The basic damage phenomena in unidirectional com­posites under on-axis tensile loads involve multiple microcracks or cracks that form in the matrix perpen­dicular to fiber direction and that are arrested by the fibers by deflection in the fiber-matrix interface. In the composites reinforced with fabrics of fiber bundles, matrix damage is influenced by a multilength scale structure.39 Furthermore, 2D CVI SiC/SiC is ahetero — geneous medium because of the presence of fibers, large pores (referred to as macropores) located between the plies or at yarn intersections within the plies, and a uniform layer of matrix over the fiber preform (referred to as the intertow matrix) (Figure 5). Much smaller

Longitudinal tow

Transversal tow

Layer

0.5mm

Figure 5 Micrograph showing the microstructure of a 2D CVI SiC/SiC composite.

pores are also present within the tows. Under on-axis tension, damage in 2D CVI SiC/SiC occurs essentially in the formation of matrix cracks perpendicular to longitudinal fiber axis and their deflection either by the tows (first and second steps) or by the fibers within the tows (third step). These steps (Figure 6) correspond to deformation increments:

Step 1: cracks initiate at macropores where stress concentrations exist (deformations between 0.025% and 0.12%);

Step 2: cracks form in the transverse yarns and in the interply matrix (deformations between 0.12% and 0.2%);

Step 3: transverse microcracks initiate in the lon­gitudinal tows (deformations larger than 0.2%). These microcracks are confined within the lon­gitudinal tows. They do not propagate in the rest of the composite. The matrix in the longi­tudinal tows experiences a fragmentation pro­cess and the crack spacing decreases as the load increases.

As mentioned earlier, the directions of principal stresses are dictated by fiber orientation rather than by the loading direction. Thus, under on-axis condi­tions, all the matrix cracks are perpendicular to the loading direction. Then, under off-axis tension, matrix cracks that are located in the tows are
perpendicular to fiber direction, whereas those located between the tows are perpendicular to the load direction. On-axis loading conditions are dis­cussed later.

The resulting Young’s modulus decrease illustrates the importance of damage in the mechanical behavior (Figure 4). The major modulus loss (70%) is caused by both the first families of cracks located on the outside of the longitudinal tows (deformations <0.2%). By contrast, the microcracks within the longi­tudinal tows are responsible for only a 10% loss. The substantial modulus drop reflects important changes in load sharing: the load gets carried essentially by the matrix-coated longitudinal tows (tow reloading). During microcracking in the longitudinal tows, load sharing is affected further, and the load becomes carried essentially by the filaments (fiber reloading). The elastic modulus reaches a minimum described by the following equation (Figure 4):

Emin = 1/2Ef Vf [10]

where Vf is the volume fraction of fibers.

Equation [10] implies that the matrix contribution is negligible. At this stage, matrix damage and debonding are complete (saturation). The load is carried by fibers only. The mechanical behavior is controlled by the fiber tows oriented in the direction of loading.