Page 215 - Mechanics Analysis Composite Materials

200 Mechanics and analysis of composite materials

For the layer under study, tan 4 = 1, E,. < 0, E.~> 0, so tan 6'< 1 and 6'< 45". But
in the plies with 4 < 45" transverse normal stresses, 02, become compressive (see
Fig. 4.67) and close the cracks. Thus, the load exceeding the level at which the
cracks appear due to shear locks the cracks and induces compression across the
fibers thus preventing material failure. Because 4' is only slightly less than 45",
material stiffness, E.r, is very low and slightly increases with the rise of strains
and decrease of 4'. For the material under study, the calculated and experimental
diagrams are shown in Fig. 4.70. Circle on the theoretical curve indicates the stress
a, that causes the cracks in the matrix. More pronounced behavior of this type is
demonstrated by glass-epoxy composites whose stress-strain curve is presented in
Fig. 4.71 (Alfutov and Zinoviev, 1982). A specific plateau on the curve and material
hardening at high strain are the result of the angle variation that is also shown in
Fig. 4.71.

'E,,%
0 0.4 0.8 1.2 1.6 2
Fig. 4.70. Experimental (solid line) and calculated (broken line) stress-strain diagrams for *45" angle-
ply carbon-epoxy layer.

0 2 4 6 8 E,,%

Fig. 4.71. Experimental dependencies of stress (I) and ply orientation angle (2) on strain for f45"angle-
ply glass-epoxy composite.