Page 364 - Mechanics Analysis Composite Materials

Chapter 7. Environmental, special loading, and manufacturing eflects 349
and find that after the failure of the second couple of fabric layers fi = 316.2 m/s.
This process is repeated until & = 0, and thus found number k determines the
minimum number of 0"/90"layers that can stop the projectile with striking velocity
V, = 320 m/s. The result of calculation is presented in Fig. 7.42 from which it
follows that k = 32. This is exactly the same number of layers that have been used to
construct the experimental plates.
Thus, it can be concluded that the high impact resistance of aramid fabrics is
determined by two main factors. First, by relatively high work of fracture which is
governed not only by the high strength, but also by the interaction of the fabric
layers. The broken line in Fig. 7.41 shows the fracture process constructed as a
result of superposition of experimental diagrams for individual 0" and 90" layers.
The solid line corresponds as was noted, to 0" and 90" layers tested together (the
ratio of the fabric strength under tension in the warp and the fill direction is 1.3).As
can be seen, the area under the solid line is much larger that under the broken one
which indicates high contribution of the layers interaction to the work of fracture. If
this conclusion is true, we can expect that for layers with higher anisotropy and for
laminates in which the principal material axes of the adjacent layers are not
orthogonal, the fracture work can be higher than for the orthotropic laminate under
study. The second factor increasing the impact resistance of aramid fabrics is
associated with a specific process of the failure during which the fabric layers fail
one after another but not at once. Plates of the same number of layers but consisting
of resin impregnated and co-cured layers that fail at once demonstrate much less
impact resistance.

I
V,,m sec
350 r

300
250

200
150

100
50
0
0 8 16 24 32
Fig. 7.42. Dependence of the residual velocity of the projectile on the number of penetrated layers.