Page 226 - Mechanics Analysis Composite Materials

Chapter 4. Mechanics of a composite layer 211
u,MPa
250 -

200 -

0 1 2 3 4
Fig. 4.84. Typical stress-strain curves for fiberglass knitted composites loaded in tension at different
angles with respect to direction indicated by the arrow Fig. 4.83.

(a) (b)
Fig. 4.85. Diamond (a) and regular (b) braided fabric structures.

strength characteristics of fabric composites (e.g., Skudra et al., 1989), for practical
design and analysis, these characteristics are usually determined by experimental
methods. Elastic constants entering constitutive equations written in the principal
material coordinates, e.g., Eqs. (4.59, are found testing strips cut out of fabric
composite plates at different angles with respect to the orthotropy axes. The 0" and
90" specimens are used to determine moduli of elasticity El, E2 and Poisson's ratios
v12, v21 (or parameters of nonlinear stress-strain diagrams), while the in-plane shear
stiffness can be obtained with the aid of off-axis tension described in Section 4.3.1.
For fabric composites, elastic constants usually satisfy conditions in Eqs. (4.80),and
there exists the angle 6 specified by Eq. (4.79) such that off-axis tension under this
angle is not accompanied with shear-extension coupling.
Because Eq. (4.79) specifying 6 includes shear modulus GI?,which is not known,
transform the results presented in Section 4.3.1. Using Eqs. (4.76) and assuming
that there is no shear-extension coupling (q,,,, = 0) we can write the following
equations: