Computational Methods for Debonding in Composites

36 R. Rolfes et al.
Unit cell
(a) Scan of NCF (b) Discretization
Fig. 2.7 Mesomechanical non-crimp fabric unit cell
resin pocket
2 Stitch yarn
Fiber orientation
1
b H
r N
5 mm
Fig. 2.8 Geometry of mesomechanical unit cell for non-crimp fabric
of the stitch yarn has no significant influence, because of its small diameter and
minor material properties of the PES-yarn compared to the glass-fibers that lie in
the same plane. The discretization of the unit cell is shown in Fig. 2.7b, it consists
of 200 × 200 × 1 elements. The fiber bundle structure in thickness direction is
neglected, hence only one element is used in this direction. Figure 2.8 shows the different
components, fiber bundle, epoxy resin pocket and stitching yarn, of the unit
cell and their dimensions. The resin pocket is LH = 3.5 mm long and bH = 0.3mm
wide, the radius of the stitch yarn is rN = 0.054 mm.
2.2.4.2 Weft-Knitted Fabrics
Compared to the non-crimp fabric described above the yarn in weft-knitted fabrics
presented here plays a bigger role, although still the yarn diameter is smaller than the
fiber bundle diameter. Firstly the loops of the yarn are denser and secondly the yarn
is stronger, because it consists of glass fibers instead of polyester and has a larger
diameter. Secondly the material structure of the weft-knitted fabric is much more
heterogeneous, the individual fiber bundles are clearly identifiable. Therefore in the
draping process layers of weft-knitted fabric penetrate each other, as can be seen in
Fig. 2.9. Thus the smallest unit cell of the weft-knitted fabric, see Fig. 2.9 includes
5 mm
L