15 



concentration factor of Z applied to the theoretical membrane stress, 

 considering the shell thickness at the hole boundary. The experimental 

 stresses given in Tables 11 and IZ vary between Z0% greater and 66% less 

 than the theoretical value. 



If a more accurate determination of stress at the boundaries of the 

 discontinuitiesis required the birefringent coating technique should be 

 used since the stresses could be obtained exactly at the hole boundary. 

 Also, the individual principal stresses could be obtained from measurements 

 of relative retardation with normal and oblique incidence without any inte- 

 gration effect since the specimen was synnmetrical. A description of the 

 birefringent coating technique is given in the Appendix for the interested 

 reader. 

 D. Accuracy of Results 



The accuracy of any reported results are as important as the 

 results themselves. The maximum deviation of the results reported in 

 these tests from the true stresses is estimated to be ±4% maximum, 

 neglecting the integration effect over the gage length and the small dis- 

 tance between the gage centerline and the boundaries of the discontinuities. 



