Page 364 - Engineered Interfaces in Fiber Reinforced Composites

Chapter 8. Improvement of interlaminar fracture toughness with interface control 345

8.3.2. Intevlenving techniques

Among the several techniques which have been attempted to suppress the onset of
free edge delamination, the interleaving technique has received significant attention
which uses a soft, tough strip interleaved between delamination-prone layers. The
interleaving technique is based on an early study of various crack arrest concepts
where integral crack arrester strips were placed at critical damage-prone regions to
give a composite structure the ability to carry the limiting load after sustaining the
damage (Hess et al., 1977). In a similar study, the use of softening strips made from
glass fiber4poxy matrix composites in place of [OO] carbon fiber-epoxy matrix plies
at the center notches reduced significantly the notch sensitivity, thereby improving
the laminate strength (Sun and Luo, 1985).
Adhesive layers having low modulus and high elongation were employed
successfully at delamination-prone free edges to suppress delamination growth by
reducing the interlaminar stresses, particularly the tensile mode I component normal
to the laminar interfaces (Chan, 1986, 1991; Chan et al., 1986). The huge reduction
in the interlaminar stresses for [ f 3Oo2/9Oo3/- + 30°2] carbon fiber-epoxy matrix
composites with interleaves is clearly seen from Fig. 8.14. This, in turn, improved
substantially the critical strength before the onset of delamination and the ultimate
strength of the laminate in in-plane tension, Fig. 8.15. In uniaxial tension of cross-
ply laminates, interfacial delamination was found to be the immediate failure mode
associated with transverse cracking, and the presence of soft interleaves could reduce
the stresses, and thereby delay the onset of delamination (Altus and Ishai, 1990).
Furthermore, it is worth noting that the interleaves effectively eliminated delam-
ination prior to final failure. The edge strips of adhesive had the same effect as the
adhesive layer placed over the whole plane.
Although the interleaving technique was originally devised mainly to suppress free
edge delamination, this technique has been employed extensively to improve the
interlaminar fracture toughness of carbon fiber composites in various fracture
modes. The interleaving strips effectively increase the composite mode 1 interlaminar
fracture toughness by almost ten times those without interleaves, depending on the
thickness and types of interlayer used (Ishai et al., 1988; Sela et al., 1989; Altus and
Ishai, 1990; Chen and Jang, 1991; Sun and Rechak, 1988; Rechak and Sun, 1990;
Lagace and Bhat, 1992; Singh and Partridge, 1995). The critical load for mode I
delamination crack is substantially higher for the laminates with interleaves,
although using adhesive strips may cause a concomitant reduction in in-plane
strength and stiffness (Sun and Norman, 1990; Norman and Sun, 1991). Further, the
mode I1 interlaminar fracture toughness of the composites interleaved with
thermoset and thermoplastic polymers are also measured experimentally and
numerically (Carlsson and Aksoy, 1991; Aksoy and Carlsson, 1992; Sohn and Hu,
1994). Both types of interleaves enhance the fracture toughness significantly, the
thermoplastic interleaves being more effective than thermmoset counterparts, due to
their higher energy absorption capability. The interlaminar fracture toughness in
both mode I and mode I1 fracture increase rapidly with increasing film thickness
when the film is relatively very thin, whereas it becomes a constant value once the