models made of high- strength steel have collapsed at pressures much 

 higher than those predicted by calculations based on the most optimistic 

 expectations. These observations have been explained on the basis that 

 the exceptionally high buckling strength possessed by these sandwich 

 cylinders permits straining of the material well into the inelastic and 

 work-hardening ranges with its beneficial effects of higher strength 

 levels. Although this phenomenon may exist, it should be viewed as more 

 of a scientific curiosity than one which could be incorporated in a design. 

 Furthermore, this strain-hardening influence may not even exist for 

 sandwich structures fabricated of the higher strength but lower modulus 

 materials like titanium, for example. 



In addition to experimental programs on sandwich cylinders, ana- 

 lytical studies have been going on concurrently in order to develop 

 rational formulas based on thin- shell theory for predicting the static 

 structural response of these type structures. In Reference 74 Pulos 

 presents an analysis of the axisymmetric elastic deformations and stresses 



in a web-stiffened sandwich cylinder under hydrostatic pressure. Raetz 



75 

 presents a similar analysis for the tube- stiffened sandwich cylinder. 



An examination of available elastic- stability analyses ' for three- 

 layered sandwich cylinders under hydrostatic pressure revealed the 

 absence of "satisfactory criteria pertinent to the thick-walled sandwich 

 hull problem in which the core possesses compression- and shear-resist- 

 ant characteristics. For this reason, a new analysis has been developed 



128 



