The study of fracture surface morphologies allows
qualitative and a quantitative analysis of crack
propagation phenomena. An SEM analyses of
the fracture surfaces at the region of stable crack
growth demonstrates that the changes in fracture
toughness discussed above are associated with
a fundamental alteration in the crack propagation
mechanisms. The predominantly unstable crack
propagation behavior of the blends for LN4 content
!20 wt% can be correlated with a brittle failure right
after the crack-tip blunting [113]. A small amount of
stable crack growth was observed ahead of an
unstable crack, characterized by dimple-like structures
formed by the coalescence of microvoids
(Section 4.4 and Fig. 9).