towards shallower, continental shelf-capable 

 vehicles was brought about by the newly 

 emerging offshore oil customer. As this cus- 

 tomer goes ever-deeper in his quest for pe- 

 troleum, the problem of defining a depth 

 limit to an oil industry-oriented vehicle be- 

 comes increasingly difficult. In essence, the 

 selection of an optimum depth is difficult, 

 and to err on the side of excess may spell the 

 difference between profit or loss for the com- 

 mercial lessor. 



MATERIALS 



Pressure hull materials are metallic and 

 non-metallic; regardless, all physical proper- 

 ties must be characterized and taken into 

 account during the process of material selec- 

 tion in order to provide a design which will 

 perform successfully in the ocean environ- 

 ment. Such materials and their welds or 

 bonding materials must be characterized to 

 account for the following during the material 

 selection phase: 

 Corrosion: The deterioration of a metal by 

 chemical or electrochemical action within 

 its evironment, 



Stress-Corrosion Cracking: Failure by flow 

 propagation under combined action of a 

 flaw and tensile stress, 



Low Cycle Fatigue: Fracture under fluc- 

 tuating stresses having a maximum value 

 less than the tensile strength of the mate- 

 rial. (Low cycle is less than 100,000 fluctua- 

 tions in pressure). 



Creep: Time dependent plastic deformation 

 (permanent change in size or shape of a 

 body) occurring under stress, 



Stress Relief Embrittlement: Reduction in 

 the normal ductility of a metal when it is 

 heated to a suitable temperature and then 

 slowly cooled to reduce residual stresses, 

 Brittle Fracture: Fracture with little or no 

 plastic deformation which occurs in some 

 metals at low temperatures. 

 High Strength to Density Ratio: (defined 

 previously). 



High Ductility: The ability to deform plas- 

 tically without fracturing. 

 Fracture Toughness: The ability to deform 

 plastically in the presence of a thru-hull 

 crack without catastrophic propagation 

 and failure. 



Weldability: Suitability of a metal for weld- 

 ing under proper conditions, 

 Fomiability: The relative ease with which 

 a metal can be shaped through plastic 

 deformation, and 



Reproducibility: The process of production 

 being such that the material can be se- 

 quentially produced to closely approximate 

 its predecessor in all properties. 



Equally important are the developmental 

 and fabrication cost and the availability of 

 the candidate material. 



Submersible pressure hull materials con- 

 sist of steel, aluminum, titanium, acrylic 

 plastic, glass and wood (Table 5.1). Steel, at 

 90 percent, constitutes the overwhelming 

 majority of pressure hulls — primarily be- 

 cause of the high degree of knowledge availa- 

 ble to the designers and fabricators and the 

 large amount of experience with respect to 

 its performance in the ocean. 



The technical literature devoted to pres- 

 sure hull material candidates and their char- 

 acteristics is voluminous. Almost all deal 

 with materials for deep (greater than 1,000 

 ft) diving, with too little attention paid to 

 shallow diving. This is unfortunate in view of 

 the present trend toward shallow, rather 

 than deep, submersibles. One might theorize 

 that sufficient is known of materials for shal- 

 low vehicles, but this is not always the 

 case. For example, a portion of DEEP DI- 

 VERTS pressure hull consists of a grade of T-1 

 steel which, when a flaw is present in a 

 tensile stress field, is subject to brittle frac- 

 ture at low temperatures. While this steel 

 might be acceptable in certain ocean areas, 

 or by employing design and fabrication tech- 

 niques which would preclude tensile stresses 

 and minimize the flaw size, it was not accept- 

 able to the U.S. Navy (2). They declined 

 material certification because of the lack of 

 material characterization (fracture mechan- 

 ics properties) and the potential existence of 

 flaws and residual tensile stresses in weld- 

 ments. 



The reason for the trend toward high 

 strength steels is their high yield stress, 

 acceptable fatigue and fracture properties 

 and fabricability. 



At the very least, submersible develop- 

 ment and operations in the decade of the 



247 



