To determine how much straining is necessary before the material 

 in the shells is in the strain-hardening range, a compression specimen 

 was taken from a ;^-in. -thick steel plate which was heat-treated to a yield 

 strength of approximately 100,000 psi. Figure 19 is the stress -strain 

 curve for this specimen tested under uniaxial connpressive loading. The 

 specimen was loaded at a slow rate; that is, 1 hr was needed to reach the 

 strain-hardening range. The observed stress-strain characteristics in- 

 dicated that strain hardening for a material similar to that used in Models 

 OV-1 and OV-2 would be reached at a strain of about 17, 000 |iin./in. If 

 the material was worked into this range, it would, in effect, correspond 

 to raising the yield strength. It is conceivable that the strains in Models 

 OV-1 and OV-2, at pressures approaching collapse, were on the order of 

 20, 000 to 30, 000 fiin./in. , thus suggesting the possibility of strain harden- 

 ing. Compression specimens taken from the shells of the models after 

 collapse, and as close to the regions of failure as possible, gave average 

 increases in yield strength of 1 5 and 19 percent over the values obtained 





1- 















1 

 1 





— 

































ISTRAIN-HARDE 



:nino 



1 



' RA 





1 













































































































2000 4000 6000 8000 10,000 12,000 14,000 16,000 18,000 20,000 22,000 



Figure 19 - Stress -Strain Curve of Specimen Taken fronn 

 i-Inch-Thick HY-80 Steel Plate 



26 



