Page 260 - Mechanics Analysis Composite Materials

Chapter 5. Mechanics of laminates 245

All quasi-isotropic laminates having different structures determined by Eqn.
(5.55) for a given number of layers, k, possess the same apparent modulus and
Poisson’s ratio specified by Eqs. (5.56). For typical advanced composites with
properties listed in Table 3.5,these characteristics are presented in Table 5.2.
As follows from Tables 5.2 and I. 1,specific stiffness of quasi-isotropic composites
with carbon and boron fibers exceeds the corresponding characteristic of traditional
isotropic structural materials - steel, aluminum, and titanium.

5.6. Symmetric laminates

Symmetric laminates are composed of layers that are symmetrically arranged with
respect to the laminate middle plane shown in Fig. 5.9. To study general properties
of symmetric laminates, consider Eqs. (5.28) and (5.29) and apply them to calculate
stiffness coefficients with some combination of subscripts, e.g., pn = 1 and n = 1.
Because coordinate of the reference plane, e, is an arbitrary parameter, we can find
it from the condition CII= 0. Then,

(5.57)

Table 5.1
Angles providing quasi-isotropic properties of the laminates.
Number of layers, Orientation angle of the ith layer
k
4; 4; 4; 4; 4; 4:
3 0 60 120 -
4 0 45 90 135 - -
5 0 36 72 I08 144 -
6 0 30 60 90 120 150

Table 5.2
Modulus of elasticity and Poisson’s ratio of quasi-isotropic laminates made of typical advanced
composites.
Property Glass-epoxy Carbon-poxy Aramid-epoxy Boron+poxy Boron-AI
Modulus, E, GPa 27.0 54.8 34.8 80.3 183.1
Poisson’s ratio, v 0.34 0.31 0.33 0.33 0.28
Specific modulus, 1290 3530 2640 3820 6910
kl; x lo3,m