 in the fabrication of the hull, the constants were experimentally determined 

 as a part of the acrylic plastic capsule research program. 



The photoelastic properties of the grade G Plexiglas acrylic plastic 

 material were determined under both polarized white light and monochro- 

 matic illumination, although monochromatic illumination is generally the 

 only one utilized for detailed experimental stress analysis. The reason for 

 determining the photoelastic properties of acrylic plastic also under white 

 light was that it would be hard to provide uniform monochromatic illumin- 

 ation for the whole 66-inch hull needed during the general overview of the 

 stress distribution in the capsule. 



The photoelastic properties of grade G Plexiglas were experimentally 

 determined by uniaxial compression testing of a 2x 4x 1/2-inch acrylic plastic 

 test specimen. The uniaxial load was applied parallel to the 4-inch dimension, 

 while the light was transmitted across the 0.5-inch thickness of the material. 

 The Polaroid polarizer and analyzer were equipped with quarterwave plates 

 and placed parallel to the faces of the acrylic plastic specimen through which 

 the light was transmitted. The light, after passing through the polarizer, was 

 circularly polarized and, after passing through the stressed acrylic plastic, was 

 analyzed by the analyzer. (The technically correct description of this photo- 

 elastic investigation technique is "determination of materials' fringe constants 

 in a light-field polariscope utilizing circularly polarized transmitted light.") 



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