When a material fractures it absorbs energy. The main form of energy absorption is in the work required to propagate a crack through the material. This amount of work represents the toughness of the material. If more work is required to propagate a crack in a material then we can say that it is a tougher material.

However, in composites there are unique factors that contribute to energy absorption and result in a tougher material. Firstly, in many composites the bond between the fibres and matrix is not particularly strong. When a crack propagates through the material, debonding occurs between the fibres and matrix. Significant amounts of energy are required to achieve this separation.

Secondly, energy is also absorbed due to fibre pullout. Work is required to pull the fibres out of their holes in the matrix. The amount of work is dependent on the level of friction between the fibres and matrix. 

Crack propagation in a composite is affected by a combination of debonding and fibre pull-out. Click on the graphic on the right to view the simultaneous occurrence of debonding and fibre pull-out.

The strength of a composite in tension is much more than it is in compression. In tension, the strong fibres bear the large majority of the load. In compression, however, the fibres tend to buckle. The surrounding matrix is generally not strong enough to support the shear stresses created and a form of cooperative buckling occurs, referred to as a kink.

Fibre fracture under tension is shown in the middle image, while fibre kinking under compression is shown in the image on the right.