Biomedical Engineering Reference
In-Depth Information
fibre interface testing. The boron-containing fibre/matrix interphase does
not rapidly oxidize (compared with the carbon fibre/matrix interphase) due
to the formation of a glass seal in the crack mouth for fast oxygen diffusion
paths during oxidation at high temperature. Such a phenomena minimizes
oxidation at fibre/matrix interface (Folsom et al., unpublished). It has been
observed that higher applied stress levels lead to greater matrix-crack
densities during the initial loading period. The increased crack density and
higher opening displacements act to accelerate the oxidation and rupture
processes. The fibre reinforcement (Nicalon fibre) exhibits several creep
characteristics, which are important to the understanding of the creep
response observed in investigations of a woven Nicalon/Si-N-C ceramic
matrix composite (Lee et al. 1988). It has been reported that
the
microstrucure of Nicalon fibre remains unchanged at
temperatures
<
C (Weber et al. 1994). Increasing creep resistance over time has
also been observed in composites that contain off-axis plies, such as [0/90]
cross-ply laminates. Fibres in the 90
1200
8
plies increase the axial creep resistance
of a composite by decreasing creep flow in the matrix (Xu and Holmes
1993). At
8
8
C, it was reported
(Kervadec and Chermant 1992, 1993) that strain accumulation in the CMC
reinforced by Nicalon fibres is dominated by damage-induced stress
redistribution within the composites, and, because of the relatively low
test temperatures, the fibre may still behave elastically in this temperature
range. Holmes and Cho (1992) reported that for fatigue tests conducted on a
unidirectional Nicalon/CAS CMC at stress levels below the proportional
limit (225MPa), matrix cracks along with interface debonding and sliding
along the fibre/matrix interface were observed.
The effect of interfacial characteristics for the improvement of tensile
creep properties in ceramic nanocomposites was studied by Nakahira and
Niihara (1993). High strength at high temperatures suggests that ceramic
nanocomposites would possess good creep resistance. They conducted
flexure creep tests for an alumina /17 vol% silicon carbide nanocomposite
and monolithic alumina at 1100-1400
temperatures in the range of 900-1000
C at a stress level of 100MPa and
revealed that the creep rate of the former was four orders of magnitude
lower than that of the latter. For both scientific and engineering aspects, it is
necessary to identify (1) why (or by which mechanism) the creep resistance is
so remarkably improved in the ceramic nanocomposite and (2) how (or in
which way) the creep and creep rupture behaviour of the ceramic
nanocomposite is different from that of the monolithic material. To deal
with these problems, uniaxial tensile testing was considered the most
suitable method.
It has been speculated that grain boundary diffusion ability in a silicon
carbide-alumina interface of a nanocomposite is significantly lower than
that
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in the alumina-alumina interface (Raj and Ashby 1971). This
