e. Any material which will enter into a rapid chemical reaction with
seawater, gaseous oxygen, or any other media to be used inside the vessel.
f. Any material whose toughness at the minimum operational temperature
is deemed insufficient (see below).
5.
MATERIAL TOUGHNESS. If a pressure vessel contained no defects or flaws,
then the ductility of the vessel's material would not be an important design
parameter so long as the stresses induced in that material were kept below
its ultimate strength in local regions and its yield strength in general
regions. The "no flaw" or "zero defect" condition is not practical because
even raw materials have flaws. Defects are also induced in a structure
during fabrication and by certain mechanisms during its operational life.
Such mechanisms include environmental attack (corrosion, erosion, stress
corrosion, etc.) and mechanical damage (unintentional gouging, secondary
loading, fatigue loading, etc.). It is therefore important to know how a
stressed material will respond to a defect, be it either geometrical or
metallurgical.
a. Notch Sensitivity. Under certain conditions, many metals and some
other materials may fail in a brittle manner even though the same materials
show good elongation or ductility in a slow-tension test. These materials
are said to be "notch sensitive" for those conditions under which they
experience brittle fracture or exhibit low energy shear. The three common
conditions which tend to promote a brittle fracture in a material normally
considered ductile are (1) high velocity stress application, as in impact,
(2) reduced operational temperature, and (3) stress concentrations, such as
exist at the root of a notch, where plastic flow cannot take place readily.
Conditions (1) and (3) can be readily evaluated by a standard Charpy "V"
Impact Test at room temperature on materials characterized by dynamic shear
or sufficient Charpy data to establish an RAD curve, Figure 2-1. For steels
and some other materials, "notch sensitivity" increases dramatically as the
temperature is reduced over certain temperature ranges. This can be
determined reliably by Nil-ductility transition (NDT) data and Charpy test
data as follows:
(1) The NDT test performed per ASTM-A-208 will demonstrate the
temperature below which the material will act brittle and notches will
propagate as cracks at low stresses (5 to 8 KSI in steel). Typical specimens
are shown in Figure 2-2.
(2) Crack arrest transition (CAT) curves (Figure 2-3) developed
from Charpy data over a range of temperatures allow the prediction of the
performance of material in the presence of a high activity flaw at different
stress levels in as-welded structures. As welded, structures contain
residual welding stresses at the yield point of the weaker component which
may be the base material, heat-affected zone (HAZ) or weld material (Figure
2-4).
(3) Dropweight tear energy versus yield strength curves from
dynamic tear tests, Figure 2-5, for low alloy high strength steels and for
aluminum alloys, Figure 2-6, allows the prediction of critical flaw sizes for
fracture-safe performance. Note the point where the flaw size becomes too
small to be consistently demonstrated by practical NDT processes.
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