Creep-fatigue of High Temperature Materials for VHTR: Effect of ...

Cycle to failure
10000
1000
100
0.3% total strain in air
0.3% total strain in helium
0.6% total strain in air
0,01 1 100 10000
Hold time (s)
Fig. 3. Cycles to failure as a function of hold time for LCF
and creep-fatigue tests in air and in helium at a 0.3% and 0.6%
total strain.
a.
b.
Fig. 4. Peak tensile stresses as a function of cycle in air and
in helium at a 0.3% total strain for a) LCF and cb) reep-fatigue
with a 180s hold time.
Typical stress relaxation curves at mid-life are shown
in Figure 5 for creep-fatigue tests with a hold time up to
1800s with a 0.3% and 0.6% total strain. During the hold
period at constant strain, the stress relaxed at the same rate
for all hold durations: it initially decreased rapidly and was
almost completely relaxed after approximately 180s into
the hold.
316
a.
b.
Proceedings of ICAPP 2011
Nice, France, May 2-5, 2011
Paper 11284
Fig. 5. Stress relaxation at mid-life during the tensile hold
for creep-fatigue specimens tested in air a) at 0.3% total strain
with a 180s, 600s, and 1800s hold time and b) at 0.6% total strain
with a 180s, and 600s hold time.
III.D. Failure mode
No noticeable difference was observed in specimen
cracking after cyclic testing in air and in helium. Hence in
the following, results from both environments are presented
together.
In continuous cycling tests, the primary crack (most
likely initiated and) propagated in a predominantly
transgranular manner, perpendicular to the stress axis.
Figure 6 shows longitudinal cross sections through the
specimen gage after LCF. The edges of the cracks were
slightly oxidized and there was little indication of an oxide
having formed on the surface of the specimens. A few wide
but short intergranular secondary cracks were observed
perpendicular to the stress axis in the specimens tested at a
0.6% total strain.
The addition of a hold time resulted in multiple larger
or primary cracks originating from the specimen surface as
well as an evolving microstructure, particularly at the
specimen surface. For the specimens tested in creepfatigue,
all of the grain boundaries close to the specimen