Many materials are processed in a specific way or introduced with a specific reinforcement to increase their toughness and prevent crack growth.
In composite materials the presence of fibres acts as a means of slowing crack growth by forming a bridge that spans the gap of the cracks and holds them shut.
The increased toughness is a result of the fibres acting as crack stoppers and the increased energy required to debond the fibres from the matrix.
The toughness of polymers may also be increased through the addition of rubber particles.
The cracks intersect the particles. The applied stress stretches the particles which acts to clamp the crack shut.