Page 366 - Engineered Interfaces in Fiber Reinforced Composites

Chapter 8. Improvement of interkuminar fracture toughness with interface control 347

(2) Adhesive strips interleaved at a certain distance away from the free edge.
(3) Adhesive layers inserted over the whole laminate plane.
(4) Termination of a critical ply(s) with a tapered end a small distance away from
the free edge.
(5) Wrapping of the laminate edges with edge caps.
In particular, the techniques based on the termination of certain plies within the
laminate has also shown promise. Static tensile tests of [30"/-30"/30"/90"], carbon-
epoxy laminates containing terminals of [90"] layers at the mid-plane show that
premature delamination is completely suppressed with a remarkable 20% improve-
ment in tensile strength, compared to those without a ply terminal. Cyclic fatigue on
the same laminates confirms similar results in that the laminate without a ply
terminal has delamination equivalent to about 40% of the laminate width after
2 x lo6 cycles, whereas the laminates with a ply terminal exhibit no evidence of
delamination even after 9 x lo6 cycles. All these observations are in agreement with
the substantially lower interlaminar normal and shear stresses for the latter
laminates, as calculated from finite element analysis. A combination of the adhesive
interleaf and the tapered layer end has also been explored by Llanos and Vizzini,
(1 992).
Regarding the use of edge cap reinforcements, Kim (1983) applied a glass fiber
cloth, and Howard et al. (1986) used a Kevlar-carbon fiber hybrid composite layer
to cap the edges of carbon fiber-epoxy matrix composites. The observed
improvement in both static and fatigue strengths in the edge capped laminates is
attributed to the reduction in the interlaminar normal stress, similar to the adhesive
interleaving technique.
interleaving strips made from ductile short fibers, notably Kevlar fiber mat, and
an adhesive (Browning and Schwartz, 1986) provide extra energy required during
delamination crack propagation due to additional toughening mechanisms such as

Tdbk 8.4
Mode I and Mode I1 interlaminar fracture toughness values, qc and GTlC, of carbon tiber-epoxy matrix
composites containing various interleaved adhesive layers.*

Types of adhesive layer Adhesive thickness (mm) Gfc (kJ/m2) GflC (kJ/m2)
~~
Control 0 0.193 0.527
Tuff-ply 0.04 0.444 1.15
Tuff-ply 0.08 0.575 1.7
Tuff-ply 0.11 0.754 2.61
FM 73 0.12 0.975 1.84
FM 300 0. I 1.14 I .77
FM 300 0.26 1.47 2.23
FM 300 0.3 1.27 2.01
FM 300 0.68 I .48 2.32
FM 300 1.1 1.78 1.65
"After Sela et al. (1989).