Page 156 - Engineered Interfaces in Fiber Reinforced Composites

138 Engineered inlerfaces in Jiber reinforced composites

(Fig. 4.28). It is worth noting that the L,,, value decreases significantly when the
fiber surface is treated to improve the interfacial bonding (and thus the interface
fracture toughness, Gic), e.g. acid treated Sic fibers versus untreated fibers. This
observation is analogous to what is expected from the fiber fragmentation test of
single fiber composites: the stronger the interface bond the shorter is the fiber
fragment length at the critical stage (see Section 4.2).

4.3.5. Characterization of interface properties

Microcomposite tests including fiber pull-out tests are aimed at generating useful
information regarding the interface quality in absolute terms, or at least in
comparative terms between different composite systems. In this regard, theoretical
models should provide a systematic means for data reduction to determine the
relevant properties with reasonable accuracy from the experimental results. The data
reduction scheme must not rely on the trial and error method. Although there are
several methods of micromechanical analysis available, little attempt in the past has
been put into providing such a means in a unified format. A systematic procedure is
presented here to generate the fiber pull-out parameters and ultimately the relevant
fiber-matrix interface properties.
In single fiber pull-out experiments, the most useful data that are readily obtained
from the load-deflection records are the maximum debond stress, 02, and the initial
frictional pull-out stress, ofr, as a function of L. If the debond process is carefully
monitored for a large embedded fiber length, L, the initial debond stress, 00, can also
be determined directly in the average sense, depending on the composite system.
Most important properties to be calculated are the fracture toughness, Gi,, at the
bonded region, and the coefficient of friction, p, and the residual clamping stress, 40,
at the debonded region, by evaluating the pull-out parameters of, i and r~. There are
several steps to be followed for this purpose.
(i) Firstly, ofr versus L data allow the initial slope at L = 0 to be determined based
on Eq. (4.103),

(4.110)

(ii) Secondly, the gradient can be taken from the linear region of the stress drop
Ao(= 02 - ofr) versus L plots for large L where the crack tip debond stress is
almost constant and independent of L, Le.,
--
d ln(Ao)
dL --A , (4.111)
where the difference between the stresses obtained immediately before and after
the load instability is given by

Ao = o: - ofr = {of + Tj[exp(-;lz,,,) - 11) exp[-A(L - zmax)] (4.112)