Page 270 - Engineered Interfaces in Fiber Reinforced Composites

Chapter 6. Interface mechanics and fracture toughness theories 2.5 1

For example, see glass fiber--PC matrix composites in Fig. 6.8. This is partly
associated with the matrix embrittlement by the constraint imposed by the stiff fibers
and the interaction between neighboring short fibers; and partly due to the loss of
volume of the tough matrix material taken up by the fibers. Matrix fracture is
predominant over other failure mechanisms, such as interfacial debonding and fiber
pull-out, because of the very short fiber length and relatively low vf that can be
accommodated by manufacturing processes like injection molding. Matrix-domi-
nant fracture behavior is promoted by weak interfacial bonding, but is discouraged
once the matrix ductility is suppressed such as at low temperatures. Experimental
results on the critical stress intensity factor, K,, of injection molded thermoplastic
composites containing glass and carbon fibers (Friedrich, 1985; Voss and Friedrich,
1986; Karger-Kocsis and Friedrich, 1988; Friedrich and Karger-Kocsis, 1989;
Karger-Kocsis, 1991) show that it is generally a function of the matrix toughness K,
and ‘microstructural efficiency factor’ (M)
Kc = MK, = (A + BQ)Km , (6.15)

where Q is a ‘reinforcing effectiveness parameter’ which is related to vf and the
geometrical arrangement of the fibers across the thickness, i.e., fiber orientation

0 20 bo 60

Wf (O/Ol
Fig. 6.8. Fracture toughness, K,, of short glass fiber-thermoplastics injection molded composites as a
function of weight fraction of fiber, Wf: (0) and (A) polyethylene terephthalate (PET) matrix; (0) and (A)
polycarbonate (PC) matrix. Notches made transverse (0, and parallel (A, A) to the mold fill direction.
0)
After Friedrich (1985).