Page 325 - Engineered Interfaces in Fiber Reinforced Composites

306 Engineered inierfaces in fiber reinforced composites

(1) weak interface-bond layer;
(2) microductile/compliant layer;
(3) compensating layer.
In the weak interface-bond layer concept, the coating layer should provide a weak
interface bonding, promoting interface debonding and subsequent fiber pull-out. A
coating material which forms a discrete interlayer between the fiber and matrix can
readily act as a physical barrier to the chemical bonding between the functional
groups present in the composite constituents. To obtain the maximum benefits of
high fracture toughness, the coating material should provide a sufficiently high
frictional bonding, while maintaining a low chemical bonding at the interface. There
must be optimum values for these conflicting requirements.
The microductile/compliant layer concept stems from the early work on
composite models containing spherical particles and oriented fibers (Broutman
and Agarwal, 1974) in that the stress around the inclusions are functions of the shear
modulus and Poisson ratio of the interlayer. A photoelastic study (Marom and
Arridge, 1976) has proven that the stress concentration in the radial and transverse
directions when subjected to transverse loading was substantially reduced when
there was a soft interlayer introduced at the fiber-matrix interface. The soft/ductile
interlayer allowed the fiber to distribute the local stresses acting on the fibers more
evenly, which, in turn, enhanced the energy absorption capability of the composite
(Shelton and Marks, 1988).
A compensating layer concept is based on the interlayer which can reduce the
residual thermal stresses, as detailed in Sections 7.3.1 and 7.5.2. This is best achieved
when the microductile/compliant layer has a high CTE so that the shrinkage stress
in the matrix around the fiber can be effectively balanced by the greater shrinkage of
the coating layer, if not completely eliminated. This concept has been originally
proposed for advanced metal matrix composites (Vedula et al., 1988; Arnold et al.,
1990, 1992; Arnold and Wilt, 1992), such as Sic fiber-Ti,Al+ Nb systems, in which
microscopic cracking in the radial and circumferential directions due to high
shrinkage stresses was a major concern during the manufacturing process.
Apart from the above three major engineered interface concepts, the ductile
coating material may also heal up the surface flaws that are often generated during
the fiber manufacturing processes, and protect the brittle fiber surface during
subsequent processing.

7.4. Control of laminar interfaces-delamination promoters

Another way of improving the energy absorption capacity of laminate composite
in the transverse direction is by promoting controlled delamination when the
interlaminar bond strength or interlaminar fracture toughness is weakened.
Depending on the orientation of the interface relative to the main crack, the
triaxial tension operating at the crack tip causes the main crack to be arrested at the
weak laminar interfaces by allowing delamination (Almond et al., 1969). Based on
the concept of crack arrest, the transverse fracture toughness of CFRPs has been