Environmental Engineering Reference
In-Depth Information
Figure 3.37
SEM fractographs of (a) transgranular SCC of austenitic stainless steel
in hot chloride solution and (b) intergranular SCC of sensitized type 316 stainless steel
in polythionic acid.
Constant strain tests
make use of U-bend, C ring, bent beam specimens or
tensile specimens in a small loading frame with nuts on the threaded ends of the
specimen (Fig. 3.38). The specimens in the stressed condition are exposed to the
stress-corroding medium and checked periodically for the appearance of cracks.
Although the stress distribution within the specimen cannot be precisely known,
such tests are most simple for the qualitative evaluation of resistance of various
alloys to SCC to different solution conditions. Time to failure,
t
f
, is the parameter
used to measure the resistance to SCC.
Tests on statically loaded smooth samples
are conducted at various fixed stress
levels, and the time to failure is measured. A dead-weight load is hung from one
end of the specimen, directly or by means of a lever arrangement, whereas the
specimen is simultaneously exposed to the test solution contained in a glass cell.
A threshold stress below which the time to failure approaches infinity is encoun-
tered in some systems, as shown in Fig. 3.34, whereas in some other systems it
is not observed.
Time to failure,
t
f
, is essentially made up of two components: the time for
crack initiation (
t
i
), and the time for crack propagation,
t
p
, so that
t
i
t
p
t
f
