c) Coupled Fibre-Matrix Level Mechanisms:

1) Fibre Pullout:

The fibre pullout takes place when the bonding between fibre and matrix is weakened and the fibres are subjected to tensile stresses. If the fibres are already broken then the fibres just slide through the matrix and come out of it. This phenomenon is called fibre pullout.

The fibre pullout is shown in Figure 6.3(a).

2) Fibre Breakage and Interfacial Debonding:

When the fibres break the interface close to the tip of broken fibre, acts as a site of stress concentration. The interface may then fail, leading to debonding of the fibre from matrix.

The fibre breakage leading to interfacial debonding is shown in Figure 6.3(b).

3) Transverse Matrix Cracking:

The interface failure causing debonding (as in fibre breaking and interfacial debonding in above case) from the matrix may act like as a stress concentration site for the in-plane transverse tensile stress. When this stress exceeds the limiting stress in matrix, it leads to through thickness transverse crack in the matrix.

The through thickness transverse matrix cracking is shown in Figure 6.3(c).

4) Fibre Failure due to Matrix Cracking:

The matrix cracks formed (as in matrix cracking case above) may terminate at fibre interface at low strains, while, at high strains, the stress at the crack tip may exceed the fracture stress of the fibres, leading fibre failure.

The fibre failure due to matrix cracking is depicted in Figure 6.3(d).

5) Interfacial Shear Failure:

The fibre fracture or fibre failure due to matrix cracking may cause the matrix crack to propagate as macro-crack under opening mode until it hits an interface. The shear stresses may cause its propagation in sliding mode leading to a progressive failure of the interface.

The interfacial shear failure is shown in Figure 6.3(e).