Page 291 - Mechanics Analysis Composite Materials

276 Mechanics and analysis of composite materials

0 40 80 120 160 200 240 280

Fig. 6.7. Failure envelope for glass-epoxy fabric composite in plane (01, ~12).(-) maximum stress
criterion, Eqs. (6.2); (0) experimental data.
the accuracy determined by the scatter of experimental results. As was already
noted, this criterion ignores the interaction of stresses. However, this interaction
takes place in fabric composites which are loaded with compression in two
orthogonal directions, because compression of the filling yarns increases the
strength in the warp direction and vice versa. The corresponding experimental
results from Belyankin et al. (1971) are shown in Fig. 6.8. As can be seen, there is a
considerable deviation of experimental data from the maximum stress criterion
shown with solid lines. However, even in such cases this criterion is sometimes used
to design composite structures, because it is simple and conservative, i.e., it
underestimates material strength increasing the safety factor for the structure under
design. There exist fabric composites for which the interaction of normal stresses
shows itself under tension as well. An example of such a material is presented in

02,MPa
-120

-100 \
\
\
-80 Y)
I
-60 I
I
-40 Io
I
-20 10
I
0 - o,,MPa
0 -20 -40 -60 -80 -100 -120
Fig. 6.8. Failure envelope for glass-phenolic fabric composite loaded with compression in plane (ul,u2).
(-)maximum stress criterion,Eqs. (6.2); (- ---) polynomialcriterion,Eqs. (6.15); (0) experimental
data.