  We study the nonlinear rheology of a novel class of transient networks, made
of surfactant micelles of tunable morphology reversibly linked by block
copolymers. We couple rheology and time-resolved structural measurements, using
synchrotron radiation, to characterize the highly nonlinear viscoelastic
regime. We propose the fluctuations of the degree of alignment of the micelles
under shear as a probe to identify a fracture process. We show a clear
signature of a brittle-to-ductile transition in transient gels, as the
morphology of the micelles varies, and provide a parallel between the fracture
of solids and the fracture under shear of viscoelastic fluids.
