failed by stress corrosion cracking when stressed at values equivalent 

 to 75 percent (94,500 psi) of its yield strength after 35, 77 and 105 

 days of exposure at the surface in the Pacific Ocean. The stress corro- 

 sion cracks occurred at the edge of the weld bead as shown in Figure 5. 



Metallographic examinations of unetched and etched sections in a 

 plane parallel to the surface of the specimen showed that a secondary 

 crack which started at the edge of the specimen in the parent metal away 

 from and parallel to the main fracture was irregular and branching in 

 nature as well as transgranular as shown in Figure 6. This is typical of 

 the main fracture. 



The Naval Applied Science Laboratory, Reference 10, reported no 

 stress corrosion cracking of unwelded and butt welded 7Al-2Cb-lTa alloy 

 stressed at 100 percent of its yield strength after 199 days of exposure 

 at a depth of 4,250 feet in the Tongue-of-the-Ocean, Atlantic Ocean. 



The 6A1-4V alloy rings stressed as high as 60,000 psi (approxi- 

 mately 50 percent of its yield strength) (Figure 4) did not fail by 

 stress corrosion cracking during 402 days of exposure at a depth of 

 2,370 feet. 



Alloys 75A, Ti-0.15Pd, 5Al-2.5Sn, 7Al-2Cb-lTa, 6A1-4V and 13V- 

 HCr-3Al were exposed with an unrelieved 3-inch diameter circular weld 

 bead in the center of 6" x 12" specimens as shown in Figure 3. Only the 

 13V-llCr-3Al alloy failed by stress corrosion cracking because of the 

 residual welding stresses. Cracking occurred during surface exposure of 

 181 days, during 403 days of exposure at a depth of 6,780 feet, during 

 751 days of exposure at a depth of 5,640 feet, and during 402 days of 

 exposure at a depth of 2,370 feet. The cracks in all cases were radial 

 across the weld beads, a typical crack is shown in Figure 7. The crack 

 in this case changed direction by 90 degrees (left side of Figure) just 

 outside the weld bead because of the redistribution of the residual 

 stresses . 



Metallographic examination of a polished section of the crack taken 

 in the plane of the sheet where it changed direction showed that the 

 path of the crack was irregular and branching in nature as shown in 

 Figure 8. After etching and reexamination it was found that the crack 

 was predominantly transgranular as shown in Figure 9. 



Mechanical Properties 



The effect of exposure in sea water at nominal depths of 2,500 and 

 6,000 feet on the mechanical properties of the alloys is given in Table 

 5. These data show that the mechanical properties of the alloys were 

 not adversely affected by exposure in sea water at nominal depths of 

 2,500 and 6,000 feet for periods of time as long as 751 days. 



