ANALYTICAL METHODS FOR TEXTILE COMPOSITES
2. Tow waviness. Nominally straight tows, which are highly desirable in stiffness
critical applications, always exhibit some degree of waviness. Fig. 2-21 shows
waviness in the through-thickness direction typical of triaxial braids, in which the axial
tows are nominally straight, and interlock weaves, which contain nominally straight
stuffers. The misalignments even in quasi-laminar textiles are usually considerably
greater than in tape laminates. Tow waviness tends to be commensurate with the
spacing of tows that traverse the subject tow, but waviness deflections are not periodic.
Tow misalignment has been described by normal distributions [2.15, 2.21] and power
spectra [2.27,2.28]. In flat panel specimens of quasi-laminar textiles, out-of-plane tow
waviness is often the larger component and is directly correlated with failure
mechanisms ([2.21, 2.23, 2.24, 2.25] and Section 4), but in-plane waviness can also
be significant.
Fig. 2-21. Waviness in nominally straight axial tows in a triaxial braid. The fine white curves mark the
centers of mass of nominally straight axial tows determined by digital image analysis.
Tow waviness can be especially pronounced in through-thickness tows in quasi-
laminar textiles, e.g., stitching tows or warp weavers in an interlock weave. Through-
thickness tows are very vulnerable to the through-thickness pressure that is favoured in
consolidating plate-like parts to enhance the in-plane fiber volume fraction (Fig. 2-22).
Waviness in through-thickness tows can only be controlled by forming the textile
preform as nearly as possible to its desired final thickness before the consolidation
process.
3. Tow pinching. In all textiles, the cross-sectional aspect ratios of tows are not
uniform along their axes. Each tow is subjected to irregular pressures imposed by
neighboring tows during both manufacture of the textile preform and the consolidation
process. Each tow is pinched in different directions at different points. The resulting
cross-sectional variations are especially pronounced in textiles with predominantly
curved tows. Fig. 2-23 shows tow outlines sketched from a photomicrograph of a 3D
braid. Cross-sectional variations are also a major form of irregularity in textiles formed
predominantly of nominally straight tows.
