46 THE NAVY OCEAN ENGINEERING PROGRAM 



MATERIAL PROPERTIES FOR DEEP-SUBMERGENCE 

 PRESSURE HULLS 



To perform its function, the iiull must withstand the pressure of the sea. 

 Demands on the hull material are thus unique, and combine high values of 

 stiffness, strength, Ughtness, toughness, and corrosion resistance. Modes 

 of the collapse failure of the pressure hull include elastic buckling, plastic 

 buckling, and fracture. Elastic buckling implies that the material recovers 

 its previous shape as the load is decreased. Plastic buckling implies non- 

 recoverable change in shape without increase in load. Fracture failure im- 

 phes catastrophic crack propagation. 



Toughness and Fracture 



To date, pressure hulls for deep submergence vehicles have been fabri- 

 cated from relatively low-strength metals such as HY 80 and HY 100 

 steels, because of their availability, fabricability, and toughness. These 

 metals satisfy the requirements of the fracture-safe philosophy; i.e., the 

 material can deform plastically in the presence of a through-the-thickness 

 crack without catastrophic crack propagation and failure. 



For high-strength steels, toughness properties improve with special proc- 

 essing such as vacuum melting. However, toughness decreases sharply with 

 increased strength. Above the 200,000 psi yield level, fracture will propa- 

 gate under essentially elastic stresses. It is possible to utilize fracture-safe 

 metals for buoyant huUs to depths of around 6000 to 9000 ft. Beyond 

 this, for a limited depth span, the high strength, relatively brittle metals 

 can be used, provided that new toughness standards which, for example, 

 relate flaw size to working stress, can be developed. It now appears that 

 nonmetaUic materials hold the greatest promise for vessels designed to go 

 to great depths. 



Since tensile stresses may occur because of local bending near stiffeners, 

 or because of elastic deformations in small cavities in the material, or 

 follow from local plastic upset in compression, it is necessary to guard 

 against brittle fracture in tension. 



Corrosion Fatigue 



Test results for HY 140 steel show that the crack-growth rate plotted 

 against total strain range falls off in salt water in the region of the propor- 

 tional Umitand approaches the rate in air. The Ti 7-2-1 alloy undergoes an 



