Utilization of fractography in the evaluation of high temperature dynamic fatigue experiments Page: 2 of 14
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terms of lifetime predictions, it is necessary to understand the relative
contributions from the different mechanisms, as each of them may develop
differently with time. The fracture mechanics framework utilized in lifetime
predictions during SCG assumes that linear elastic fracture mechanics is valid,
but if creep occurs simultaneously, then corrections must be made. The best way
to decouple the mechanisms is to perform experiments such that only one process
is active at any time, e.g., creep and SCG in Si3N4 was studied by performing
experiments in air, argon (Ar) and nitrogen (N2) atmospheres at elevated
temperatures.5-7 These experiments, combined with microstructural analyses,
indicated significant differences in the damage mechanisms due to creep and
SCG. Zeng et al.8,9 performed stressing-rate experiments on A1203 and SiC
whisker-reinforced Al2O3 in water at room temperature and found a clear
difference in fracture mode between SCG and fast fracture. Differences in
fracture propagation paths were also observed by Pletka and Wiederhornl who
compared slow crack growth in several ceramics and glass ceramics by the
dynamic fatigue method and by crack velocity measurements using double
torsion experiments. The ceramics with the coarser microstructures (presumably
the ones exhibiting R-curve behavior) had different crack velocities measured by
the two methods, showing that the crack propagation mode, e.g. transgranular
vs. intergranular, is important and must be determined in these types of
experiments. These examples show the importance of the fractographic analysis
for determination of SCG. In the present work, several ceramic materials have
been compared and different SCG paths will be shown.
MATERIALS AND EXPERIMENTAL PROCEDURE
Six ceramic materials were compared in this work. They were
Siliconized SiC (Si-SiC) NT230 from Saint-Gobain Norton,10-12 p-SiC from
Coors Ceramics Company,12 Lanxide DIMOX, a SiC particulate reinforced
A1203 from Lanxide Composites Inc.(SiCp/A1203),12 hot isostatically pressed
Si3N4 with yttria as a sintering aid (PY6) from GTE Laboratories Inc.,5,6 and
AIN and SiC whisker reinforced AIN from Advanced Refractory Technologies
Inc.13 Further details about these materials can be found in the cited references.
These materials are candidate structural ceramics for applications in which
lifetime predictions are important.
The dynamic fatigue experiment is commonly referred to as a stressing
rate or a strength technique, to distinguish it from direct crack velocity
measurement methods such as the double torsion experiment.1 The
measurements can be performed by testing in three- or four-point bending,
biaxial bending or in tension. The basic principle is the assumption that there is a
relationship between the crack velocity and the applied stress intensity, such as is
expressed in the power law form:
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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/m1/2/: accessed April 19, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.