Page 292 - Engineered Interfaces in Fiber Reinforced Composites

Chapter 6. Intetjace mechanics and fracture toughness theories 273
ends in aligned short fiber-thermoplastic composites. Depending on the interfacial
bond strength and matrix ductility, the main crack propagates in a fiber avoidance
mode, as illustrated clearly in Fig. 6.5. The cracks bypass the regions of
agglomeration of locally aligned fibers without breaking them (Mandell et al.,
1981, 1982). It follows therefore that a critical distance from the main crack where
the stress is just enough to initiate debonding (i.e. fiber debond stress, Qd) can be
regarded as the size of damage zone, co, in these composites. It is proposed that in
the absence of large scale yielding, the effective length of these long fibers that are
exceeded by 5% of the total fibers in injection molded thermoplastic composites is
approximately equal to co (Mandell et al., 1981,1982). Assuming that the stress field
near the crack tip varies linearly with ~/JY as in isotropic materials, where Y is the
distance from the crack tip, the stress in the damage zone is given by

(6.37)

It is assumed that the maximum stress in the next fiber located at a distance d/&
away is approximately equivalent to the composite tensile strength, 0;. It follows
then that the damage zone size, cg, for aligned short fiber composites is given by
(Lauke et al., 1985)

(6.38)

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