Page 317 - Engineered Interfaces in Fiber Reinforced Composites

298 Engineered interfaces in fiber reinforced composites

which in turn control the specific failure modes. Jao and McGarry (1992b) have also
used an elastomer for injection molded glass fiber-nylon matrix composites, showing
that a thin rubber coating mitigates significantly the stress concentration at the fiber
ends. The CTEs of composites are calculated to determine the effect of the interphase
which depends on the interfacial bond strength (Siderisodis, 1994). Using a three-
cylinder model, Gao (1993) also studied the effect of interface bond strength on global
failure of carbon fiber-epoxy matrix composite under multi-directional loading.
Stress distributions are estimated based on two typical three cylinder phase
models with both uniform and varying interphase properties and with the interlayer
thickness being 15% of the fiber diameter (Gardener et al., 1993a, b). The major
results are compared in Fig. 7.10 for a carbon-epoxy system with a fiber volume
fraction of 36%. The stresses are normalized with the matrix shrinkage stress
(a = Emam AT, see Eq. (7.10)) which is the product of the matrix Young’s modulus,
matrix CTE and the temperature change. It is noted that both models predicted a
constant axial stress within each phase, which is consistent with previous results
(Pagan0 and Tandon, 1988; Benveniste et al., 1989).
Driven mainly by aerospace industries for applications to engine components and
high temperature structures, many researchers studied interlayers that were designed
to reduce the residual stresses in MMCs. The deformation behavior and the strength
of unidirectional MMCs were modeled taking into account the yielding of the
matrix material in an elasto-plastic analysis of the three-phase model (Craddock and
Savides, 1994), and in compression (Waas, 1992). The effect of plastic deformation
of the interlayer on matrix stress reduction was found to be equivalent to increasing
the CTE of the layer by 1.5 times. The failure of composite materials containing
interlayers was also predicted based on different failure criteria (Walpole, 1978;
Aboudi, 1991; Mitaka and Taya, 1986). The elastic constant and CTE of the Ni and
Sic interlayer in carbon fiber-aluminium matrix composites were assumed to be
linear functions of the radial coordinate (Mitaka and Taya, 1985a). It was found
that the variability of thermo-elastic constants of the interlayer had little direct
influence on the stress distributions in the fiber and matrix. However, the maximum
shear stress occurred at the interlayer when its modulus was comparable to the
matrix. Ni coating was found to be advantageous over Sic coating from the fracture
mechanics viewpoint (Mitaka and Taya, 1985a). The Young’s modulus of the
interphase was treated as a variable for a three-cylinder model of carbon fiber-
aluminum matrix composites (Vedula et al., 1988; Jansson and Leckie, 1992; Doghri
et a]., 1990). It was proposed that the compliant layer in MMCs with a high CTE
was much more efficient for reducing the residual thermal stresses than the
compliant layer with a Young’s modulus lower than the other composite
constituents. A compliant interlayer was found to be beneficial mainly for reducing
the tensile residual stresses in the matrix. This result has formed a sound basis for
the establishment of the compliant/compensating interlayer concept where the
residual thermal stresses could be minimized for a variety of metal matrix
composites. The details are presented in Section 7.3.2. The optimum compliant
layer for a SiC-Ti3A1 + Nb system was found to have a modulus value about 15%
that of the composite without an interlayer (Caruso et al., 1990).