Page 144 - Engineered Interfaces in Fiber Reinforced Composites

Chapter 4. Micromechanics of stress transfer 127

Piggott (1987, 1990) have equated the increase in debond length with the energy
changes arising from the fiber axial tension and matrix shear. Gao et al. (1988) also
presented a debond criterion using the assumptions similar to those made previously
(Wells and Beaumont, 1985). Based on the relationship between the debond and
post-debond frictional pull-out stresses versus embedded fiber length established in
the work of Gao et al. (1988), it has been demonstrated (Kim et al., 1992) that the
model is able to determine the inherent interfacial properties including interfacial
fracture toughness Gi,, coefficient of friction, p, and the residual fiber clamping
stress, 40. Hutchinson and Jensen (1990) and Keran and Parthasarathy (1991)
considered the effect of residual stresses in the fiber axial direction in a thermo-
mechanical analysis, giving a solution for the pull-out stresses similar to that
obtained earlier by Gao et al. (1988). Other recent studies using the fracture
mechanics approach include those of Pally and Stevens (1989), Sigl and Evans
(1989), Marshall et al. (1992) and Jiang and Penn (1992), the latter based on the
stress solutions derived earlier by Piggott (1987). Zhou and Mai (1993) also took
into account the anisotropy of the embedded fiber for the fiber pull-out problem.
As opposed to the common perception of perfectly cylindrical surface of the fiber,
several investigators, including Jero and Keran (1990), Jero et al. (1991), Carter et al.
(1991), Waren et al. (1992), Mackin et al. (1992a, b), have noted substantial surface
roughness of some ceramic fibers, notably the SCS-6 Sic fibers and sapphire fibers.
They found that surface roughness contributes significantly to the frictional
resistance of fiber pull-out (and fiber push-out). It is assumed that the fiber and
matrix geometry, once removed from their original position, would create a uniform
asperity pressure, as schematically illustrated in Fig. 4.20, that simply adds to the
existing radial clamping stress. Assuming the separation of fiber and matrix during
frictional sliding is equivalent to the roughness amplitude, Keran and Parthasarathy

+2R+
Fig. 4.20. Schematic presentation of rough fiber surface in a fiber push-out test. After Mackin et al.
(1992a. b).