Utilization of fractography in the evaluation of high temperature dynamic fatigue experiments Page: 5 of 14
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Specimens in this
region have cavities No cavitation
00 SCG and Creep failure
I- SCG failure
U , / _________
/ - -- Strength in Ar and N2
V Strength in Ar and N2
Q 100 - ' adjusted for creep
Strength in air
1370 C - - - Strength In air
adjusted for creep
0.0001 0.001 0.01 0.1 1 10 100
STRESSING RATE [MPa/s]
Figure 2. Dynamic fatigue results for HIP'ed Si3N4 measured in four-point
flexure in air and inert environment at 1370*C.
Figure 3 shows strength as a function of stressing rate for AIN and AIN
reinforced with 20% SiC whiskers measured in air at 1200*C. The addition of
SiC whiskers did not strengthen the AIN significantly at this temperature, but the
creep deformation was considerably lower. The creep exponents for these
experimental materials were not known, so a strength adjustment to account for
the creep was not performed. The presence of SCG must be sought in the
fractography results as the dynamic fatigue results showed no evidence for such.
Figure 4 shows the fracture surface of the D-SiC fractured at 420 MPa at
a stressing rate of 0.0001 MPa/s at 1400*C. The fracture initiation point was a
pore and there was no evidence of SCG around this. This is consistent with the
strength results, which showed no strength degradation with time and no
difference in strength between room temperature and 1400 C.
A typical fracture surface of the Si-SiC fractured at 40 MPa/s at 1400*C is
shown in Fig. 5 a). The fracture initiation point was a pore. Figure 5 b)
shows the fracture surface of the Si-SiC fractured at 1400*C and 0.0001 MPa/s.
It can be seen that the fracture origin was a Si-rich area which enlarged during the
test. The enlargement was probably due to subcritical (slow) crack growth. This
type of failure origin was typical for the Si-SiC tested at 1400*C, and the
increased variability in the strength results was believed to be due to the
variability in the size and composition of these Si-rich areas.11
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Breder, K.; Wereszczak, A.A.; Tennery, V.J. & Mroz, T.J. Utilization of fractography in the evaluation of high temperature dynamic fatigue experiments, article, December 31, 1995; Tennessee. (https://digital.library.unt.edu/ark:/67531/metadc670054/m1/5/: accessed May 23, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.