When a sodium vapor monochromatic light source was utilized, the 

 successive dark fringes were observed in grade G Plexiglas material to be 

 approximately separated by 1 ,600-psi principal stress or 800-psi maximum 

 shear intervals. When placed into the fringe constant equation 



{a-, - a 2 )h 

 f = — ■ B-1 



where f = fringe constant (lb/in. /fringe order) 



o, = maximum principal stress (psi) 



a 2 = minimum principal stress (psi) 



h = thickness of test specimen (in.) 



n = order of interference 



the fringe constant was determined to be f = 800 lb/in. /fringe order. Since 

 the modulus of elasticity of grade G Plexiglas is approximately 450,000 psi, 

 the fringe constant when expressed in terms of strain is about 1 ,770 juin./ 

 in. /order for a 1-inch specimen thickness. 



In a light-field circular polariscope utilizing monochromatic transmitted 

 light, the first dark fringe appears in 0.5-inch-thick acrylic plastic at the 800-psi 

 principal stress magnitude while the first light band appears at the 1 ,600-psi 

 principal stress magnitude (800-psi maximum shear). That is because in a 

 light-field circular polariscope, isochromatics (black fringes) represent half 

 orders of interference. If the polariscope was set to produce a dark field, the 

 dark field would be present at zero loading, while the first black isochromatic 

 would appear only after the 1 ,600-psi principal stress (800-psi maximum shear) 

 was reached. 



When white light was used in the light-field polariscope utilizing circu- 

 larly polarized transmission, colored isochromatics replaced the black fringes. 

 Since it is rather difficult to convey in words the subtle changes in color that 

 accompany the loading of acrylic plastic, color photographs were taken at 

 many loading intervals (Figure B-1). With the aid of the colors, it is possible 

 to determine rather readily without any elaborate experimental setup the 

 magnitude of maximum shear stress in the completed acrylic plastic capsules. 

 Using sunlight as the source of white light only two sheets of Polaroid with 

 bonded quarterwave plates are needed to perform a photoelastic stress anal- 

 ysis of an acrylic plastic structure. 



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