However, its inability to similarly undergo crazing reduces the amount of deformation post-yield, resulting in brittle behaviour . Fracturing this highly cross-linked network requires the breaking of primary (covalent) bonds, since the molecules are chemically and physically prevented from sliding past each other. As such failure will originate at, and propagate from, defect sites in the polymer network, where local bonds are under increased stress . A secondary 'filler' phase can be introduced into the epoxy to help dissipate energy, delaying crack propagation, through a transfer of stress from the matrix.