Page 299 - Engineered Interfaces in Fiber Reinforced Composites

280 Engineered interfaces in jiber reinforced composites

interlaminar fractures and low energy impact where the fracture toughness and
ductility of the matrix material play a dominant role. Comprehensive reviews of the
failure mechanisms have been given for thermoplastics (Mascia, 1989) and
toughened epoxies (Kinloch, 1993; Garg and Mai, 1988a, b). Tough adhesives
and/or composite strips are often interleaved between plies as delamination resister
or arrester to improve the interlaminar fracture toughness. Details of these
techniques are given in Chapter 8.
(3) The use of large diameter fibers can also result in improved fracture toughness
of brittle fiber-ductile metal matrix composites, such as tungsten wir-opper
matrix systems (Cooper and Kelly, 1967; Tetelman, 1969). This can be explained in
terms of increased volume of ductile matrix involved in shear flow at the interface
region, which gives rise to the fiber pull-out force proportional to the fiber diameter.
A large diameter fiber is also found to be beneficial for brittle fiber-brittle matrix
composites (Piggott, 1970; Wells and Beaumont, 1985): it increases the debond and
fiber pull-out lengths by increasing the critical transfer length, &, given the
mechanical properties of the fiber and the fiber-matrix interface. However, care
should be taken in using this technique because the tensile strength and modulus of
many fibers show a systematic decrease with increasing fiber diameter (Metcalfe and
Schmitz, 1964; Kelly, 1970). This problem may be overcome by using bundle fibers
that are impregnated with polymers prior to being incorporated in a resin matrix
(Fila et al., 1972; Kim and Mai, 1993b).
(4) If the fiber is coated intermittently along its length with an appropriate coating
material before being embedded in a matrix so that there are regions of both strong
and weak interfacial bonds, high transverse fracture toughness can be achieved
without deteriorating the composite strength and stiffness. The triaxial stress
distribution at the advancing crack tip allows easy debonding and crack tip blunting
at the weak interface due to the tensile debonding mechanism (Cook and Gordon,
1964). Simultaneously, a good composite strength is maintained through the
interface with strong bonding. The intermittent bonding concept has been further
extended to laminate composites where diferent kinds of thin films with perforated
holes are inserted between plies as delamination promoters.
(5) The energy absorption capability of composites can be enhanced significantly
by promoting interface debonding and fiber pull-out, while maintaining a ductile
interphase. This method is most effective if fibers are coated with an appropriate
material for both polymer, metal and ceramic matrices composites. A review has
recently been given of fiber coating methods, coating materials and associated
toughening mechanisms of the interlayer for polymer matrix composites (Labronici
and Ishida, 1994).
(6) Reduction of residual stresses that arise from the differential thermal shrinkage
between the fiber and matrix materials when cooling from the processing
temperature has a beneficial effect of enhancing the fracture resistance of
composites. This can be achieved by applying a soft, compliant coating onto the
reinforcing fibers (Marom and Arridge, 1976) and/or by adding an expanding
monomer into the matrix material (Piggott and Woo, 1986). Reduction of thermal
residual stresses may also have the benefits of reducing the tendency of fiber