Utilization of fractography in the evaluation of high temperature dynamic fatigue experiments Page: 3 of 14
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v = A ( aC) ' (1)
where v is the crack velocity, A and n are environmentally determined material
constants, KIC is the material fracture toughness and Ka is the applied stress
intensity. The crack velocity exponent n is determined in the dynamic fatigue
experiment by measuring the fracture stress as a function of stressing rate:
a = B +1, (2)
where a is the strength in the given environment, a is the stressing rate and B is
a constant that is a function of the material toughness, crack velocity, inert
strength, and crack geometry. With the knowledge of n and A (from B) failure
prediction diagrams can be determined from well established fracture mechanics
The present experiments were conducted in ambient air at temperatures
between 1100 and 1400*C. SiC four-point-bend fixtures with 20 mm inner and
40 mm outer span were used in a hydraulic flexure test system with load control.
The loading rates were set so as to correspond to stressing rates ranging between
40 MPa/s and 0.0001 MPa/s. Resulting times to failure ranged from a few
seconds up to 1200 h. In order to decouple the effects of SCG, oxidation, and
creep, the Si3N4 was also tested in argon and nitrogen atmospheres at 1370*C.5,6
The flexure strengths were calculated according to classical beam theory, and in
the cases where creep was observed by virtue of observed specimen curvature
after the test, the stress was as a first approximation adjusted according to the
analysis by Hollenberg et al.14 The fractography was performed using a stereo
microscope with a video set-up, a digital image analysis system and a SEM
RESULTS AND DISCUSSION
The dynamic fatigue graphs, i.e., strength as a function of stressing rate,
are shown in Figs. 1 through 3. Figure 1 shows Si-SiC, P-SiC and SiCp/A1203
measured at 1400*C. The -SiC exhibited no strength degradation as a function
of time at this temperature. This result is consistent with results from the
literature for this material measured in a dry, oxidizing environment.15,16
Further, the strength level of this SiC was unchanged from room temperature,
which is also consistent with previous results.15,16 The Si-SiC ceramic exhibited
a reduction of fracture strength with decreasing stressing rates. Other work
showed increased variability in the strength as the temperature and time-to-failure
were increased. The n-value for this material, calculated according to Eq. (2),
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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/m1/3/: accessed April 25, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.