7.3. How to judge whether the crack is dormant or
active?
For a long term service pipe, the oxide film on the crack path has been
built up for perhaps many years. The crack tip may be produced newly and
should indicate whether the crack is active or dormant rather than the
crack sides. That is, it is more important to examine the details of the
region at the very crack tip than the bulk of the crack surface.
Fig. 15. Three types of crack tip.
Consequently, if we want to judge whether the crack in
the service pipe is active or dormant, it is reasonable to compare the
crack tip of the service pipe crack and the fresh crack produced in the
laboratory LIST sample. If the crack tip of the LIST sample is similar to
that of the service pipe, it means the crack in the service pipe is
probably active.
7.4. Laboratory samples JQ1 & JQ2
The loading procedure for JQ2 provided a longer time for a crack to
propagate, so JQ2 did have longer cracks than JQ1. The crack size was less
than 10 mm for JQ1, and about 50 mm for JQ2.
As shown in Fig. 11(a), the small cracks were open. If the two sides of
the crack sides are closed again, they meet together very well. It means
there was no serious dissolution at these crack sides.
The big cracks for JQ2 were more open, which is attributed to the
overload. If the two crack sides of each crack are closed again, they meet
very well when the crack size is small as shown in Fig. 11(a), and also
meet nearly exactly as shown in Fig. 11(b) and (c). It means the crack
surface was not subjected to significant corrosion. A crack close to the
overload fracture surface is shown in Fig. 11(d). The crack tip front was
broad. The outer layer of the sample was weakened by cracks. When the
sample was overloaded, the stress concentrated at the crack tips and the
crack tip front was torn to be broad. Although there was corrosion and a
significant oxide on the specimen surface caused by the 10 days of the SCC
test, there was clear IGSCC at the sides of the newly created cracks and
furthermore, there were few corrosion products on the sides of these newly
created cracks.