Stress corrosion cracking of candidate waste container materials; Final report Page: 74 of 86
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66
correlation. The observed CGRs for Types 304L and 316L SS and Incoloy 825 are generally
lower than the predicted rates. Because the code correlation represents a 95% confidence
limit upper bound for the observed data base, it is expected to be conservative for most
heats of material in the absence of environmental effects.
Crack growth rate under cyclic loads in a corrosive environment (da/dt) may be
expressed as a sum of contributions by (1) stress corrosion cracking, (da/dt)scc;
(2) corrosion fatigue, (da/dt)cF, representing the additional crack growth under cyclic
loading due to the environment; and (3) mechanical fatigue, (da/dt)air, representing the
fatigue growth In air:
(da/dt) = (da/dt)scc + (da/dt)cF + (da/dt)air . (20)
The first two terms on the right side of the equation are environment-sensitive.
They depend on loading history variables, such as rise-, unload- and hold-time, as well as
on frequency. In oxygenated-water environments, the environment-sensitive terms can
contribute significantly to crack growth rates of austenitic stainless steels.17-19 Under
low-R and high-frequency loading, mechanical fatigue dominates. Environmental
contributions would be expected to become more significant as the frequency decreases.
Crack-growth rates as a function of cyclic frequency and CGRs per cycle versus rise time
for the current tests are shown in Figs. 65-68. Figures 65 and 66 show that the time-
based growth rates are proportional to frequency over the entire range of the frequencies
used in the tests. This indicates that no environmental acceleration of crack growth is
present for the test conditions considered. Figures 67 and 68 show that the growth rate
per cycle is independent of rise time. This too indicates that no environmentally assisted
crack growth occurred, and that the crack-growth mechanism is pure mechanical fatigue.
For comparison, the environmentally accelerated behavior observed in high-temperature
oxygenated environments18'19 is shown schematically by the curved lines in Figs. 65 and
66.
5 Summary and Conclusions
A series of slow-strain-rate tensile tests on six candidate nuclear waste container
materials was conducted under both crevice and noncrevice conditions in simulated well
J-13 water at 93*C at strain rates of l0U- and 10-H s-i. The tests were performed under
well-characterized environmental conditions. Similar tests were also performed on
weldment specimens of Types 304L and 316L SS, Incoloy 825, Cu, Cu-7%Al, and Cu-
30%Ni at a strain rate of 10-7 s-1. The specimens contained small-diameter
through-holes (with or without pins of a matching material) to facilitate observation of
cracks by scanning electron microscopy (SEM). The SEM observations showed cracking in
virtually all of the materials under the severe testing conditions employed. A stress ratio
was formulated on the basis of the ratio of the increase in stress following the initiation of
local yielding for the material in water and the corresponding stress difference for an
identical test in air at the same elongation. A ratio of plastic strain in air to the plastic
strain in the environment (strain ratio), both evaluated at the same stress, was also
formulated to describe the cracking susceptibility. Higher values of stress or strain ratio
imply greater cracking or SUC susceptibility. Tile stress or strain ratio appears to be useful
in screening the materials for SCC, even though the crack depths are small (<100 m). On
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Park, J. Y.; Maiya, P. S.; Soppet, W. K.; Diercks, D. R.; Shack, W. J. & Kassner, T. F. Stress corrosion cracking of candidate waste container materials; Final report, report, June 1, 1992; Illinois. (https://digital.library.unt.edu/ark:/67531/metadc625520/m1/74/: accessed July 16, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.