ANALYTICAL METHODS FOR TEXTILE COMPOSITES
In the traditional textile industry, yarns are twisted to provide structural integrity
and the ability to hold shape. In forming structural composites, in contrast, softer yarns are
desirable, since this allows compaction to maximize the total volume fraction or flattening
(especially in braids) to maximize coverage. Furthermore, twist would reduce the axial
stiffness of yarns, which is paramount in airframe applications. Therefore, yarns with
minimal or nominally zero twist (tows) are preferred.
Epoxy resins have been the predominant matrix in textile composites manufactured
by RTM or RFI for commercial aircraft structures. Epoxies meet the requirement of having
a low-viscosity state (100-500 cps), which is crucial for flow through a low permeability
preform with complete wetting of the fibers. Some epoxies that have been used in
prepregs, e.g. Hercules 3501-6 and 3502, have also been used for the RTM process (see
Section 2.1.3). The extensive data available from their use in prepregs has made them
attractive choices. There are now also a number of resins that have been especially
formulated for RTM, including 3M PR-500, BP Chemical E-905L, and Shell RSL-1895.
PEEK is a typical thermoplastic for manufacture via commingling or spinning a
matrix onto yarns, as described in Section 2.1.3.
Table 2.2 Typical Yarn Weights and Dimensions
Fiber Filament
count
Area of
Fibers
(mm
2
)
Diam. of Circular
Yarn with p
d
=
0.75
(mm)
Denier
(g/9000m)
Hercules AS4 3K 0.12 0.45 1990
Dia = 7.1 µm 6K 0.24 0.64 3980
Density = 1850 kg/m
3
12K 0.48 0.90 7960
30K 1.19 1.42 19900
75K 2.98 2.25 49800
Hercules IM6 3K 0.07 0.35 1190
Dia = 5.6 µm 6K 0.15 0.50 2380
Density = 1800 kg/m
3
12K 0.29 0.71 4770
30K 0.74 1.12 11900
75K 1.84 1.77 29800
Du Pont Kevlar 49 1K 0.11 0.44 1450
Dia=11.9 µm 2K 0.22 0.62 2900
Density = 1440 kg/m
3
3K 0.34 0.75 4350
4K 0.45 0.87 5800
