Characterization and control of the fiber-matrix interface in ceramic ...

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contraction have been ignored, and more effort is needed to thoroughly
examine the value of the tensile test and its results.
11.4 Flexure Testinz of ComDosite Materials
The usefulness of applying a flexure test to a composite has been
a topic of debate in recent years. Samples of unidirectional Nicalonglass
ceramic, tested using a bending technique, were shown to fail in
compression and shear but never in tension (87,165). This demonstrates
that calculation of tensile strength from this loading configuration is
clearly invalid.
The computation of tensile strength is based on the
propagation of a single coplanar crack that initiates at the tensile
surface and ends at the compressive face of the specimen.
At this point,
the specimen fails.
The flexure strength of a beam of fiber-composite material can be up
to three tirncs that which would be predicted by linear elastic analysis
for the ultimate tensile strength of the same material (141-143). The
main reason for this discrepancy is the quasi-plastic stress-strain
behavior of a composite material after matrix cracking.
Postcracking
behavior is dominated by fiber pull-out, and conventional beam theory is
not applicable.
The stress block used to calculate flexural strength for an elastic
material is inaccurate for fiber composites. A mare realistic stress
block for describing composite behavior in bending has been developed for
cement and concrete composites. After matrix fracture, the neutral axis
is shifted toward the compressive surface (166,167). A conservative
estimate places the axis one-fourth the specimen thickness from the
compressive surface (Figure 11.7). This changes the bending moment of