The Naval Research Laboratory (NRL) defines the three (3) critical
points on crack arrest curves as follows:
(a) Nil-Ductility Transition (NDT) Temperature. Below this
temperature the steel does not deform (around a defect of sufficient size)
prior to fracturing, and the fracture occurs immediately upon reaching the
yield point. The fracture propagation is unstable and it propagates easily
through material stressed in the elastic region. (See Reference 3, Pellini,
Fracture Safe Design of Steel Structures, 1969.)
(b) Fracture Transition for Elastic Loading (FTE). Below
this temperature and above NDT Temperature, the defect size necessary to
initiate fracture will increase greatly but once fracture initiates it will
propagate through material stressed in the elastic zone. At temperatures
above FTE, fractures will propagate only through material stressed in the
plastic zone. At FTE large amounts of plastic deformation are required to
initiate fracture. As the temperature is lowered toward the NDT Temperature,
the amount of plastic deformation and hence the energy required to initiate
the fracture decreases.
(c) Fracture Transition for Plastic Loading (FTP). Above
this temperature, brittle or cleavage fractures do not take place in an
otherwise ductile material. That is, brittle fracture cannot take place even
though the material is severely plastically deformed.
For more detailed information on notch toughness and design, see the
applicable reports available from NRL, Code 6300, Washington, D.C. (See
References 4 and 5, Pellini, Fracture Analysis.)
6.
CORROSION CONSIDERATIONS. Some shore-based byperbaric facilities will
have pressure vessels containing seawater and/or gasses which are either pure
oxygen or which have a high percentage of oxygen as one of their
constituents. The materials from which these pressure vessels are fabricated
could be subject to "wet" corrosion and/or to "dry" oxidation. Corrosion
effects are considered during the initial selection of a material, and when
determining the dimensional changes and/or defect-producing phenomenon
applicable to the environment and the life expectancy of the vessel. As
specific corrosion problems arise, they should be referred to an expert in
corrosion engineering.
Materials for containment of gasses containing over 40% oxygen at
pressures above 270 psig or which could contribute fuel in case of a fire,
should be stainless steel type 310 or 316, monel, bronze, or copper. Bronze
and copper should be used in dry gas systems only. Unless justified by
design use parameters, hyperbaric chambers of ferritic materials which are
exposed to seawater (saltwater or other electrolyte) or sea spray environment
shall be provided with a 1/16-inch corrosion allowance.
If protective materials are used, they may be organic in nature, such as
paint, or may be metallic coatings which in themselves are not subject to
corrosion, such as nickel, gold, chromium, etc. The main problem encountered
in the use of coatings is the near impossibility of maintaining the integrity
of the coating. Small faults in the film, such as pin holes, allow corrosion
to occur in the base metal under the film in the vicinity of the hole,
leading
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