Design 



The stacked-ring and the segmented-wall pressure vessel models 

 (Figures 12 and 13) were designed to represent in 1:10 scale the full scale 

 10-foot-diameter, 10,000-psi vessels (operating pressure) made from maraging 

 steel. Since little was known on the magnitude of stress concentrations in 

 such vessels, they were designed on the basis of ordinary engineering calcula- 

 tions. It was calculated that the failure of a given structural member was 

 initiated when the maximum tensile stress in the member became equal to the 

 ultimate tensile stress of the material under uniaxial tension, without taking 

 the stress raisers into consideration. Since the distribution of forces acting 

 on individual members of the vessel was not completely understood in many 

 cases, engineering assumptions were made in their place. 



The two vessels were designed to fail at 40,000 psi if they were 

 constructed from maraging steel with 300,000-psi ultimate tensile strength. 

 A design failure pressure of 40,000 psi would give the vessels an apparent safety 

 factor of 4 based on an operating pressure of 10,000 psi while the use of 

 300,000-psi steel would give the vessel the lightest structure made possible by 

 existing steel alloys applicable to construction of 10-foot-diameter pressure 

 vessels. 



Fabrication 



Although the actual dimensioning of vessels was based on the 300,000- 

 psi steel, the material selected for actual fabrication of models was not maraging 

 steel, but acrylic plastic. The reasons for using plastic material were twofold. 

 First, small forgings of 18% nickel maraging steel were not available at reasonable 

 cost; and second, fabrication of the models from a material that had half the 

 ductility of maraging steel would make the model much more sensitive to stress 

 concentrations, causing it to fail when the stresses in the material at the stress 

 raiser reached its ultimate strength. If the model was made from steel, it 

 would probably only yield locally at the stress raiser without any external indi- 

 cations of yielding. Yet, in full-scale vessels, local yielding would in many cases 

 cause the vessel to fail at lower cyclic pressure than predicted on the basis of 

 static failure pressure. 



Since acrylic plastic has a tensile strength of about 9,000 psi, while that 

 of 18% nickel maraging steel is about 300,000 psi, the failure pressure of the 

 model is scaled down to 1 ,200 psi in direct proportion to the lower tensile 

 strength. The operational pressure of the acrylic vessel would be 300 psi instead 

 of the 10,000-psi value for a steel vessel. 



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