Page 373 - Engineered Interfaces in Fiber Reinforced Composites

354 Engineered interfaces in fiber reinforced composites

Fig. 8.24. Weft knit with laid-in weft and warp yarns. After KO (1989).

8.4.2. Improvement of interlaminar fracture toughness

This section examines the advantages and disadvantages of using three-dimen-
sional textile preforms, especially through-the-thickness stitches, as the reinforce-
ments for composites. Their major mechanical properties are compared with those
of conventional two-dimensional composites, such as strength, stiffness, interlam-
inar properties, impact resistance and tolerance, etc. Dransfield et al. (1994) have
recently given a useful review on the improvement of interlaminar fracture
toughness of stitched composites.
Huang et al. (1978) were among the first researchers who introduced a technique
designed to reduce delamination, which, in turn, enhanced the local shear strength
of carbon fiber-epoxy matrix composites. Steel wires of 0.33 mm in diameter were
placed by hand at an angle of +45" to the laminate surface. Holt (1982) employed
the stitching technique in composite joining for aircraft structural components. In a
subsequent study by Mignery et al. (1985), Kevlar threads were stitched along the
edges of the laminates to mitigate the free edge delamination and ultimately to
improve the tensile strength of carbon fiber composites. Stitching along the free edge
improves the mode I interlaminar fracture toughness by 85%, while also enhancing
the flexural strength by up to 30% for carbon fiber-epoxy matrix composites
fabricated from prepregs, as summarized in Table 8.5 (Chung et al., 1989). Stitches
also give enhanced interlaminar shear strength (Adanur et al., 1995). The unstitched
fiber composites fail normally by interlaminar shear, while the stitched counterparts
fail predominantly by tension due to the restriction of shear achieved by the stitches.
The load-displacement curves for the orthogonal interlock fabric composites
show a non-linear unloading sequence and an appreciable permanent deformation
after unloading, with the crack tip not completely closed (Guenon et al., 1987).
These observations are attributed to the crack closure process of the three-
dimensional fabric composites where through-the-thickness yams break near the
outer surface of the specimen.