in every part of the structure, no matter where this part may be located on 

 the structure, or how complex it may be. Because of the expensive model 

 and equipment and the time required for machining of slices, it was decided 

 instead to apply the two-dimensional photoelastic strain-investigation technique. 



The two-dimensional photoelastic strain-investigation technique 

 requires either photoelastic coatings on structural members under investigation, 

 or biaxially loaded transparent structural members with surface boundaries 

 at right angles to the polarized light source. In the first case, polarized light 

 is reflected from the backside of the photoelastic coating, while in the second, 

 light is transmitted through the structural member. In both cases, a camera 

 records the number and location of photoelastic fringes in the photoelastically 

 active material while it is stressed. The only severe limitation on the use of 

 two-dimensional photoelastic technique is that it only provides information 

 on the biaxial strains located in a plane perpendicular to the path of polarized 

 light. This technique is incapable of detecting strains parallel to the light path 

 and thus is somewhat limited in the evaluation of three-dimensional strains in 

 a pressure vessel. It was felt, however, that by placement of photoelastic coat- 

 ings on two-dimensional models of three-dimensional structural parts suspected 

 of having stress concentrations, enough information could be obtained to alert 

 the design engineer to the magnitude of stress concentrations that may be 

 encountered in the vessel structure. 



EXPERIMENTAL PROCEDURE 



The two-dimensional photoelastic strain investigations were all conducted 

 with reflected polarized light, but two kinds of test models were employed. The 

 models consisted either of an epoxy-coated metallic shape, representing the cross 

 section of the actual part, or of the actual structural part made out of epoxy 

 painted on one side with a reflecting paint. The decision on whether to use the 

 coatings on metallic models or actual structural parts made out of epoxy for 

 investigation of strains in a particular part of the vessel structure was based 

 primarily on the ease with which the particular structural part could be loaded 

 sufficiently to generate a high number of photoelastic fringes to make the photo- 

 elastic analysis more reliable. 



The structural parts of the vessel that lent themselves to the two- 

 dimensional modeling without much trouble were the end-closure tie rods and 

 flanges. For the strain investigation of the tie rods, special two-dimensional 

 metallic models were made which represented the longitudinal cross section 

 of the tie rod (Figure C-1 ). Since many different tie-rod heads can be used in 

 pressure vessel fabrication, several kinds of heads were investigated besides the 



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