Page 119 - 3D Fibre Reinforced Polymer Composites

108 30 Fibre Reinforced Polymer Composites

and interlaminar fracture toughening mechanisms are outlined in Section 5.4 and the
impact damage tolerance in Section 5.5. The properties of 3D woven sandwich
composites made using distance fabric are given in Section 5.6.

5.2 MICROSTRUCTURAL PROPERTIES OF 3D WOVEN COMPOSITES

The microstructure of a 3D woven composite is determined largely by the fibre
architecture to the woven preform and weaving process, and to a lesser extent by the
consolidation process. Various types of microstructural defects are inadvertently
produced during 3D weaving that can degrade the in-plane, through-thickness and
impact properties. The main types of defects are abrasion, breakage and distortion of
the in-plane and z-binder yarns as well as resin-rich and resin-starved regions.
Abrasion and breakage of the warp, weft and z-binder fibres’ are common types of
damage incurred in 30 weaving that are difficult to avoid. This damage occurs by the
bending of yarns in the weaving process and as yams slide against the loom machinery
(Lee et al., 2001,2002). For example, Figure 5.1 shows broken filaments in a yarn that
is passing through the guide to a 3D weaving loom. Figure 5.2 shows fragments of
broken fibre caused by 3D weaving. This damage from the weaving process can cause a
large reduction to the tensile strength of brittle yarns. Figure 5.3 shows cumulative
probability distribution plots by Lee et al. (2002) of the failure strength of an E-glass
yarn after different stages of weaving. It is seen that the tensile strength decreases
progressively after the tensioning, warping and take-up stages, causing an overall
strength reduction of about 30%. The loss in yarn strength is dependent on a number of
factors, such as the yam diameter, 3D fibre architecture, and type of loom. It is also
strongly influenced by the brittleness of the fibre, with glass yarns experiencing a
greater loss in strength than carbon or Kevlar yams. It is worth noting that the fibre
damage and loss in strength shown here for 3D woven fabric is also experienced with
2D fabric during conventional (single-ply) weaving.
In addition to abrasion and fracture, the fibres are distorted and crimped by 3D
weaving. The warp and weft yarns in 3D woven preforms have a large amount of
waviness, and typically the fibres are misaligned from the in-plane direction by 4 to 12”
(Cox et al., 1994, Callus et al., 1999; Kuo and KO, 2000). In extreme cases, the
misalignment can be greater than 12O, particularly in fibre segments close to the z-
binders. The fibres in 3D preforms show much greater waviness than in 2D prepreg
laminates, where the waviness is under 2-3”. The fibres in 3D preforms also experience
extreme localised distortion, known as crimping, at the surface regions where the z-
binder yarns cross-over the in-plane tows. The crimping of a filler tow is shown
schematically in Figure 5.4. This pinching by the z-binder crimps the surface yarns,
thus causing them to collimate (or bunch together) which creates pockets rich in resin
bet ween them.
The z-binder yarns can also experience excessive distortion in 3D woven
composites. This distortion can occur by a high tensile force applied to the z-binder in
the weaving process, as discussed earlier in Chapter 2. It can also occur during

Different terminology is used to describe the fibres in 3D woven composites. The warp yarns
are also known as ‘load-bearing yarns’ or ‘stuffers’ while weft yarns can be called ‘transverse
yams’ or ‘fillers’. The z-binder yam is also known as a ‘weaver’.