b) Matrix Level Damage Mechanisms:

There are two main damage mechanisms in matrix. These are: Matrix cracking and fibre interfacial debonding. These are explained below.

1) Matrix Cracking:

When the stress in the matrix exceeds the strength of the matrix, matrix cracks are developed. There are two types of matrix cracks that are developed in a unidirectional lamina. The cracks are either perpendicular or parallel to the fibre direction. In the first type, the cracks are developed when axial stress in the lamina is tensile in nature. In the second type, the cracks are developed when the in-plane transverse stress in the lamina is tensile in nature.

It is generally seen that the matrix cracks develop along the preferred directions in unidirectional lamina. The matrix cracks which are parallel to the fibre direction cause significant modulus degradation whereas the matrix cracks which are perpendicular to the fibre direction cause less degradation in modulus. The first mode of damage is very critical as one of them causes significant degradation. The second mode can go undetected sometimes. This is very dangerous from safety point of view. For example, for gas pipes leakage is an important criterion. If such damage is not detectable, it can lead to a catastrophe. This damage is shown in Figure 6.2(a), (b).

2) Fibre Interfacial Cracking:

When the in-plane transverse stresses in matrix are tensile in nature, the weaker interface between fibre and matrix is broken. A crack in the matrix region at this location is initiated. This crack grows along the fibre length. This leads to the debonding of the interphase between fibre and matrix. This mode of damage is also called “transverse fibre debonding”. This damage is shown in Figure 6.2(c).