  We investigate the nucleation and propagation of cracks in self-assembled
viscoelastic fluids, which are made of surfactant micelles reversibly linked by
telechelic polymers. The morphology of the micelles can be continuously tuned,
from spherical to rod-like to wormlike, thus producing transient double
networks when the micelles are sufficiently long and entangled, and transient
single networks otherwise. For a single network, we show that cracks nucleate
when the sample deformation rate involved is comparable to the relaxation time
scale of the network. For a double network, by contrast, significant
rearrangements of the micelles occur as a crack nucleates and propagates. We
show that birefringence develops at the crack tip over a finite length, $\xi$,
which corresponds to the length scale over which micelles alignment occur. We
find that $\xi$ is larger for slower cracks, suggesting an increase of
ductility.
