Page 259 - Engineered Interfaces in Fiber Reinforced Composites

240 Engineered interfaces in fiber reinforced composites

composite is not simply the sum of the weighted contributions by the constituents,
but is governed more importantly by the extent of synergistic energy absorption
processes through various toughening mechanisms, depending on the nature of
physicochemical bonding and elemental constitutions at the fiber-matrix interface
region.
There are many theoretical and experimental studies carried out on the fracture
behavior and toughening mechanisms in fiber reinforced composites. When a
composite having internal cracks is loaded, there is a highly strained region at the
crack tip, the so-called fracture process zone (FPZ) or damage zone, where failure
mechanisms of various kinds take place before the cracks propagate. Summaries of
the failure mechanisms in polymer matrix composites can be found in many
references including Kelly (1973), Marston et al. (1974), Atkins (1975) and Harris
(1980), and these are reviewed recently by Kim and Mai (1991a, b, 1993). Reviews
on failure mechanisms are also available for MMCs (Ochiai, 1989; Taya and
Arsenault, 1989; Clyne and Withers, 1993), CMCs (Davidge, 1989; Warren and
Sarin, 1989; Evans, 1989; Ruhle and Evans, 1989; Chawla, 1993), and cementitious
fiber composites (Mai, 1985; Cotterell and Mai, 1996).
Many fracture toughness theories of composite materials have been developed
mainly for those with unidirectional fibers. The various origins of fracture toughness
in composites may be characterized by considering the sequence of microscopic
fracture events that lead to crack propagation macroscopically under monotonic
increasing loads. The cracks in composites can propagate preferentially along the
fiber-matrix and laminar interfaces (i.e. longitudinal splitting) or transversely right
through the fiber and matrix (i.e. transverse cracking), depending on the properties
of the interface relative to the fiber and matrix. The criteria for these two opposing
fracture phenomena are given in Section 6.4. Consideration of a microcomposite
model shown in Fig. 6.1 (Harris, 1980) makes it most convenient to isolate the
individual micromechanisms of toughening. When a crack present in the matrix
approaches an isolated fiber, the following failure mechanisms may be expected to
take place:
(1) matrix fracture,
(2) fiber-matrix interface debonding,
(3) post-debonding friction,
(4) fiber fracture,
(5) stress redistribution,
(6) fiber pullout.
The underlying physical bases of these toughening mechanisms are presented in
the following sections, and the corresponding equations are summarized in Table
6.1. All these mechanisms, except fracture of fibers and matrix, are a direct
consequence of shear failure at the imperfectly bonded fiber-matrix interface. In
conjunction with these mechanisms, fiber bridging, crack deflection and bifurcation,
and microcracking also take place depending on the strength of the constituents
relative to that of the interface, microstructure of the composite constituents, and
the loading configuration of the composite structure.