derived on purely theoretical grounds are those inherent in the fabrication 

 process itself since: 



1. Initial stresses, such as those due to rolling and welding of flat 

 plating into the cylindrical form, can be of such magnitude so as to cause 

 premature yielding and thus appreciably influence static collapse strength. 



2. Even the residual stresses which are introduced during the metal- 

 fabrication process of rolling billets into flat plating and eventual heat 

 treatment may also be of considerable magnitude. 



3. Initial imperfect circularity (out-of-roundness) and axisymmetric 

 longitudinal imperfections (deviations from straightness of the cylinder 

 generators) introduce localized stresses which can lead to premature 

 yielding and thus influence static collapse strength. 



4. Even so-called isotropic and homogeneous materials (such as 

 steel, and aluminum and titanium alloys) may not be so, to varying degrees 

 depending on the plating thickness. 



Finally, the elasto-plastic restraints afforded the shell plating by the 

 stiffening ring frames are of a highly complicated nature due to varying 

 degrees of localized yielding prior to collapse so that the actual boundary 

 conditions may be different for each case, and would certainly deviate 

 considerably from the ideal conditions (i.e., simple supports, clamped. 



* These considerations can have a great influence on the shape of the 



stress-strain curve for the material as it exists in the fabricated struct- 

 ure. They can lead to an effect commonly referred to as the 

 Bauschinger Effect. 



16 



