Page 75 - Engineered Interfaces in Fiber Reinforced Composites

58 Engineered interfaces in fiber reinforced Composites
where i? = 2EfS/of is the debonded length estimated from the displacement of the
fiber end, 6, at an average external stress, bf, applied to the fiber.
In contrast to the thick specimens used in the above studies, very thin slice
specimens of known embedded fiber lengths (Fig 3.12(c)) are also employed (Bright
et al., 1989) to distinguish debonding and post-debond frictional push-out in a
continuous loading test. The latter fiber push-out technique has become most
popular in recent years among the variations of specimen geometry and loading
methods. Rigorous micromechanics analyses dealing with interface debonding and
fiber push-out responses are detailed in Chapter 4.
The above test techniques have been developed initially and used extensively for
polymer matrix composites (Grande et al., 1988; Herrera-Franco and Drzal, 1992;
Desaeger and Verpoest, 1993; Chen and Croman, 1993). Its usefulness has been
extended to ceramic matrix composites (Grande et al., 1988; Brun and Singh, 1988;
Netravali et al., 1989a, b; Morscher et al., 1990; Weihs and Nix, 1991; Wang et al.,
1992; Watson and Clyne, 1992a, b; Ferber et al., 1993) where difficulties of specimen
preparation and testing associated with fiber misalignment, breakage of high
modulus fibers in grips, etc. are frequently experienced in fiber pull-out tests. Other
major advantages include the ability to test real composites and the speed and
simplicity of the test, once automated instruments are equipped with the testing
machine. The main questions associated with this test method are concerned with its
physical significance and the interpretation of experimental data. Other drawbacks
are the inability to monitor the failure process during the test of opaque composites;
problems associated with crushing and splitting of fibers by the sharp indentor
under compression (Desaeger and Verpoest, 1993); and radial cracks within the
matrix near the fiber-matrix interface (Kallas et al., 1992).

3.2.6. Slice compression test

The slice compression test is a modified version of the indentation test and was
developed specifically for ceramic matrix composites utilizing the difference in elastic
modulus between the fiber and the matrix material. This test involves compression
of a polished slice of a unidirectional fiber composite cut perpendicularly to the fiber
axis between two plates (Fig 3.13). The applied load is increased to a desired peak
stress and then unloaded. At the critical load, interfacial debonding and sliding
occur near the top surface of the specimen where the elastic mismatch is at its
maximum and the fibers protrude against the soft top plate (e.g. pure aluminum)
with known work-hardening characteristics. At the same time, the hard bottom
plate (e.g. Si3N4) ensures a constant strain in the specimen bottom. Upon removing
the load, the fibers partially relax back into the matrix, retaining a residual
protrusion. Fig 3.14 schematically shows the sequence of the slice compression test
based on a single fiber model composite (Hsueh, 1993). Therefore, the interface
properties can be estimated from the fiber protrusion, 6, under a peak load and the
residual fiber protrusion after unloading, 6,. Shafry et al., (1989) derived
approximate solutions for the relationship between the fiber protrusion length
and the applied stress for a constant interface friction along the embedded fiber