Page 326 - Engineered Interfaces in Fiber Reinforced Composites

Chapter 7. improvement of transverse fracture toughness with interface control 307

Number of nylon sheets

and
Fig. 7.16. Impact fracture toughness (0) interlaminar shear strength (ILSS, 0) of carbon fiber-epoxy
matrix composites with varying number of nylon sheets as delamination promoters. After Favre (1977).

increased successfully by three times with embedded nylon sheets, at the expense of
some 25% reduction in ILSS (Favre, 1977) (Fig. 7.16). Perforated films were found
to be more effective than unperforated films because the perforated films could
provide both the weak and strong bonding in the regions of film and perforation,
respectively, similar to the intermittent interlaminar bond concept (Mai et al.,
1982a). The failure mechanisms underlying the delamination promoter concept are
schematically shown in Fig. 7.17. In the weak regions, delamination occurred
allowing the main crack front to be bifurcated and the sub-cracks to propagate
along the laminar interfaces. These mechanisms promote energy absorption, as
envisaged from the tensile debonding theory (Cook and Gordon, 1964). However,
excessive delamination was effectively discouraged and the shear stress transfer was
permitted between laminae in the strong regions, enabling the original bond strength
to be maintained. While the tensile strength dropped about 20%, and the Young’s
modulus remained the same, the transverse fracture toughness of CFRPs increased a

I nterlarninar -Strong bonding
subrrack shown

: I transverse crack

(a) I bl
Fig. 7.17. Schematic illustrations of sub-crack propagation along the laminar interface of the composite
intermittently bonded with perforated films. After Jea and Felbeck (1980).