50 



The shear stresses in a structure can be fairly accurately deter- 

 mined by visual observation of the birefringent coating and utilizing a 

 strain- stress color chart'^ ''. A method commonly used to more accurately, 

 determine whole field shear stresses is the Tardy Method^ -i-(goniometric 

 compensation), which utilizes fringe shift from analyzer rotation. Beginning 

 with a dark field circular polariscope, which has crossed polarizers and 

 crossed quarter wave plates or parallel polarizers and parallel quarter 

 wave plates, rotating the analyzer 90° causes the fringes to shift to a light 

 field, and an additional 90® rotation in the same direction causes the fringes 

 to shift further to a dark field again. Knowing the fringe constant, f, the stress 

 at some point between adjacent fringes can then be determined within an 

 accuracy of f/180. 



Fringe multiplication* ' is another method useful in deter- 



naining whole field shear stresses. This technique has been successfully 

 used in experimental work with birefringent coatings^ but the limitations are 

 in- plane loading and studies of flat surface So The surface area being 

 studied has to have a specular finish and be optically flat, but this can be 

 easily achieved by various electropolish or electroplating procedures. 

 With a system of mirrors, the number of fringes can be increased as many 

 as five timeSj, dependent upon the quality of the specinnen surface. 



The Babinet Compensator is a means of adding a known birefringence, 

 since the compensator is a calibrated quartz wedge which is optically flat 

 on each side. The compensator constant is the nunaber of divisions between 



