Appendix C 

 PHOTOELASTIC INVESTIGATION OF STRESS CONCENTRATIONS 



INTRODUCTION 



Since both the stacked-ring and the segmented-wall pressure vessel 

 models failed at lower internal hydrostatic pressures than could be predicted 

 by the nominal stress magnitude, it appeared desirable to investigate the mag- 

 nitude of stress concentrations at locations where failures were initiated. To 

 accomplish this, the magnitude of stresses and stress concentrations in these 

 vessels had to be determined before meaningful recommendations could be 

 formulated for redesigning the vessels. Two approaches were available: the 

 analytical and the experimental. Although these approaches complement 

 each other, the limited funding and time available for the determination of 

 stress concentrations in the stacked-ring and structural-module (segmented- 

 wall) vessels made two simultaneous investigations unfeasible. The experimental 

 approach was chosen because it was felt that with the limited time and funding 

 allowed for the hydrostatic pressure vessel study, experimentation would yield 

 more exploratory engineering design data than would analysis. 



BACKGROUND INFORMATION 



Although many different methods are available for the measurement 

 of strains in a structure with stress raisers, only one of them lends itself easily 

 to quantitative interpretation. This method is the photoelastic strain-measuring 

 technique.* Ideally, a three-dimensional photoelastic frozen-strain technique 

 supplies the most detailed and accurate strain information for every part of a 

 stressed structure. It is a cumbersome and expensive method requiring for its 

 success not only an epoxy model of the vessel but also an oven for heating the 

 vessel while it is internally pressurized. In addition, extremely fine slices must 

 be taken out of the epoxy model after the strains have been frozen in; these 

 slices are, after precision machining to a uniform thickness and polishing for 

 uniform light transmissivity, photoelastically investigated under transmitted 

 polarized light. The advantage of the frozen strain technique is, of course, its 

 ability to present visually the distribution magnitude and orientation of strains 



* For brevity, the materials, coatings, and techniques are all described as "photoelastic' 



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