Figure 4-5. Schematic of delamination and shear failure events in a typical 2D weave.
Because tows in 2D weaves pass over and under one another, the primary load
bearing fibers suffer unavoidably large misalignments with respect to the applied load axis.
Therefore, axial shear stresses are high under nominally aligned loads and the critical load
for plasticity (probably leading to kinking) is relatively low. In triaxial braids, the axial
tows can be straight in principle, but in practice they are much more wavy than plies in a
tape laminate [4.2]. Critical loads for kinking are again relatively low [4.2,4.17].
Table 4.1 Compression Data for Gr/Ep Triaxial Braids (from [4.16]).
Braid Modulus
(GPa)
Ultimate Strength
(MPa)
Ultimate Strain
(%)
[0
6k
/±45
15k
]
12%
32.4 ± 1.0 353 ± 37 1.56 ± 0.22
[0
36k
/±45
15k
]
46%
69.7 ± 1.8 621 ± 17 1.03 ± 0.04
[0
30k
/±70
6k
]
46%
62.0 ± 0.3 503 ± 43 0.86 ± 0.08
[0
75k
/±70
15k
]
46%
63.4 ± 2.1 439 ± 752 0.77 ± 0.14
Once kinking or other shear failure occurs, inter-ply delamination usually ensues
immediately. If inter-ply delamination is already present, as after impact, failure may occur
via delamination crack growth and Euler buckling without kink band formation. Because
the delamination stress following impact or the kinking stress otherwise is low, 2D weaves
and braids appear to be inferior choices for applications involving significant in-plane
compression.
