Damage Mechanisms in Fibrous Composites:

The damage mechanisms in a fibrous composite are broadly categorized as:

1. Micro-level damage mechanisms

2. Macro-level damage mechanisms and

3. Coupled micro-macro-level damage mechanisms

The local level mechanisms are further subcategorized based on constituent level as

1. Fibre level damage mechanisms

2. Matrix level damage mechanisms and

3. Coupled fibre-matrix level damage mechanisms

A. Micro-level Damage Mechanisms:

First, we will look at the micro-level mechanisms in detail as follows:

a) Fibre Level Damage Mechanisms:

The fibre failure mode is considered to be the most catastrophic mode of failure in laminates. This is because the fibre is the load carrying constituent. The failure of fibres can take place due to various stress components. The damage mechanisms for fibre are explained below in detail.

1) Fibre Fracture/Breaking:

The fibre breaks into two or more pieces along its length when the axial tensile stress (or strain) in the fibre exceeds the axial strength (or maximum allowable strain) of the fibre. This kind of fracture occurs in brittle fibres. Such fractures are more catastrophic in nature than other modes of fibre failure.
The fibre fracture may also take place in shearing when the shear stress or strain exceeds the maximum allowable stress or strain.
The fibre fracture is depicted in Figure 6.1(a).

2) Fibre Buckling or Kinking:

This type of failure occurs when the axial load on the fibre is compressive in nature. The axial compressive stress causes the fibre to buckle. This form of fibre failure is also called as fibre kinking. The critical stress at which the kinking takes place is function of material properties of fibre and matrix properties and the distribution of fibres in the matrix. In general, the fibre kinking first starts at the site of fibre misalignment or local defects.

It is seen that the kinking of fibres takes place in a sharply defined region. This region is called as kink band. In general, the kink band is oriented at an angle with respect to fibre direction.
This mechanism is one of the key failure mechanisms for laminates under compression. This failure mechanism triggers the other failure mechanisms leading to a complex and inter-related mechanisms.

The fibre kinking is depicted in Figure 6.1(b).

3) Fibre Bending:

The bending of fibre can take place under flexural load. The bending of fibres also depends upon the properties of fibre and matrix along with the fibre arrangement.
The fibre bending is shown in Figure 6.1(c).

4) Fibre Splitting:

The fibre fails in this mode when the transverse or hoop stresses in the fibre exceeds the maximum allowable value. Further, this can also happen when these stresses in the interface/interphase region (region in matrix very close to the fibre) exceed the maximum allowable stress. The fibre splitting is elucidated in Figure 6.1(d).

5) Fibre Radial Cracking:

The hoop stresses can also cause the radial cracking of the fibre. This type of cracking is seen in some of the fibres. The radial cracking of a fibre is shown in Figure 6.1(e).