Page 334 - Engineered Interfaces in Fiber Reinforced Composites

Chapter I. Improvement of transverse ,fracture toughness with interface control 315

7.5.2. Control of residual stresses
7.5.2.1. Compensating interlayer
Marom and Arridge (1976) were among the early researchers who demonstrated
that a soft interlayer present at the interface between the stiff reinforcements and
brittle matrix increased the composite strength in the transverse direction. This is
attributed to the reduction of the stress concentration around the inclusion, which is
influenced by the shear modulus and Poisson ratio of the compliant interlayer. A
plasticized epoxy resin and a silicone rubber coating (of thickness less than 10% the
reinforcement diameter) on the steel wire produces almost zero shrinkage stresses in
the radial and hoop directions, as measured photoelastically. The crack initiated
within the soft coating is blunted at the interface region, becoming stable under the
applied transverse loading, similar to the observations for other fiber composites
(Kardos, 198 1).
Apart from PMCs, highly complex residual stresses are also introduced due to
thermal mismatch in MMCs during manufacturing. This often causes cracking of
the matrix, especially those with brittle matrices (Chou et al., 1985). The tensile
residual stresses in the longitudinal and hoop directions are the major cause of the
observed matrix cracking (Vedula et al., 1988; Ghosn and Lerch, 1989).
Micrographs are shown in Fig. 7.22 of a transverse section with radial cracking at
the fiber-matrix interface for a Sic fiber- Ti3Al--Nb matrix composite after
fabrication and after 1000 thermal cycles. Aiming specifically to tackle the matrix
cracking issue, the properties of an interlayer necessary to minimize the local tensile
residual stresses in the matrix have been studied extensively (Ghosn and Lerch,
1989; Caruso et al., 1990; Doghri et al., 1990; Morel et al., 1991; Jansson and Leckie,
1992; Arnold et al., 1992). Based on the elastic analysis of a three-cylindrical model
with temperature-dependent properties of the fiber and matrix, the following
summary is given (Ghosn and Lerch, 1989)
(1) A well designed interface layer with controlled Young’s modulus, CTE and
thickness can reduce the tensile residual stresses in a MMC system.

Fig. 7.22. Radial cracking in a SIC fiher/Ti-24Al-l INb matrix composite (a) after fabrication and
(h) after additional 1000 thermal cycles. After Arnold et al. (1992).