Page 250 - Plastics Engineering

Mechanical Behaviour of Composites 233

A variety of methods have been suggested to deal with the prediction of
failure under multi-axial stresses and some of these have been applied to
composites. The main methods are
(i) Maximum Stress Criterion: This criterion suggests that failure of the
composite will occur if any one of five events happens

oI 2 CTT or 01 5 &c or 02 2 62T or a2 5 & or t12 2 312
That is, if the local tensile, compressive or shear stresses exceed the materials
tensile, compressive or shear strength then failure will occur. Some typical
values for the strengths of uni-directional composites are given in Table 3.5.

Table 3.5
Typical strength properties of unidirectional fibre reinforced plastics

Fibre
volume
fraction, 3117 32r 62 &IC 32c
Material Vf (GN/m2) (GN/m2) (GN/m*) (GN/m2) (GN/m*)

GFRP 0.6 1.4 0.05 0.04 0.22 0.1
(E glasdepoxy)
GFRP 0.42 0.52 0.034 - - -
(E glasdpolyester)
KFRP 0.6 1.5 0.027 0.047 0.24 0.09
(Kevlar 49/epoxy)
CFRP 0.6 1.8 0.08 0.1 1.57 0.17
(Carbodepoxy)
CFRP 0.62 1.24 0.02 0.04 0.29 0.03
(Carbon HWepoxy)

GFRP - Glass fibre reinforced plastic
KFRP - Kevlar fibre reinforced plastic
CRFP - Carbon fibre reinforced plastic

(ii) Maximum Strain Criterion: This criterion is similar to the above only
it uses strain as the limiting condition rather than stress. Hence, failure is
predicted to occur if

(iii) Tsai-Hill Criterion: This empirical criterion defines failure as occur-

The values in this equation are chosen so as to correspond with the nature of
the loading. For example, if (TI is compressive, then 6~c is used and so on.