and a yield strength ranging from 104, 000 psi to 108, 000 psi. The outer 

 shell had an average thickness of 0.259 in. and a yield strength ranging 

 from 109, 000 psi to 113, 000 psi. Yield strength values of 104, 000 psi 

 and 109, 000 psi were estimated for the original flat plates and as close 

 as possible to the regions of failure observed in the inner and outer shells 

 of the model, respectively. 



MODEL OV-5 



From the tests of the titanium models, OV-3 and OV-4, it was con- 

 clusively determined that inelastic general instability is the critical mode 

 of failure for the original design. Both of these naodels had the same typi- 

 cal interbay configuration. The only difference was in the overall length; 

 OV-3 was one diameter long, whereas OV-4 was four diameters long. As 

 a consequence of its shorter length. Model OV-3 had an elastic general- 

 instability pressure twice that of the longer model. The observed collapse 

 pressures of these two models reflected this difference. When extrap- 

 olated to the specified yield strength of 120, 000 psi, the shorter model 

 (OV-3) failed at a pressure 10 percent above the minimum specified col- 

 lapse pressure and the longer model (OV-4) failed 3.5 percent below this 

 value . 



It appeared that a redesign of the basic sandwich configuration to 

 increase the general-instability strength was necessary to ensure that 

 the fabricated structure would meet the minimum design pressure. One 

 redesign possibility would be to increase the depth of the webs at the ex- 

 pense of making them thinner so as to retain the same overall weight of 

 hull structure. Another possible redesign would be to decrease the over- 

 all stress intensity through the sandwich cross section by adding additional 

 material to the two cylindrical shells. Either of these two alternatives, or 

 a combination of them, may be necessary to attain the stated objective. 



A small-scale, four -diameter -long, machined model (OV75) incor- 

 porating the first alternative was designed and constructed. The overall 

 depth of the sandwich cross section in this design corresponds to 12.9 in. 

 as compared with 10 in. in the original design (see Figure 2); the web 

 thickness was reduced to l| in. in order to retain, as closely as possible. 



13 



