2.12.6.1 Tensile Stress-Strain Behavior

Figures 1 and 2 summarize the typical stress-strain behavior of 2D CVI SiC/SiC composites. The behav­ior is initially linear under strains below 0.03%. Then,

Source: Aubard, X.; Lamon, J.; Allix, O. J. Am. Ceram. Soc. 1994, 77, 2118-2126.

Property

the nonlinear deformations result essentially from transverse cracking in the matrix (the cracks are per­pendicular to fibers oriented in the loading direction). Saturation of matrix damage is indicated by the end of the curved domain marked by a point of inflection. Then the ultimate portion of the curve reflects the deformation of fibers. Fiber failures may initiate prior to ultimate fracture. Such mechanical behavior is essentially damage-sensitive.

A damage-sensitive stress-strain behavior is obtained when the initial contribution of the matrix to load carrying is significant. The elastic modulus of the matrix (Em) is not negligible when compared to that of the fiber (Ef). Its contribution to the modulus of the composite (Ec) is illustrated by the mixtures law, which provides satisfactory trends for continuous fiber-reinforced composites:

Ec = EmVm + Ef Vf [9]

where Vm is the volume fraction of matrix and Vf is the volume fraction of fibers oriented in the loading direction in a 2D woven composite.

In 2D CVI SiC/SiC composites, Em («410 GPa) > Ef (200-380 GPa), Vm ~ Vf the initial contribution of the matrix to Ec is significant. Then, as it decreases when the matrix cracks, the behavior becomes con­trolled by the tows. The 2D SiC/SiC composites exhibit an elastic damageable behavior (Figure 3). This means that the response of the damaged mate­rial is elastic as indicated by the linear portion of the curves on reloading. Figure 4 shows the dependence of the elastic modulus on damage.