Page 214 - Engineered Interfaces in Fiber Reinforced Composites

196 Engineered interfaces in jiber reinforced composites

Table 5.8
Summary of Mode I and Mode I1 interlaminar fracture toughness values for unidirectional carbon fiber-
828 mPDA epoxy matrix composites with different fiber surface treatmentsa
Interlaminar properties AU-4 fiber AS-4 fibcr AS-4C fiber mPDA epoxy
Mode I interlaminar Unable to 0.33 iz 0.02 0.39 =k 0.03 0.13
fracture toughness, GI, (kJ/m2) determine
Mode I1 interlaminar fracture 0.40 It 0.08 1.04 f 0.17 1.15 f 0.13 -
toughness, GII, (kJ/rnZ)
“After Drzal and Madhukar (1993)

interfacial adhesion is expected in view of the analogy in loading geometry of the
two test methods. Enhancement of the interfacial bond strength changes the failure
mode from “interface-dominated’’ to “matrix-dominated”, which is mainly
responsible for the increase in mode I1 interlaminar fracture toughness. The above
finding suggests that when the interfacc bond strength approaches the matrix shear
strength, further increase in interface bonding may not impart additional improve-
ment in mode I1 interlaminar fracture toughness.

5.4. Polymeric fibers

5.4.1. Aramid,fihers

5.4.1 .I. Structure and properties of aramid fibers
The commercial aramid fibers are currently produced by several companies,
including du Pont (Kevlar), AKZO (Twaron) and Tenjin (Technora). Table 5.9
(Morgan and Allred, 1993) presents the chemical structures of aromatic polyamides
that are developed for commercial fiber production. Among these, Kevlar fibers
were the first developed and are now most popular. Thus, discussion in the following
will be mainly concerned with this fiber type, unless otherwise specified. The Kevlar
fibers are produced from liquid crystal dopes through a dry-jet wet-spinning process.
It is a polycondensation product of terephthaoyl chloride and p-phenylene diamine.
The molecules form a planar array with interchain hydrogen bonding as shown in
Fig. 5.19(a). The stacking sheets form a crystalline array whose bonding is rather
weak. Electron microscopy and diffraction studies show (Dobb et al., 1977) that
Kevlar fibers consist of radially arranged and axially pleated crystalline supramo-
lecular sheets, as schematically presented in Fig. 5.19(b).
Kevlar fibers are available in four forms: Kevlar, Kevlar 29, Kevlar 49 and the
recently developed Kevlar 149. Kevlar is designed specifically for reinforcements of
elastomers (e.g. tires and belts), while Kevlar 29 is used primarily for tensile
members such as ropes, cables, webbings and ballistic cloth. Kevlar 49 and 149 are
designed for reinforcement of high performance PMCs. Kevlar 149 offers a 40%