ABSTRACT 



A series of structural model tests was conducted to evaluate a pre- 

 liminary sandwich design of the pressure hull for a proposed 15, 000-ft 

 operating-depth oceanographic vehicle. The model studies were limited to 

 an evaluation of the static strength of a typical section of the web- stiffened 

 sandwich configuration. It is intended that the prototype be fabricated of 

 high- strength, a-titanium alloy having a nominal yield strength of approx- 

 imately 120, 000 psi. 



Model tests in the series indicated that the original design would 

 not prove adequate to meet the minimum specified collapse pressure of 

 10, 000 psi; this is attributed primarily to a premature inelastic general- 

 instability mode of failure. A redesign has been suggested with the ob- 

 jective of increasing the overall stability of the hull without increasing 

 the weight. 



A small-scale model, machined from a nascent bar of titanium alloy, 

 was also tested to determine whether the suggested redesign represents a 

 significant improvement in static strength over the original design. Upon 

 extrapolation of the observed collapse pressure for the redesigned model 

 to a yield strength of 120, 000 psi, a pressure of 1 1 , 290 psi is obtained. 

 Here again, failure occurred by general instability at a reduced modulus 

 but at a nnuch higher pressure than for the original design. The results 

 indicate that the static strength depends on the stress-strain characteris- 

 tics of the material in the hull structure. Built-in stresses, such as those 

 that might be induced by rolling and welding of flat plates into the cylindri- 

 cal form, could significantly alter the shape of the stress -strain curve so 

 that the collapse strength of a fabricated structure may be considerably 

 lower than that realized from tests with perfectly circular and initially 

 stress -free machined models. 



Before any final conclusions can be drawn concerning the adequacy 

 of the suggested redesign, it will be necessary to test larger scale titani- 

 um models which are fabricated to be identical in all respects, except 

 size, to those anticipated in the prototype. In this way all factors which 

 appear to influence static and fatigue strength will be adequately consid- 

 ered. 



