ANALYTICAL METHODS FOR TEXTILE COMPOSITES
Data for triaxial braids occasionally show variations that are not readily correlated
with architecture or waviness. For example, Table 3.2 (from [3.8]) shows that the tensile
strengths for {[0
°
18k
/±66.5°
6k
]37.6% Axial} and {[0°
18k
/±70°
6k
]34% Axial} braids differ
by far more (30%) than the small differences in their fiber distributions would imply. No
significant differences in the degree of waviness were observed on cut sections [3.9]. It is
possible that the small change in orientation and packing of the bias tows caused more
damage during braiding to the axial tows in the braid with 70
° bias orientation. Further
research must define the process parameters involved, so that consistent strength can be
achieved. Bringing quality control to the levels common in tape laminates remains a
challenge in textile processing, although well made textiles show that it is an attainable
goal.
Table 3.2 Comparison of Unnotched Tensile Strengths for Various
AS4/1895 Triaxial Braids.
Longitudinal Tension Transverse Tension
BRAID Strength Ultimate Strength Ultimate
(MPa) Strain, % (MPa) Strain, %
[0°
24k
/±63°
12k
]
31.5%
432 ± 25 1.16 ± 0.13 243 0.67
[0°
18k
/±66.5°
6k
]
37.6%
556 ± 10 1.36 ± 0.07 288 ± 25 0.67 ± 0.12
[0°
18k
/±70°
6k
]
34%
394 ± 10 0.96 ± 0.12 321 ± 39 0.70 ± 0.14
3.4.3 Stitched Laminates
The unnotched and open hole tension strengths of some typical stitched and
unstitched laminates are compared in Figure 3-5, while Fig. 3-6 shows similar data for
compression, including compression after impact (CAI). The results show that stitching
hardly affects tensile strength, but reduces the unnotched compression strength by 10-20%.
However, stitching dramatically improves the CAI strength, being even more effective than
the toughened resin system IM7/8551-7.
Compression strength is reduced by stitching because the stitching misaligns in-plane
fibers near the surface [3.10]. Misalignment always facilitates fiber kinking (Sect. 4). If it
affects entire plies of fibers, it will also soften those plies and increase the stress born by
straighter plies. On the positive side, stitching provides preform debulking before the fabric
is placed into a tool, which helps maximize fiber volume fraction.
