NONLINEAR STRESS-STRAIN BEHAVIOUR AND STRENGTH
6-7
present no certified methods of monitoring misalignments in textile composites. Design
limits for compression currently depend on being able to place upper bounds on
misalignment angles if certain processing conditions are met, such as control of tow tension
during manufacture of a fiber preform. This is clearly an area for further work in
developing manufacturing technologies.
6.3.3 Shear Strength
Just as for tape laminates, the in-plane shear strength of a quasi-isotropic textile
composite is dominated by the strength of tows (or plies) deforming in tension or
compression. To estimate shear strength is then to estimate the critical axial stress in tows
for rupture in tension or kink band formation in compression.
4
In contrast, shear strength in composites reinforced in two orthogonal directions
only is determined by the resistance of the matrix within tows or plies to axial shear. For
example, a tensile test of a ±45° laminate, such as that reported in Fig. 4-2(a), creates
conditions of pure deviatoric shear within plies. Figure 4-2(a) implies an ultimate shear
strength of 75 MPa for the typical aerospace resin, Shell 1895. Compact shear tests have
also been reported for 3D interlock weave composites containing the same resin [6.15].
The shear response of these composites is dominated by alternating layers of orthogonal
stuffers and fillers. The ultimate shear strength in this test configuration was found to be
65-80 MPa for a range of different interlock architectures and filler and stuffer filament
counts. The coincidence of these strengths with the shear flow stress of Fig. 4-2(a)
suggests that shear failure also occurs in the interlock weaves of [6.15] when the axial
shear in stuffers and fillers reaches the critical value,
τ
c
.
Thus shear strength can be approached via the critical shear flow stress,
τ
c
.
Micromechanical arguments suggest that this strength should be influenced negligibly by
the fiber stiffness, provided the fibers are much stiffer than the matrix [6.16,6.17]. It is
also unlikely to vary much with the fiber volume fraction for the ranges of fiber packing
expected in well consolidated composites.
6.3.4 Multi-Axial Loads
Figure 6-2 shows feasible failure loci for individual tows in a textile composite
under multi-axial loads. Since these are the failure criteria that would be applied to
individual tow segments when the textile composite is represented as a tessellation of
unidirectional grains, the failure boundaries are similar to those that might be expected in a
unidirectional polymer composite.
Figure 6-2(a) shows the failure locus for combined aligned loads and axial shear.
The local fiber direction is the x-axis. Combined axial compression and shear lead to kink
band formation at a reduced critical axial stress given approximately by [6.18]
4
Experimental evidence confirms that shear strength rises with the volume fraction, V
θ
, of off-axis fibers
for small to moderate values of V
θ
. However, when V
θ
rises above approximately 45%, the strength
saturates and no longer increases [6.14]. This saturation has been attributed tentatively to easier kink band
formation in the off-axis plies when they are dominant; they are conjectured to kink more easily as plies of
other orientation become thinner and impose less constraint. However, most aerospace applications require
material stiffness under load states that are predominantly compressive or tensile, with modest shear loads.
Most fibers must then be aligned with the primary load axis and the saturation of shear strength at large V
θ
will not be relevant.
