49 



The isostatics, the principal stress trajectories, can be readily- 

 obtained from the isoclinics, the locus of points along which the principal 

 stresses have parallel directions. The whole field isostatics are deter- 

 mined by rotating the analyzer with respect to the polarizer in a plane 

 polariscope, usually using white light. 



A common procedure for determining the individual principal stresses 

 is by first determining the isopachics, the sum of the principal stresses. 

 Knowing the isopachic and the isochromatic, the principal stress dif- 

 ferences at a point, the individual principal stresses can then be readily 

 calculated. 



Another method for determining the individual principal stresses is 

 by integrating Filon's transformations of the Lame -Maxwell equations, as 

 illustrated in Reference 13, page 287. The principal stress at a boundary 

 equals the isochromaticj since the other principal stress, perpendicular 

 to and at the boundarys is zero in a plane stress problem. From a con- 

 sideration of the boundary stresses, the isoclinics, and the isochromatics, 

 an approximate solution may then by obtained by graphical integration. 

 Other means sometimes used for determining the individual principal 

 stresses is the shear difference method and the slope equilibrium method, 

 as illustrated in Reference 13, pages 215 and 270, respectively. 



The method most commonly used for determining the individual prin- 

 cipal stresses is by the consideration of oblique incidence'^ ^» ^°'. Mathe- 

 matical relationships exist for finding the principal stresses from normal 

 and oblique incidence measurements of relative retardation. 



