NONLINEAR STRESS-STRAIN BEHAVIOUR AND STRENGTH
6-5
length dependent, the more consistent estimates of textile strength will be those based on
the measured strengths of equivalent unidirectional composites. From this datum, strength
knockdowns of 20-30% seem typical.
Various reasons for the lower measured strengths were summarized in Section 4.3.
The most important is probably damage suffered by the fibers in tows during the textile
process. Irregularity in geometry, including waviness and pinching in aligned tows, has a
relatively small influence on tensile strength [6.11]. Correlation between tensile strength
and degree of waviness has been claimed in one report [6.12], but based on rather few,
noisy data points; while it did not appear elsewhere [6.7].
Table 6.1. Predicted and measured ultimate strengths of
some triaxial glass/urethane braids and 3D carbon/epoxy
interlock weaves (from [6.1,6.7]; see these references for
nomenclature and detailed descriptions of composites).
Composite Measured Predicted Ratio
(MPa) (MPa)
AS4/1895 3D interlock weaves:
h-L-1 980 1350 0.73
h-L-2 935 1200 0.78
h-T-1 840 1300 0.65
h-T-2 895 1250 0.72
h-O-1 1070 1360 0.79
h-O-2 850 1220 0.70
triaxial glass/urethane braids:
45-1-G 200 350 0.57
45-1-A 270 400 0.68
40-1-G 180 300 0.54
35-2-G 155 220 0.70
55-0.5-G 280 470 0.60
45-0.5-G 275 360 0.76
30-0.5-G 290 400 0.73
6.3.2 Compressive Strength
For aligned loads, compressive failure is either by delamination and Euler buckling
of delaminated plies, especially in 2D textile composites, or, if delamination is suppressed,
by kink band formation (Section 4.2). Predicting strength for delaminating composites is
essentially the same problem for 2D textiles as it is for conventional tape laminates. It is
therefore not an appropriate topic for this handbook. Delamination models and codes for
tape laminates abound in the literature. They can be employed just as well for 2D textile
composites, with the stiffness of individual plies calculated by the models described in
Section 5.2 for quasi-laminar textiles.
Delamination and Euler buckling of delaminated layers can also be the mode of
failure of quasilaminar 3D textiles in compression, especially following impact damage.
However, if adequate yet modest volume fractions of through-thickness reinforcement are
analyzing the statistics of flaw distributions [6.10]. Since the strength of a material with a Weibull
distribution of flaws rises as the gauge length shortens, a stiff matrix can raise the composite strength
above that of the dry fiber bundle.
