Page 319 - Engineered Interfaces in Fiber Reinforced Composites

300 Engineered interfaces in fiber reinforced composites
Finite element analysis has been a popular tool for examining the mechanical
response of coated fiber composites (Fan and Hsu, 1992; Daabin et al., 1992; Mital
et al., 1993; Daoust et al., 1993; Kim et al., 1994b; Kim and Mai, 1996a; Ho and
Drzal, 1995a, b). The use of the finite element method allows a more accurate
description of the interactions between neighboring fibers in practical composites
containing multiple fibers, and especially of the interface shear stress fields near the
singularity. The presence of an elastomeric soft interlayer reduced the shear stress
concentration at the fiber ends, and thereby reducing the load transfer efficiency
(Daoust et al., 1993), and this effect became more prominent as the interlayer
thickness increased. Increasing Young’s modulus of the fiber increased the load
transfer of the fiber at the expense of increasing shear stress at the interphase;
whereas increasing the Young’s modulus of the matrix had exactly the opposite
effect (Daabin, 1992).
On the contrary, when the interphase is stiffer than the matrix material as for
some uncoated carbon-poxy systems, increasing the interphase modulus does not
always increase the efficiency of stress transfer, and there is an optimum Young’s
modulus ratio of the interphase to the matrix (Ho and Drzal 1995a, b). Increase in
the interphase thickness was found to have a much larger effect on the interphase
shear stress distribution than on the fiber axial stress for both compliant and brittle
interphases. It was also noted that the maximum shear stress at the fiber-coating
interface was larger than the coating-matrix interface, which was later confirmed for
a carbon-epoxy system (Kim et al., 1994b). Energy distribution within the single
fiber composite and the strain energy release rate for interfacial crack propagation
has also been analyzed (Di Anselmo et al., 1992) using finite element method. The
presence of a compliant interlayer between fiber and matrix resulted in a lower strain
energy release rate, an indication of enhanced fracture toughness of the composite.
Based on a shear-lag model for CMCs, crack propagation was studied across the
fiber as opposed to interfacial debonding (Popejoy and Dharani, 1992). Coating
thickness was found to have little effect on crack growth although the interfacial
debonding was slightly favored when the thickness was small, an indication of high
fracture toughness of CMCs with the thinnest possible coating.
In summary, based on the previous studies as reviewed above, the variables which
affect most the mechanical performance of composites have been identified:
(1) Type and nature of interlayer.
(2) Modulus, CTE and glass transition temperature of interlayer.
(3) Thickness of interlayer.
(4) Modulus of matrix relative to interlayer.
(5) Interaction at the interface region.

7.3.2. Engineered interface concepts with fiber coating

It is shown in Section 7.2 that the PVAL coating applied onto Kevlar and carbon
fibers is potentially beneficial for improving the transverse fracture toughness of the
composites made therefrom. Encouraged by this promising result, further studies
were conducted on the effects of the compliant interlayer on the stress distributions,