1-1/2-in. steel 

 tie rods - 



stacked rings 



Figure 25. Test arrangement for 

 hydrostatically testing 

 stacked-ring cylinder to 

 failure. 



forecast by rosette 4 located 

 approximately 1 inch above frac- 

 ture plane. This rosette had shown 

 that the maximum principal stress 

 was oriented along the hemisphere's 

 meridian and that it was approxi- 

 mately 50% higher than the hoop 

 membrane stresses measured at 

 other locations on the end closure. 

 Since rosette 4 was 1 inch away 

 from the fracture plane, it did not 

 show the actual stress at the fracture 

 plane that caused the failure. Some 

 exploratory investigations of this 

 tensile stress concentration con- 

 ducted subsequently have shown 

 that its magnitude in the meridi- 

 onal plane is approximately 3.3 

 (Appendix C). 



6. The failure of the rods was not forecast by the strain gages as they were not 

 located in areas of the highest stress on the rods. The tie rods failed in tension 

 at the very base of their heads where the abrupt change in cross section acted 

 as a stress raiser of unknown magnitude. The failure that took place at approx- 

 imately 1/3 of calculated failure pressure in the vessel indicated that there must 

 exist in the tie rod at the base of the rod head a stress approximately 3 times 

 higher than the average tensile stress in the middle of the rod's length. Some 

 exploratory investigations of this stress concentration conducted subsequently 

 have to a large measure confirmed this (Appendix C). 



7. The fracture of the stacked rings at 1 ,200 psi showed that the rings were 

 free of stress raisers as they were the only structural components of the vessel 

 to fail at design failure pressure based on the approximately 9,000-psi tensile 

 strength of acrylic. Thus, it appears that the ring is the only structural compo- 

 nent of the stacked-ring pressure vessel whose failure can be truly determined 

 on the basis of engineering calculations that do not take stress raisers into 

 consideration. For the other structural components, combinations of stresses 

 and stress raisers must be taken into consideration at otherwise the actual 

 strength of the structural members will be considerably below the calculated 

 one. 



31 



